Commit Graph

690 Commits

Author SHA1 Message Date
Paul E. McKenney
9036c2ffd5 rcu: Improve non-root rcu_cbs_completed() accuracy
When rcu_cbs_completed() is invoked on a non-root rcu_node structure,
it unconditionally assumes that two grace periods must complete before
the callbacks at hand can be invoked.  This is overly conservative because
if that non-root rcu_node structure believes that no grace period is in
progress, and if the corresponding rcu_state structure's ->gpnum field
has not yet been incremented, then these callbacks may safely be invoked
after only one grace period has completed.

This change is required to permit grace-period start requests to use
funnel locking, which is in turn permitted to reduce root rcu_node ->lock
contention, which has been observed by Nick Piggin.  Furthermore, such
contention will likely be increased by the merging of RCU-bh, RCU-preempt,
and RCU-sched, so it makes sense to take steps to decrease it.

This commit therefore improves the accuracy of rcu_cbs_completed() when
invoked on a non-root rcu_node structure as described above.

Reported-by: Nicholas Piggin <npiggin@gmail.com>
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Tested-by: Nicholas Piggin <npiggin@gmail.com>
2018-05-15 10:29:07 -07:00
Paul E. McKenney
5b4c11d54b rcu: Add leaf-node macros
This commit adds rcu_first_leaf_node() that returns a pointer to
the first leaf rcu_node structure in the specified RCU flavor and an
rcu_is_leaf_node() that returns true iff the specified rcu_node structure
is a leaf.  This commit also uses these macros where appropriate.

Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Tested-by: Nicholas Piggin <npiggin@gmail.com>
2018-05-15 10:28:07 -07:00
Paul E. McKenney
cee4393989 rcu: Rename cond_resched_rcu_qs() to cond_resched_tasks_rcu_qs()
Commit e31d28b6ab ("trace: Eliminate cond_resched_rcu_qs() in favor
of cond_resched()") substituted cond_resched() for the earlier call
to cond_resched_rcu_qs().  However, the new-age cond_resched() does
not do anything to help RCU-tasks grace periods because (1) RCU-tasks
is only enabled when CONFIG_PREEMPT=y and (2) cond_resched() is a
complete no-op when preemption is enabled.  This situation results
in hangs when running the trace benchmarks.

A number of potential fixes were discussed on LKML
(https://lkml.kernel.org/r/20180224151240.0d63a059@vmware.local.home),
including making cond_resched() not be a no-op; making cond_resched()
not be a no-op, but only when running tracing benchmarks; reverting
the aforementioned commit (which works because cond_resched_rcu_qs()
does provide an RCU-tasks quiescent state; and adding a call to the
scheduler/RCU rcu_note_voluntary_context_switch() function.  All were
deemed unsatisfactory, either due to added cond_resched() overhead or
due to magic functions inviting cargo culting.

This commit renames cond_resched_rcu_qs() to cond_resched_tasks_rcu_qs(),
which provides a clear hint as to what this function is doing and
why and where it should be used, and then replaces the call to
cond_resched() with cond_resched_tasks_rcu_qs() in the trace benchmark's
benchmark_event_kthread() function.

Reported-by: Steven Rostedt <rostedt@goodmis.org>
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Tested-by: Nicholas Piggin <npiggin@gmail.com>
2018-05-15 10:27:29 -07:00
Paul E. McKenney
25f3d7effa rcu: Parallelize expedited grace-period initialization
The latency of RCU expedited grace periods grows with increasing numbers
of CPUs, eventually failing to be all that expedited.  Much of the growth
in latency is in the initialization phase, so this commit uses workqueues
to carry out this initialization concurrently on a rcu_node-by-rcu_node
basis.

This change makes use of a new rcu_par_gp_wq because flushing a work
item from another work item running from the same workqueue can result
in deadlock.

Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Tested-by: Nicholas Piggin <npiggin@gmail.com>
2018-05-15 10:25:44 -07:00
Paul E. McKenney
ad7c946b35 rcu: Create RCU-specific workqueues with rescuers
RCU's expedited grace periods can participate in out-of-memory deadlocks
due to all available system_wq kthreads being blocked and there not being
memory available to create more.  This commit prevents such deadlocks
by allocating an RCU-specific workqueue_struct at early boot time, and
providing it with a rescuer to ensure forward progress.  This uses the
shiny new init_rescuer() function provided by Tejun (but indirectly).

This commit also causes SRCU to use this new RCU-specific
workqueue_struct.  Note that SRCU's use of workqueues never blocks them
waiting for readers, so this should be safe from a forward-progress
viewpoint.  Note that this moves SRCU from system_power_efficient_wq
to a normal workqueue.  In the unlikely event that this results in
measurable degradation, a separate power-efficient workqueue will be
creates for SRCU.

Reported-by: Prateek Sood <prsood@codeaurora.org>
Reported-by: Tejun Heo <tj@kernel.org>
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Acked-by: Tejun Heo <tj@kernel.org>
2018-02-23 15:14:40 -08:00
Matthew Wilcox
a32e01ee68 rcu: Use wrapper for lockdep asserts
Commits c0b334c5bf and ea9b0c8a26 introduced new sparse warnings
by accessing rcu_node->lock directly and ignoring the __private
marker.  Introduce a new wrapper and use it.  Also fix a similar problem
in srcutree.c introduced by a3883df393.

Signed-off-by: Matthew Wilcox <mawilcox@microsoft.com>
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
2018-02-20 16:12:26 -08:00
Paul E. McKenney
d07aee2c03 rcu: More clearly identify grace-period kthread stack dump
It is not always obvious that the stack dump from a starved grace-period
kthread isn't instead that of a CPU stalling the current grace period.
This commit therefore adds a pr_err() flagging these dumps.

Reported-by: Peter Zijlstra <peterz@infradead.org>
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
2018-02-20 16:10:29 -08:00
Paul E. McKenney
d62df57370 rcu: Remove obsolete force-quiescent-state statistics for debugfs
The debugfs interface displayed statistics on RCU-pending checks but
this interface has since been removed.  This commit therefore removes the
no-longer-used rcu_state structure's ->n_force_qs_lh and ->n_force_qs_ngp
fields along with their updates.  (Though the ->n_force_qs_ngp field
was actually not used at all, embarrassingly enough.)

If this information proves necessary in the future, the corresponding
event traces will be added.

Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
2018-02-20 16:10:29 -08:00
Paul E. McKenney
01c495f72a rcu: Remove obsolete __rcu_pending() statistics for debugfs
The debugfs interface displayed statistics on RCU-pending checks
but this interface has since been removed.  This commit therefore
removes the no-longer-used rcu_data structure's ->n_rcu_pending,
->n_rp_core_needs_qs, ->n_rp_report_qs, ->n_rp_cb_ready,
->n_rp_cpu_needs_gp, ->n_rp_gp_completed, ->n_rp_gp_started,
->n_rp_nocb_defer_wakeup, and ->n_rp_need_nothing fields along with
their updates.

If this information proves necessary in the future, the corresponding
event traces will be added.

Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
2018-02-20 16:10:28 -08:00
Paul E. McKenney
62df63e048 rcu: Remove obsolete callback-invocation statistics for debugfs
The debugfs interface displayed statistics on RCU callback invocation but
this interface has since been removed.  This commit therefore removes the
no-longer-used rcu_data structure's ->n_cbs_invoked and ->n_nocbs_invoked
fields along with their updates.

If this information proves necessary in the future, the corresponding
event traces will be added.

Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
2018-02-20 16:10:27 -08:00
Lihao Liang
398953e62c rcu: Remove unnecessary spinlock in rcu_boot_init_percpu_data()
Since rcu_boot_init_percpu_data() is only called at boot time,
there is no data race and spinlock is not needed.

Signed-off-by: Lihao Liang <lianglihao@huawei.com>
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
2018-02-15 15:40:36 -08:00
Paul E. McKenney
1dfa55e019 Merge branches 'cond_resched.2017.12.04a', 'dyntick.2017.11.28a', 'fixes.2017.12.11a', 'srbd.2017.12.05a' and 'torture.2017.12.11a' into HEAD
cond_resched.2017.12.04a: Convert cond_resched_rcu_qs() to cond_resched()
dyntick.2017.11.28a: Make RCU dynticks handle interrupts from NMI
fixes.2017.12.11a: Miscellaneous fixes
srbd.2017.12.05a: Remove now-redundant smp_read_barrier_depends()
torture.2017.12.11a: Torture-testing update
2017-12-11 09:21:58 -08:00
Paul E. McKenney
efd88b02bb rcu: Add comment giving debug strategy for double call_rcu()
The following statement has for some reason proven non-intuitive:

	WARN_ON_ONCE(rcu_segcblist_empty(&rdp->cblist) != (count == 0));

This commit therefore adds a comment that states that this warning
usually triggers in response to a double call_rcu(), which is sort
of like a double free.  The comment also suggests building with
CONFIG_DEBUG_OBJECTS_RCU_HEAD=y to track down the double call_rcu().

Reported-by: David Howells <dhowells@redhat.com>
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
2017-12-11 09:17:39 -08:00
Paul E. McKenney
e68bbb266d rcu: Simplify rcu_eqs_{enter,exit}() non-idle task debug code
The code that checks for non-idle non-nohz_idle-usermode tasks invoking
rcu_eqs_enter() and rcu_eqs_exit() prints a considerable quantity of
helpful information.  However, these checks fire rarely, so the extra
complexity is no longer worth it.  This commit therefore replaces this
debug code with simple WARN_ON_ONCE() statements.

Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
2017-11-28 15:51:21 -08:00
Paul E. McKenney
9dd238e286 rcu: Fold rcu_eqs_exit_common() into rcu_eqs_exit()
There is now only one call to rcu_eqs_exit_common() and there is no other
reason to keep it separate.  This commit therefore inlines it into its
sole call site, saving a few lines of code in the process.

Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
2017-11-28 15:51:21 -08:00
Paul E. McKenney
215bba9f59 rcu: Fold rcu_eqs_enter_common() into rcu_eqs_enter()
There is now only one call to rcu_eqs_enter_common() and there is no other
reason to keep it separate.  This commit therefore inlines it into its
sole call site, saving a few lines of code in the process.

Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
2017-11-28 15:51:20 -08:00
Paul E. McKenney
2342172fd6 rcu: Avoid ->dynticks_nesting store tearing
Although ->dynticks_nesting is updated only by process level, it is
accessed from hardirq to check for interrupt-from-idle quiescent states.
Store tearing is thus possible, so this commit applies WRITE_ONCE()
to ->dynticks_nesting stores.

Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
2017-11-28 15:51:20 -08:00
Paul E. McKenney
914955e18c rcu: Stop duplicating lockdep checks in RCU's idle-entry code
The three RCU_LOCKDEP_WARN() calls in rcu_eqs_enter_common() are
redundant with other lockdep checks, so this commit removes them.

Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
2017-11-28 15:51:19 -08:00
Paul E. McKenney
dec98900ea rcu: Add ->dynticks field to rcu_dyntick trace event
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
2017-11-28 15:51:19 -08:00
Paul E. McKenney
84585aa8b6 rcu: Shrink ->dynticks_{nmi_,}nesting from long long to long
Because the ->dynticks_nesting field now only contains the process-based
nesting level instead of a value encoding both the process nesting level
and the irq "nesting" level, we no longer need a long long, even on
32-bit systems.  This commit therefore changes both the ->dynticks_nesting
and ->dynticks_nmi_nesting fields to long.

Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
2017-11-28 15:51:18 -08:00
Paul E. McKenney
bd2b879a1c rcu: Add tracing to irq/NMI dyntick-idle transitions
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
2017-11-28 15:51:18 -08:00
Paul E. McKenney
844ccdd7dc rcu: Eliminate rcu_irq_enter_disabled()
Now that the irq path uses the rcu_nmi_{enter,exit}() algorithm,
rcu_irq_enter() and rcu_irq_exit() may be used from any context.  There is
thus no need for rcu_irq_enter_disabled() and for the checks using it.
This commit therefore eliminates rcu_irq_enter_disabled().

Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
2017-11-27 08:42:03 -08:00
Paul E. McKenney
51a1fd30f1 rcu: Make ->dynticks_nesting be a simple counter
Now that ->dynticks_nesting counts only process-level dyntick-idle
entry and exit, there is no need for the elaborate segmented counter
with its guard fields and overflow checking.  This commit therefore
makes ->dynticks_nesting be a simple counter.

Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
2017-11-27 08:42:03 -08:00
Paul E. McKenney
58721f5da4 rcu: Define rcu_irq_{enter,exit}() in terms of rcu_nmi_{enter,exit}()
RCU currently uses two different mechanisms for tracking irqs and NMIs.
This is unnecessary complexity: Given that NMIs can nest and given that
RCU's tracking handles such nesting, the NMI tracking mechanism can also
be used to track irqs.  This commit therefore defines rcu_irq_enter()
in terms of rcu_nmi_enter() and rcu_irq_exit() in terms of rcu_nmi_exit().

Unfortunately, callers must still distinguish between the irq and NMI
functions because additional actions are taken when an irq interrupts
idle or nohz_full usermode execution, and these actions cannot always
be taken from NMI handlers.

Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
2017-11-27 08:42:03 -08:00
Paul E. McKenney
6136d6e48a rcu: Clamp ->dynticks_nmi_nesting at eqs entry/exit
In preparation for merging dyntick-idle irq handling into the NMI
algorithm, clamp ->dynticks_nmi_nesting value to allow for interrupts
that enter but never leave and vice versa.

It is important that the clamping happen outside of the extended quiescent
state.  Otherwise, there will be short windows where irqs and NMIs fail
to convince RCU to start watching.

Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
2017-11-27 08:40:10 -08:00
Paul E. McKenney
fd581a91ac rcu: Move rcu_nmi_{enter,exit}() to prepare for consolidation
This is a code-motion-only commit that prepares to define rcu_irq_enter()
in terms of rcu_nmi_enter() and rcu_irq_exit() in terms of rcu_irq_exit().

Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
2017-11-27 08:40:10 -08:00
Paul E. McKenney
a0eb22bf64 rcu: Reduce dyntick-idle state space
Both extended-quiescent-state entry and exit first update the nesting
counter and then adjust the dyntick-idle state.  This means that there
are four states: (1) Both nesting and dyntick idle indicate idle,
(2) Nesting indicates idle but dyntick idle does not, (3) Nesting indicates
non-idle and dyntick idle does not, and (4) Both nesting and dyntick
idle indicate non-idle.  This commit simplifies the state space by
eliminating #3, reversing the order of updates on exit from extended
quiescent state.

Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
2017-11-27 08:40:10 -08:00
Paul E. McKenney
2e672ab2d4 rcu: Avoid ->dynticks_nmi_nesting store tearing
NMIs can nest, and store tearing could in theory happen on carries
from one byte to the next.  This commit therefore adds the WRITE_ONCE()
macros preventing this.

Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
2017-11-27 08:40:09 -08:00
Linus Torvalds
8e9a2dba86 Merge branch 'locking-core-for-linus' of git://git.kernel.org/pub/scm/linux/kernel/git/tip/tip
Pull core locking updates from Ingo Molnar:
 "The main changes in this cycle are:

   - Another attempt at enabling cross-release lockdep dependency
     tracking (automatically part of CONFIG_PROVE_LOCKING=y), this time
     with better performance and fewer false positives. (Byungchul Park)

   - Introduce lockdep_assert_irqs_enabled()/disabled() and convert
     open-coded equivalents to lockdep variants. (Frederic Weisbecker)

   - Add down_read_killable() and use it in the VFS's iterate_dir()
     method. (Kirill Tkhai)

   - Convert remaining uses of ACCESS_ONCE() to
     READ_ONCE()/WRITE_ONCE(). Most of the conversion was Coccinelle
     driven. (Mark Rutland, Paul E. McKenney)

   - Get rid of lockless_dereference(), by strengthening Alpha atomics,
     strengthening READ_ONCE() with smp_read_barrier_depends() and thus
     being able to convert users of lockless_dereference() to
     READ_ONCE(). (Will Deacon)

   - Various micro-optimizations:

        - better PV qspinlocks (Waiman Long),
        - better x86 barriers (Michael S. Tsirkin)
        - better x86 refcounts (Kees Cook)

   - ... plus other fixes and enhancements. (Borislav Petkov, Juergen
     Gross, Miguel Bernal Marin)"

* 'locking-core-for-linus' of git://git.kernel.org/pub/scm/linux/kernel/git/tip/tip: (70 commits)
  locking/x86: Use LOCK ADD for smp_mb() instead of MFENCE
  rcu: Use lockdep to assert IRQs are disabled/enabled
  netpoll: Use lockdep to assert IRQs are disabled/enabled
  timers/posix-cpu-timers: Use lockdep to assert IRQs are disabled/enabled
  sched/clock, sched/cputime: Use lockdep to assert IRQs are disabled/enabled
  irq_work: Use lockdep to assert IRQs are disabled/enabled
  irq/timings: Use lockdep to assert IRQs are disabled/enabled
  perf/core: Use lockdep to assert IRQs are disabled/enabled
  x86: Use lockdep to assert IRQs are disabled/enabled
  smp/core: Use lockdep to assert IRQs are disabled/enabled
  timers/hrtimer: Use lockdep to assert IRQs are disabled/enabled
  timers/nohz: Use lockdep to assert IRQs are disabled/enabled
  workqueue: Use lockdep to assert IRQs are disabled/enabled
  irq/softirqs: Use lockdep to assert IRQs are disabled/enabled
  locking/lockdep: Add IRQs disabled/enabled assertion APIs: lockdep_assert_irqs_enabled()/disabled()
  locking/pvqspinlock: Implement hybrid PV queued/unfair locks
  locking/rwlocks: Fix comments
  x86/paravirt: Set up the virt_spin_lock_key after static keys get initialized
  block, locking/lockdep: Assign a lock_class per gendisk used for wait_for_completion()
  workqueue: Remove now redundant lock acquisitions wrt. workqueue flushes
  ...
2017-11-13 12:38:26 -08:00
Frederic Weisbecker
b04db8e19f rcu: Use lockdep to assert IRQs are disabled/enabled
Lockdep now has an integrated IRQs disabled/enabled sanity check. Just
use it instead of the ad-hoc RCU version.

Signed-off-by: Frederic Weisbecker <frederic@kernel.org>
Acked-by: Thomas Gleixner <tglx@linutronix.de>
Acked-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Cc: David S . Miller <davem@davemloft.net>
Cc: Lai Jiangshan <jiangshanlai@gmail.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Tejun Heo <tj@kernel.org>
Link: http://lkml.kernel.org/r/1509980490-4285-15-git-send-email-frederic@kernel.org
Signed-off-by: Ingo Molnar <mingo@kernel.org>
2017-11-08 11:13:55 +01:00
Ingo Molnar
72bc286b81 Merge branch 'for-mingo' of git://git.kernel.org/pub/scm/linux/kernel/git/paulmck/linux-rcu into core/rcu
Pull RCU updates from Paul E. McKenney:

 - Documentation updates
 - Miscellaneous fixes
 - RCU CPU stall-warning updates
 - Torture-test updates

Signed-off-by: Ingo Molnar <mingo@kernel.org>
2017-10-24 10:49:44 +02:00
Paul E. McKenney
ad4e25a3a1 Merge branches 'doc.2017.10.20a', 'fixes.2017.10.19a', 'stall.2017.10.09a' and 'torture.2017.10.09a' into HEAD
doc.2017.10.20a: Documentation updates.
fixes.2017.10.19a: Miscellaneous fixes.
stall.2017.10.09a: RCU CPU stall-warning updates.
torture.2017.10.09a: Torture-test updates.
2017-10-20 11:11:15 -07:00
Paul E. McKenney
27fdb35fe9 doc: Fix various RCU docbook comment-header problems
Because many of RCU's files have not been included into docbook, a
number of errors have accumulated.  This commit fixes them.

Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
2017-10-19 22:26:11 -04:00
Paul E. McKenney
c0da313e09 rcu: Add extended-quiescent-state testing advice
If you add or remove calls to rcu_idle_enter(), rcu_user_enter(),
rcu_irq_exit(), rcu_irq_exit_irqson(), rcu_idle_exit(), rcu_user_exit(),
rcu_irq_enter(), rcu_irq_enter_irqson(), rcu_nmi_enter(), or
rcu_nmi_exit(), you should run a full set of tests on a kernel built
with CONFIG_RCU_EQS_DEBUG=y.

Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
2017-10-19 12:13:36 -07:00
Paul E. McKenney
9b9500da81 rcu: Make RCU CPU stall warnings check for irq-disabled CPUs
One common question upon seeing an RCU CPU stall warning is "did
the stalled CPUs have interrupts disabled?"  However, the current
stall warnings are silent on this point.  This commit therefore
uses irq_work to check whether stalled CPUs still respond to IPIs,
and flags this state in the RCU CPU stall warning console messages.

Reported-by: Steven Rostedt <rostedt@goodmis.org>
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
2017-10-09 14:25:17 -07:00
Paul E. McKenney
f79c3ad618 sched,rcu: Make cond_resched() provide RCU quiescent state
There is some confusion as to which of cond_resched() or
cond_resched_rcu_qs() should be added to long in-kernel loops.
This commit therefore eliminates the decision by adding RCU quiescent
states to cond_resched().  This commit also simplifies the code that
used to interact with cond_resched_rcu_qs(), and that now interacts with
cond_resched(), to reduce its overhead.  This reduction is necessary to
allow the heavier-weight cond_resched_rcu_qs() mechanism to be invoked
everywhere that cond_resched() is invoked.

Part of that reduction in overhead converts the jiffies_till_sched_qs
kernel parameter to read-only at runtime, thus eliminating the need for
bounds checking.

Reported-by: Michal Hocko <mhocko@kernel.org>
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Cc: Peter Zijlstra <peterz@infradead.org>
[ paulmck: Keep PREEMPT=n cond_resched a no-op, per Peter Zijlstra. ]
2017-10-09 14:25:17 -07:00
Linus Torvalds
013a8ee628 Two updates.
- A memory fix with left over code from spliting out ftrace_ops
    and function graph tracer, where the function graph tracer could
    reset the trampoline pointer, leaving the old trampoline not to
    be freed (memory leak).
 
  - The update to Paul's patch that added the unnecessary READ_ONCE().
    This removes the unnecessary READ_ONCE() instead of having to rebase
    the branch to update the patch that added it.
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Merge tag 'trace-v4.14-rc1-3' of git://git.kernel.org/pub/scm/linux/kernel/git/rostedt/linux-trace

Pull tracing fixlets from Steven Rostedt:
 "Two updates:

   - A memory fix with left over code from spliting out ftrace_ops and
     function graph tracer, where the function graph tracer could reset
     the trampoline pointer, leaving the old trampoline not to be freed
     (memory leak).

   - The update to Paul's patch that added the unnecessary READ_ONCE().
     This removes the unnecessary READ_ONCE() instead of having to
     rebase the branch to update the patch that added it"

* tag 'trace-v4.14-rc1-3' of git://git.kernel.org/pub/scm/linux/kernel/git/rostedt/linux-trace:
  rcu: Remove extraneous READ_ONCE()s from rcu_irq_{enter,exit}()
  ftrace: Fix kmemleak in unregister_ftrace_graph
2017-10-04 08:34:01 -07:00
Paul E. McKenney
f39b536ce9 rcu: Remove extraneous READ_ONCE()s from rcu_irq_{enter,exit}()
The read of ->dynticks_nmi_nesting in rcu_irq_enter() and rcu_irq_exit()
is currently protected with READ_ONCE().  However, this protection is
unnecessary because (1) ->dynticks_nmi_nesting is updated only by the
current CPU, (2) Although NMI handlers can update this field, they reset
it back to its old value before return, and (3) Interrupts are disabled,
so nothing else can modify it.  The value of ->dynticks_nmi_nesting is
thus effectively constant, and so no protection is required.

This commit therefore removes the READ_ONCE() protection from these
two accesses.

Link: http://lkml.kernel.org/r/20170926031902.GA2074@linux.vnet.ibm.com

Reported-by: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Signed-off-by: Steven Rostedt (VMware) <rostedt@goodmis.org>
2017-10-03 10:27:32 -04:00
Linus Torvalds
ac0a36461f Stack tracing and RCU has been having issues with each other and lockdep
has been pointing out constant problems. The changes have been going into
 the stack tracer, but it has been discovered that the problem isn't
 with the stack tracer itself, but it is with calling save_stack_trace()
 from within the internals of RCU. The stack tracer is the one that
 can trigger the issue the easiest, but examining the problem further,
 it could also happen from a WARN() in the wrong place, or even if
 an NMI happened in this area and it did an rcu_read_lock().
 
 The critical area is where RCU is not watching. Which can happen while
 going to and from idle, or bringing up or taking down a CPU.
 
 The final fix was to put the protection in kernel_text_address() as it
 is the one that requires RCU to be watching while doing the stack trace.
 
 To make this work properly, Paul had to allow rcu_irq_enter() happen after
 rcu_nmi_enter(). This should have been done anyway, since an NMI can
 page fault (reading vmalloc area), and a page fault triggers rcu_irq_enter().
 
 One patch is just a consolidation of code so that the fix only needed
 to be done in one location.
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Merge tag 'trace-v4.14-rc1-2' of git://git.kernel.org/pub/scm/linux/kernel/git/rostedt/linux-trace

Pull tracing fixes from Steven Rostedt:
 "Stack tracing and RCU has been having issues with each other and
  lockdep has been pointing out constant problems.

  The changes have been going into the stack tracer, but it has been
  discovered that the problem isn't with the stack tracer itself, but it
  is with calling save_stack_trace() from within the internals of RCU.

  The stack tracer is the one that can trigger the issue the easiest,
  but examining the problem further, it could also happen from a WARN()
  in the wrong place, or even if an NMI happened in this area and it did
  an rcu_read_lock().

  The critical area is where RCU is not watching. Which can happen while
  going to and from idle, or bringing up or taking down a CPU.

  The final fix was to put the protection in kernel_text_address() as it
  is the one that requires RCU to be watching while doing the stack
  trace.

  To make this work properly, Paul had to allow rcu_irq_enter() happen
  after rcu_nmi_enter(). This should have been done anyway, since an NMI
  can page fault (reading vmalloc area), and a page fault triggers
  rcu_irq_enter().

  One patch is just a consolidation of code so that the fix only needed
  to be done in one location"

* tag 'trace-v4.14-rc1-2' of git://git.kernel.org/pub/scm/linux/kernel/git/rostedt/linux-trace:
  tracing: Remove RCU work arounds from stack tracer
  extable: Enable RCU if it is not watching in kernel_text_address()
  extable: Consolidate *kernel_text_address() functions
  rcu: Allow for page faults in NMI handlers
2017-09-25 15:22:31 -07:00
Paul E. McKenney
28585a8326 rcu: Allow for page faults in NMI handlers
A number of architecture invoke rcu_irq_enter() on exception entry in
order to allow RCU read-side critical sections in the exception handler
when the exception is from an idle or nohz_full CPU.  This works, at
least unless the exception happens in an NMI handler.  In that case,
rcu_nmi_enter() would already have exited the extended quiescent state,
which would mean that rcu_irq_enter() would (incorrectly) cause RCU
to think that it is again in an extended quiescent state.  This will
in turn result in lockdep splats in response to later RCU read-side
critical sections.

This commit therefore causes rcu_irq_enter() and rcu_irq_exit() to
take no action if there is an rcu_nmi_enter() in effect, thus avoiding
the unscheduled return to RCU quiescent state.  This in turn should
make the kernel safe for on-demand RCU voyeurism.

Link: http://lkml.kernel.org/r/20170922211022.GA18084@linux.vnet.ibm.com

Cc: stable@vger.kernel.org
Fixes: 0be964be0 ("module: Sanitize RCU usage and locking")
Reported-by: Steven Rostedt <rostedt@goodmis.org>
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Signed-off-by: Steven Rostedt (VMware) <rostedt@goodmis.org>
2017-09-23 16:49:42 -04:00
Alexey Dobriyan
9b130ad5bb treewide: make "nr_cpu_ids" unsigned
First, number of CPUs can't be negative number.

Second, different signnnedness leads to suboptimal code in the following
cases:

1)
	kmalloc(nr_cpu_ids * sizeof(X));

"int" has to be sign extended to size_t.

2)
	while (loff_t *pos < nr_cpu_ids)

MOVSXD is 1 byte longed than the same MOV.

Other cases exist as well. Basically compiler is told that nr_cpu_ids
can't be negative which can't be deduced if it is "int".

Code savings on allyesconfig kernel: -3KB

	add/remove: 0/0 grow/shrink: 25/264 up/down: 261/-3631 (-3370)
	function                                     old     new   delta
	coretemp_cpu_online                          450     512     +62
	rcu_init_one                                1234    1272     +38
	pci_device_probe                             374     399     +25

				...

	pgdat_reclaimable_pages                      628     556     -72
	select_fallback_rq                           446     369     -77
	task_numa_find_cpu                          1923    1807    -116

Link: http://lkml.kernel.org/r/20170819114959.GA30580@avx2
Signed-off-by: Alexey Dobriyan <adobriyan@gmail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
2017-09-08 18:26:48 -07:00
Paul E. McKenney
656e7c0c0a Merge branches 'doc.2017.08.17a', 'fixes.2017.08.17a', 'hotplug.2017.07.25b', 'misc.2017.08.17a', 'spin_unlock_wait_no.2017.08.17a', 'srcu.2017.07.27c' and 'torture.2017.07.24c' into HEAD
doc.2017.08.17a: Documentation updates.
fixes.2017.08.17a: RCU fixes.
hotplug.2017.07.25b: CPU-hotplug updates.
misc.2017.08.17a: Miscellaneous fixes outside of RCU (give or take conflicts).
spin_unlock_wait_no.2017.08.17a: Remove spin_unlock_wait().
srcu.2017.07.27c: SRCU updates.
torture.2017.07.24c: Torture-test updates.
2017-08-17 08:10:04 -07:00
Paul E. McKenney
16c0b10607 rcu: Remove exports from rcu_idle_exit() and rcu_idle_enter()
The rcu_idle_exit() and rcu_idle_enter() functions are exported because
they were originally used by RCU_NONIDLE(), which was intended to
be usable from modules.  However, RCU_NONIDLE() now instead uses
rcu_irq_enter_irqson() and rcu_irq_exit_irqson(), which are not
exported, and there have been no complaints.

This commit therefore removes the exports from rcu_idle_exit() and
rcu_idle_enter().

Reported-by: Peter Zijlstra <peterz@infradead.org>
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
2017-08-17 07:26:25 -07:00
Paul E. McKenney
d4db30af51 rcu: Add warning to rcu_idle_enter() for irqs enabled
All current callers of rcu_idle_enter() have irqs disabled, and
rcu_idle_enter() relies on this, but doesn't check.  This commit
therefore adds a RCU_LOCKDEP_WARN() to add some verification to the trust.
While we are there, pass "true" rather than "1" to rcu_eqs_enter().

Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
2017-08-17 07:26:25 -07:00
Peter Zijlstra (Intel)
3a60799269 rcu: Make rcu_idle_enter() rely on callers disabling irqs
All callers to rcu_idle_enter() have irqs disabled, so there is no
point in rcu_idle_enter disabling them again.  This commit therefore
replaces the irq disabling with a RCU_LOCKDEP_WARN().

Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
2017-08-17 07:26:24 -07:00
Paul E. McKenney
2dee9404fa rcu: Add assertions verifying blocked-tasks list
This commit adds assertions verifying the consistency of the rcu_node
structure's ->blkd_tasks list and its ->gp_tasks, ->exp_tasks, and
->boost_tasks pointers.  In particular, the ->blkd_tasks lists must be
empty except for leaf rcu_node structures.

Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
2017-08-17 07:26:23 -07:00
Masami Hiramatsu
35fe723bda rcu/tracing: Set disable_rcu_irq_enter on rcu_eqs_exit()
Set disable_rcu_irq_enter on not only rcu_eqs_enter_common() but also
rcu_eqs_exit(), since rcu_eqs_exit() suffers from the same issue as was
fixed for rcu_eqs_enter_common() by commit 03ecd3f48e ("rcu/tracing:
Add rcu_disabled to denote when rcu_irq_enter() will not work").

Signed-off-by: Masami Hiramatsu <mhiramat@kernel.org>
Acked-by: Steven Rostedt (VMware) <rostedt@goodmis.org>
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
2017-08-17 07:26:23 -07:00
Paul E. McKenney
d8db2e86d8 rcu: Add TPS() protection for _rcu_barrier_trace strings
The _rcu_barrier_trace() function is a wrapper for trace_rcu_barrier(),
which needs TPS() protection for strings passed through the second
argument.  However, it has escaped prior TPS()-ification efforts because
it _rcu_barrier_trace() does not start with "trace_".  This commit
therefore adds the needed TPS() protection

Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Acked-by: Steven Rostedt (VMware) <rostedt@goodmis.org>
2017-08-17 07:26:22 -07:00
Luis R. Rodriguez
d5374226c3 rcu: Use idle versions of swait to make idle-hack clear
These RCU waits were set to use interruptible waits to avoid the kthreads
contributing to system load average, even though they are not interruptible
as they are spawned from a kthread. Use the new TASK_IDLE swaits which makes
our goal clear, and removes confusion about these paths possibly being
interruptible -- they are not.

When the system is idle the RCU grace-period kthread will spend all its time
blocked inside the swait_event_interruptible(). If the interruptible() was
not used, then this kthread would contribute to the load average. This means
that an idle system would have a load average of 2 (or 3 if PREEMPT=y),
rather than the load average of 0 that almost fifty years of UNIX has
conditioned sysadmins to expect.

The same argument applies to swait_event_interruptible_timeout() use. The
RCU grace-period kthread spends its time blocked inside this call while
waiting for grace periods to complete. In particular, if there was only one
busy CPU, but that CPU was frequently invoking call_rcu(), then the RCU
grace-period kthread would spend almost all its time blocked inside the
swait_event_interruptible_timeout(). This would mean that the load average
would be 2 rather than the expected 1 for the single busy CPU.

Acked-by: "Eric W. Biederman" <ebiederm@xmission.com>
Tested-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Signed-off-by: Luis R. Rodriguez <mcgrof@kernel.org>
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
2017-08-17 07:26:15 -07:00
Paul E. McKenney
09efeeee17 rcu: Move callback-list warning to irq-disable region
After adopting callbacks from a newly offlined CPU, the adopting CPU
checks to make sure that its callback list's count is zero only if the
list has no callbacks and vice versa.  Unfortunately, it does so after
enabling interrupts, which means that false positives are possible due to
interrupt handlers invoking call_rcu().  Although these false positives
are improbable, rcutorture did make it happen once.

This commit therefore moves this check to an irq-disabled region of code,
thus suppressing the false positive.

Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
2017-07-25 13:04:50 -07:00
Paul E. McKenney
f2dbe4a562 rcu: Localize rcu_state ->orphan_pend and ->orphan_done
Given that the rcu_state structure's >orphan_pend and ->orphan_done
fields are used only during migration of callbacks from the recently
offlined CPU to a surviving CPU, if rcu_send_cbs_to_orphanage() and
rcu_adopt_orphan_cbs() are combined, these fields can become local
variables in the combined function.  This commit therefore combines
rcu_send_cbs_to_orphanage() and rcu_adopt_orphan_cbs() into a new
rcu_segcblist_merge() function and removes the ->orphan_pend and
->orphan_done fields.

Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
2017-07-25 13:04:49 -07:00
Paul E. McKenney
21cc248384 rcu: Advance callbacks after migration
When migrating callbacks from a newly offlined CPU, we are already
holding the root rcu_node structure's lock, so it costs almost nothing
to advance and accelerate the newly migrated callbacks.  This patch
therefore makes this advancing and acceleration happen.

Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
2017-07-25 13:04:48 -07:00
Paul E. McKenney
537b85c870 rcu: Eliminate rcu_state ->orphan_lock
The ->orphan_lock is acquired and released only within the
rcu_migrate_callbacks() function, which now acquires the root rcu_node
structure's ->lock.  This commit therefore eliminates the ->orphan_lock
in favor of the root rcu_node structure's ->lock.

Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
2017-07-25 13:04:48 -07:00
Paul E. McKenney
9fa46fb8c9 rcu: Advance outgoing CPU's callbacks before migrating them
It is possible that the outgoing CPU is unaware of recent grace periods,
and so it is also possible that some of its pending callbacks are actually
ready to be invoked.  The current callback-migration code would needlessly
force these callbacks to pass through another grace period.  This commit
therefore invokes rcu_advance_cbs() on the outgoing CPU's callbacks in
order to give them full credit for having passed through any recent
grace periods.

This also fixes an odd theoretical bug where there are no callbacks in
the system except for those on the outgoing CPU, none of those callbacks
have yet been associated with a grace-period number, there is never again
another callback registered, and the surviving CPU never again takes a
scheduling-clock interrupt, never goes idle, and never enters nohz_full
userspace execution.  Yes, this is (just barely) possible.  It requires
that the surviving CPU be a nohz_full CPU, that its scheduler-clock
interrupt be shut off, and that it loop forever in the kernel.  You get
bonus points if you can make this one happen!  ;-)

Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
2017-07-25 13:04:47 -07:00
Paul E. McKenney
b1a2d79fe7 rcu: Make NOCB CPUs migrate CBs directly from outgoing CPU
RCU's CPU-hotplug callback-migration code first moves the outgoing
CPU's callbacks to ->orphan_done and ->orphan_pend, and only then
moves them to the NOCB callback list.  This commit avoids the
extra step (and simplifies the code) by moving the callbacks directly
from the outgoing CPU's callback list to the NOCB callback list.

Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
2017-07-25 13:04:47 -07:00
Paul E. McKenney
95335c0355 rcu: Check for NOCB CPUs and empty lists earlier in CB migration
The current CPU-hotplug RCU-callback-migration code checks
for the source (newly offlined) CPU being a NOCBs CPU down in
rcu_send_cbs_to_orphanage().  This commit simplifies callback migration a
bit by moving this check up to rcu_migrate_callbacks().  This commit also
adds a check for the source CPU having no callbacks, which eases analysis
of the rcu_send_cbs_to_orphanage() and rcu_adopt_orphan_cbs() functions.

Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
2017-07-25 13:04:46 -07:00
Paul E. McKenney
c47e067a3c rcu: Remove orphan/adopt event-tracing fields
The rcu_node structure's ->n_cbs_orphaned and ->n_cbs_adopted fields
are updated, but never read.  This commit therefore removes them.

Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
2017-07-25 13:04:46 -07:00
Paul E. McKenney
313517fc44 rcu: Make expedited GPs correctly handle hardware CPU insertion
The update of the ->expmaskinitnext and of ->ncpus are unsynchronized,
with the value of ->ncpus being incremented long before the corresponding
->expmaskinitnext mask is updated.  If an RCU expedited grace period
sees ->ncpus change, it will update the ->expmaskinit masks from the new
->expmaskinitnext masks.  But it is possible that ->ncpus has already
been updated, but the ->expmaskinitnext masks still have their old values.
For the current expedited grace period, no harm done.  The CPU could not
have been online before the grace period started, so there is no need to
wait for its non-existent pre-existing readers.

But the next RCU expedited grace period is in a world of hurt.  The value
of ->ncpus has already been updated, so this grace period will assume
that the ->expmaskinitnext masks have not changed.  But they have, and
they won't be taken into account until the next never-been-online CPU
comes online.  This means that RCU will be ignoring some CPUs that it
should be paying attention to.

The solution is to update ->ncpus and ->expmaskinitnext while holding
the ->lock for the rcu_node structure containing the ->expmaskinitnext
mask.  Because smp_store_release() is now used to update ->ncpus and
smp_load_acquire() is now used to locklessly read it, if the expedited
grace period sees ->ncpus change, then the updating CPU has to
already be holding the corresponding ->lock.  Therefore, when the
expedited grace period later acquires that ->lock, it is guaranteed
to see the new value of ->expmaskinitnext.

On the other hand, if the expedited grace period loads ->ncpus just
before an update, earlier full memory barriers guarantee that
the incoming CPU isn't far enough along to be running any RCU readers.

This commit therefore makes the required change.

Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
2017-07-25 13:04:45 -07:00
Paul E. McKenney
a58163d8ca rcu: Migrate callbacks earlier in the CPU-offline timeline
RCU callbacks must be migrated away from an outgoing CPU, and this is
done near the end of the CPU-hotplug operation, after the outgoing CPU is
long gone.  Unfortunately, this means that other CPU-hotplug callbacks
can execute while the outgoing CPU's callbacks are still immobilized
on the long-gone CPU's callback lists.  If any of these CPU-hotplug
callbacks must wait, either directly or indirectly, for the invocation
of any of the immobilized RCU callbacks, the system will hang.

This commit avoids such hangs by migrating the callbacks away from the
outgoing CPU immediately upon its departure, shortly after the return
from __cpu_die() in takedown_cpu().  Thus, RCU is able to advance these
callbacks and invoke them, which allows all the after-the-fact CPU-hotplug
callbacks to wait on these RCU callbacks without risk of a hang.

While in the neighborhood, this commit also moves rcu_send_cbs_to_orphanage()
and rcu_adopt_orphan_cbs() under a pre-existing #ifdef to avoid including
dead code on the one hand and to avoid define-without-use warnings on the
other hand.

Reported-by: Jeffrey Hugo <jhugo@codeaurora.org>
Link: http://lkml.kernel.org/r/db9c91f6-1b17-6136-84f0-03c3c2581ab4@codeaurora.org
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Cc: Ingo Molnar <mingo@kernel.org>
Cc: Anna-Maria Gleixner <anna-maria@linutronix.de>
Cc: Boris Ostrovsky <boris.ostrovsky@oracle.com>
Cc: Richard Weinberger <richard@nod.at>
2017-07-25 13:03:43 -07:00
Paul E. McKenney
96036c4306 rcu: Add last-CPU to GP-kthread starvation messages
This commit augments the grace-period-kthread starvation debugging
messages by adding the last CPU that ran the kthread.

Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
2017-07-24 16:04:18 -07:00
Paul E. McKenney
fe5ac724d8 rcu: Remove nohz_full full-system-idle state machine
The NO_HZ_FULL_SYSIDLE full-system-idle capability was added in 2013
by commit 0edd1b1784 ("nohz_full: Add full-system-idle state machine"),
but has not been used.  This commit therefore removes it.

If it turns out to be needed later, this commit can always be reverted.

Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Cc: Frederic Weisbecker <fweisbec@gmail.com>
Cc: Rik van Riel <riel@redhat.com>
Cc: Ingo Molnar <mingo@kernel.org>
Acked-by: Linus Torvalds <torvalds@linux-foundation.org>
2017-06-08 18:52:39 -07:00
Paul E. McKenney
f7a10a9750 rcu: Remove the RCU_KTHREAD_PRIO Kconfig option
Anything that can be done with the RCU_KTHREAD_PRIO Kconfig option can
also be done with the rcutree.kthread_prio kernel boot parameter.
This commit therefore removes this Kconfig option.

Reported-by: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Cc: Frederic Weisbecker <fweisbec@gmail.com>
Cc: Rik van Riel <riel@redhat.com>
2017-06-08 18:52:39 -07:00
Paul E. McKenney
90040c9e30 rcu: Remove *_SLOW_* Kconfig options
The RCU_TORTURE_TEST_SLOW_PREINIT, RCU_TORTURE_TEST_SLOW_PREINIT_DELAY,
RCU_TORTURE_TEST_SLOW_PREINIT_DELAY, RCU_TORTURE_TEST_SLOW_INIT,
RCU_TORTURE_TEST_SLOW_INIT_DELAY, RCU_TORTURE_TEST_SLOW_CLEANUP,
and RCU_TORTURE_TEST_SLOW_CLEANUP_DELAY Kconfig options are only
useful for torture testing, and there are the rcutree.gp_cleanup_delay,
rcutree.gp_init_delay, and rcutree.gp_preinit_delay kernel boot parameters
that rcutorture can use instead.  The effect of these parameters is to
artificially slow down grace period initialization and cleanup in order
to make some types of race conditions happen more often.

This commit therefore simplifies Tree RCU a bit by removing the Kconfig
options and adding the corresponding kernel parameters to rcutorture's
.boot files instead.  However, this commit also leaves out the kernel
parameters for TREE02, TREE04, and TREE07 in order to have about the
same number of tests slowed as not slowed.  TREE01, TREE03, TREE05,
and TREE06 are slowed, and the rest are not slowed.

Reported-by: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
2017-06-08 18:52:38 -07:00
Paul E. McKenney
fa3c664769 rcu: Improve __call_rcu() debug-objects error message
The "__call_rcu(): Leaked duplicate callback" error message from
__call_rcu() has proven to be unhelpful.  This commit therefore changes
it to "__call_rcu(): Double-freed CB" and adds the value of the pointer
passed in.  The value of the pointer improves debuggability by allowing
correlation with tracing output, for example, the rcu:rcu_callback trace
event.

Reported-by: Vegard Nossum <vegard.nossum@oracle.com>
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
2017-06-08 18:52:31 -07:00
Paul E. McKenney
791875d16e rcu: Eliminate the unused __rcu_is_watching() function
The __rcu_is_watching() function is currently not used, aside from
to implement the rcu_is_watching() function.  This commit therefore
eliminates __rcu_is_watching(), which has the beneficial side-effect
of shrinking include/linux/rcupdate.h a bit.

Reported-by: Ingo Molnar <mingo@kernel.org>
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
2017-06-08 18:52:30 -07:00
Paul E. McKenney
a68a2bb28b rcu: Move docbook comments out of rcupdate.h
The include/linux/rcupdate.h file is included by more than 200
files, so shrinking it should provide some build-time benefits.
This commit therefore moves several docbook comments from rcupdate.h to
kernel/rcu/update.c, kernel/rcu/tree.c, and kernel/rcu/tree_plugin.h, thus
reducing the number of times that the compiler has to scan these comments.
This likely provides only a small benefit, but every little bit helps.

This commit also fixes a malformed bulleted list noted by the 0day
Test Robot.

Reported-by: Ingo Molnar <mingo@kernel.org>
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
2017-06-08 18:52:27 -07:00
Paul E. McKenney
c0b334c5bf rcu: Add lockdep_assert_held() teeth to tree.c
Comments can be helpful, but assertions carry more force.  This
commit therefore adds lockdep_assert_held() and RCU_LOCKDEP_WARN()
calls to enforce lock-held and interrupt-disabled preconditions.

Reported-by: Peter Zijlstra <peterz@infradead.org>
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
2017-06-08 08:25:37 -07:00
Paul E. McKenney
17c7798bea rcu: Update rcu_bootup_announce_oddness()
This commit updates rcu_bootup_announce_oddness() to check additional
Kconfig options and module/boot parameters.

Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
2017-06-08 08:25:35 -07:00
Paul E. McKenney
f4687d2637 rcu: Add preemptibility checks in rcu_sched_qs() and rcu_bh_qs()
This commit adds WARN_ON_ONCE() calls that trigger if either
rcu_sched_qs() or rcu_bh_qs() are invoked with preemption enabled.
In the immortal words of Peter Zijlstra: "these are much harder to ignore
than comments".

Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
2017-06-08 08:25:34 -07:00
Paul E. McKenney
e28371c891 rcu: Remove obsolete reference to synchronize_kernel()
The synchronize_kernel() primitive was removed in favor of
synchronize_sched() more than a decade ago, and it seems likely that
rather few kernel hackers are familiar with it.  Its continued presence
is therefore providing more confusion than enlightenment.  This commit
therefore removes the reference from the synchronize_sched() header
comment, and adds the corresponding information to the synchronize_rcu(0
header comment.

Reported-by: Peter Zijlstra <peterz@infradead.org>
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
2017-06-08 08:25:25 -07:00
Paul E. McKenney
5b72f9643b rcu: Complain if blocking in preemptible RCU read-side critical section
Although preemptible RCU allows its read-side critical sections to be
preempted, general blocking is forbidden.  The reason for this is that
excessive preemption times can be handled by CONFIG_RCU_BOOST=y, but a
voluntarily blocked task doesn't care how high you boost its priority.
Because preemptible RCU is a global mechanism, one ill-behaved reader
hurts everyone.  Hence the prohibition against general blocking in
RCU-preempt read-side critical sections.  Preemption yes, blocking no.

This commit enforces this prohibition.

There is a special exception for the -rt patchset (which they kindly
volunteered to implement):  It is OK to block (as opposed to merely being
preempted) within an RCU-preempt read-side critical section, but only if
the blocking is subject to priority inheritance.  This exception permits
CONFIG_RCU_BOOST=y to get -rt RCU readers out of trouble.

Why doesn't this exception also apply to mainline's rt_mutex?  Because
of the possibility that someone does general blocking while holding
an rt_mutex.  Yes, the priority boosting will affect the rt_mutex,
but it won't help with the task doing general blocking while holding
that rt_mutex.

Reported-by: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
2017-06-08 08:25:24 -07:00
Paul E. McKenney
f92c734f02 rcu: Prevent rcu_barrier() from starting needless grace periods
Currently rcu_barrier() uses call_rcu() to enqueue new callbacks
on each CPU with a non-empty callback list.  This works, but means
that rcu_barrier() forces grace periods that are not otherwise needed.
The key point is that rcu_barrier() never needs to wait for a grace
period, but instead only for all pre-existing callbacks to be invoked.
This means that rcu_barrier()'s new callbacks should be placed in
the callback-list segment containing the last pre-existing callback.

This commit makes this change using the new rcu_segcblist_entrain()
function.

Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
2017-06-08 08:25:22 -07:00
Linus Torvalds
de4d195308 Merge branch 'core-rcu-for-linus' of git://git.kernel.org/pub/scm/linux/kernel/git/tip/tip
Pull RCU updates from Ingo Molnar:
 "The main changes are:

   - Debloat RCU headers

   - Parallelize SRCU callback handling (plus overlapping patches)

   - Improve the performance of Tree SRCU on a CPU-hotplug stress test

   - Documentation updates

   - Miscellaneous fixes"

* 'core-rcu-for-linus' of git://git.kernel.org/pub/scm/linux/kernel/git/tip/tip: (74 commits)
  rcu: Open-code the rcu_cblist_n_lazy_cbs() function
  rcu: Open-code the rcu_cblist_n_cbs() function
  rcu: Open-code the rcu_cblist_empty() function
  rcu: Separately compile large rcu_segcblist functions
  srcu: Debloat the <linux/rcu_segcblist.h> header
  srcu: Adjust default auto-expediting holdoff
  srcu: Specify auto-expedite holdoff time
  srcu: Expedite first synchronize_srcu() when idle
  srcu: Expedited grace periods with reduced memory contention
  srcu: Make rcutorture writer stalls print SRCU GP state
  srcu: Exact tracking of srcu_data structures containing callbacks
  srcu: Make SRCU be built by default
  srcu: Fix Kconfig botch when SRCU not selected
  rcu: Make non-preemptive schedule be Tasks RCU quiescent state
  srcu: Expedite srcu_schedule_cbs_snp() callback invocation
  srcu: Parallelize callback handling
  kvm: Move srcu_struct fields to end of struct kvm
  rcu: Fix typo in PER_RCU_NODE_PERIOD header comment
  rcu: Use true/false in assignment to bool
  rcu: Use bool value directly
  ...
2017-05-10 10:30:46 -07:00
Paul E. McKenney
933dfbd7c4 rcu: Open-code the rcu_cblist_n_lazy_cbs() function
Because the rcu_cblist_n_lazy_cbs() just samples the ->len_lazy counter,
and because the rcu_cblist structure is quite straightforward, it makes
sense to open-code rcu_cblist_n_lazy_cbs(p) as p->len_lazy, cutting out
a level of indirection.  This commit makes this change.

Reported-by: Ingo Molnar <mingo@kernel.org>
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
2017-05-02 09:22:48 -07:00
Paul E. McKenney
4b27f20b40 rcu: Open-code the rcu_cblist_n_cbs() function
Because the rcu_cblist_n_cbs() just samples the ->len counter, and
because the rcu_cblist structure is quite straightforward, it makes
sense to open-code rcu_cblist_n_cbs(p) as p->len, cutting out a level
of indirection.  This commit makes this change.

Reported-by: Ingo Molnar <mingo@kernel.org>
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
2017-05-02 09:21:59 -07:00
Paul E. McKenney
8ef0f37efb rcu: Open-code the rcu_cblist_empty() function
Because the rcu_cblist_empty() just samples the ->head pointer, and
because the rcu_cblist structure is quite straightforward, it makes
sense to open-code rcu_cblist_empty(p) as !p->head, cutting out a
level of indirection.  This commit makes this change.

Reported-by: Ingo Molnar <mingo@kernel.org>
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
2017-05-02 08:18:40 -07:00
Paul E. McKenney
7f6733c3c6 srcu: Make rcutorture writer stalls print SRCU GP state
In the past, SRCU was simple enough that there was little point in
making the rcutorture writer stall messages print the SRCU grace-period
number state.  With the advent of Tree SRCU, this has changed.  This
commit therefore makes Classic, Tiny, and Tree SRCU report this state
to rcutorture as needed.

Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Tested-by: Mike Galbraith <efault@gmx.de>
2017-04-26 11:23:28 -07:00
Paul E. McKenney
f2094107ac Merge branches 'doc.2017.04.12a', 'fixes.2017.04.19a' and 'srcu.2017.04.21a' into HEAD
doc.2017.04.12a: Documentation updates
fixes.2017.04.19a: Miscellaneous fixes
srcu.2017.04.21a: Parallelize SRCU callback handling
2017-04-21 06:00:13 -07:00
Paul E. McKenney
bcbfdd01dc rcu: Make non-preemptive schedule be Tasks RCU quiescent state
Currently, a call to schedule() acts as a Tasks RCU quiescent state
only if a context switch actually takes place.  However, just the
call to schedule() guarantees that the calling task has moved off of
whatever tracing trampoline that it might have been one previously.
This commit therefore plumbs schedule()'s "preempt" parameter into
rcu_note_context_switch(), which then records the Tasks RCU quiescent
state, but only if this call to schedule() was -not- due to a preemption.

To avoid adding overhead to the common-case context-switch path,
this commit hides the rcu_note_context_switch() check under an existing
non-common-case check.

Suggested-by: Steven Rostedt <rostedt@goodmis.org>
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
2017-04-21 05:59:27 -07:00
Paul E. McKenney
da915ad5cf srcu: Parallelize callback handling
Peter Zijlstra proposed using SRCU to reduce mmap_sem contention [1,2],
however, there are workloads that could result in a high volume of
concurrent invocations of call_srcu(), which with current SRCU would
result in excessive lock contention on the srcu_struct structure's
->queue_lock, which protects SRCU's callback lists.  This commit therefore
moves SRCU to per-CPU callback lists, thus greatly reducing contention.

Because a given SRCU instance no longer has a single centralized callback
list, starting grace periods and invoking callbacks are both more complex
than in the single-list Classic SRCU implementation.  Starting grace
periods and handling callbacks are now handled using an srcu_node tree
that is in some ways similar to the rcu_node trees used by RCU-bh,
RCU-preempt, and RCU-sched (for example, the srcu_node tree shape is
controlled by exactly the same Kconfig options and boot parameters that
control the shape of the rcu_node tree).

In addition, the old per-CPU srcu_array structure is now named srcu_data
and contains an rcu_segcblist structure named ->srcu_cblist for its
callbacks (and a spinlock to protect this).  The srcu_struct gets
an srcu_gp_seq that is used to associate callback segments with the
corresponding completion-time grace-period number.  These completion-time
grace-period numbers are propagated up the srcu_node tree so that the
grace-period workqueue handler can determine whether additional grace
periods are needed on the one hand and where to look for callbacks that
are ready to be invoked.

The srcu_barrier() function must now wait on all instances of the per-CPU
->srcu_cblist.  Because each ->srcu_cblist is protected by ->lock,
srcu_barrier() can remotely add the needed callbacks.  In theory,
it could also remotely start grace periods, but in practice doing so
is complex and racy.  And interestingly enough, it is never necessary
for srcu_barrier() to start a grace period because srcu_barrier() only
enqueues a callback when a callback is already present--and it turns out
that a grace period has to have already been started for this pre-existing
callback.  Furthermore, it is only the callback that srcu_barrier()
needs to wait on, not any particular grace period.  Therefore, a new
rcu_segcblist_entrain() function enqueues the srcu_barrier() function's
callback into the same segment occupied by the last pre-existing callback
in the list.  The special case where all the pre-existing callbacks are
on a different list (because they are in the process of being invoked)
is handled by enqueuing srcu_barrier()'s callback into the RCU_DONE_TAIL
segment, relying on the done-callbacks check that takes place after all
callbacks are inovked.

Note that the readers use the same algorithm as before.  Note that there
is a separate srcu_idx that tells the readers what counter to increment.
This unfortunately cannot be combined with srcu_gp_seq because they
need to be incremented at different times.

This commit introduces some ugly #ifdefs in rcutorture.  These will go
away when I feel good enough about Tree SRCU to ditch Classic SRCU.

Some crude performance comparisons, courtesy of a quickly hacked rcuperf
asynchronous-grace-period capability:

			Callback Queuing Overhead
			-------------------------
	# CPUS		Classic SRCU	Tree SRCU
	------          ------------    ---------
	     2              0.349 us     0.342 us
	    16             31.66  us     0.4   us
	    41             ---------     0.417 us

The times are the 90th percentiles, a statistic that was chosen to reject
the overheads of the occasional srcu_barrier() call needed to avoid OOMing
the test machine.  The rcuperf test hangs when running Classic SRCU at 41
CPUs, hence the line of dashes.  Despite the hacks to both the rcuperf code
and that statistics, this is a convincing demonstration of Tree SRCU's
performance and scalability advantages.

[1] https://lwn.net/Articles/309030/
[2] https://patchwork.kernel.org/patch/5108281/

Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
[ paulmck: Fix initialization if synchronize_srcu_expedited() called first. ]
2017-04-21 05:59:26 -07:00
Paul E. McKenney
bfd090be14 rcu: Fix typo in PER_RCU_NODE_PERIOD header comment
This commit just changes a "the the" to "the" to reduce repetition.

Reported-by: Michalis Kokologiannakis <mixaskok@gmail.com>
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
2017-04-19 09:29:20 -07:00
Nicholas Mc Guire
50dc7def4a rcu: Use bool value directly
The beenonline variable is declared bool so there is no need for an
explicit comparison, especially not against the constant zero.

Signed-off-by: Nicholas Mc Guire <der.herr@hofr.at>
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
2017-04-19 09:29:19 -07:00
Paul E. McKenney
deb34f3643 rcu: Improve comments for hotplug/suspend/hibernate functions
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
2017-04-19 09:29:18 -07:00
Paul E. McKenney
d1e4f01d09 rcu: Remove obsolete comment from rcu_future_gp_cleanup() header
The rcu_nocb_gp_cleanup() function is now invoked elsewhere, so this
commit drags this comment into the year 2017.

Reported-by: Michalis Kokologiannakis <mixaskok@gmail.com>
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
2017-04-19 09:29:17 -07:00
Paul E. McKenney
e95d68d212 srcu: Make num_rcu_lvl[] array be external
This commit makes the num_rcu_lvl[] array external so that SRCU can
make use of it for initializing its upcoming srcu_node tree.

Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
2017-04-18 11:38:21 -07:00
Paul E. McKenney
41f5c63178 rcu: Remove redundant levelcnt[] array from rcu_init_one()
The levelcnt[] array is identical to num_rcu_lvl[], so this commit
removes levelcnt[].

Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
2017-04-18 11:38:21 -07:00
Paul E. McKenney
2b34c43cc1 srcu: Move rcu_init_levelspread() to rcu_tree_node.h
This commit moves the rcu_init_levelspread() function from
kernel/rcu/tree.c to kernel/rcu/rcu.h so that SRCU can access it.  This is
another step towards enabling SRCU to create its own combining tree.
This commit is code-movement only, give or take knock-on adjustments.

Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
2017-04-18 11:38:20 -07:00
Paul E. McKenney
2e8c28c2dd srcu: Move rcu_seq_start() and friends to rcu.h
This commit moves rcu_seq_start(), rcu_seq_end(), rcu_seq_snap(),
and rcu_seq_done() from kernel/rcu/tree.c to kernel/rcu/rcu.h.
This will allow SRCU to use these functions, which in turn will
allow SRCU to move from a single global callback queue to a
per-CPU callback queue.

Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
2017-04-18 11:38:19 -07:00
Paul E. McKenney
900b1028ec srcu: Allow SRCU to access rcu_scheduler_active
This is primarily a code-movement commit in preparation for allowing
SRCU to handle early-boot SRCU grace periods.

Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
2017-04-18 11:38:18 -07:00
Paul E. McKenney
15fecf89e4 srcu: Abstract multi-tail callback list handling
RCU has only one multi-tail callback list, which is implemented via
the nxtlist, nxttail, nxtcompleted, qlen_lazy, and qlen fields in the
rcu_data structure, and whose operations are open-code throughout the
Tree RCU implementation.  This has been more or less OK in the past,
but upcoming callback-list optimizations in SRCU could really use
a multi-tail callback list there as well.

This commit therefore abstracts the multi-tail callback list handling
into a new kernel/rcu/rcu_segcblist.h file, and uses this new API.
The simple head-and-tail pointer callback list is also abstracted and
applied everywhere except for the NOCB callback-offload lists.  (Yes,
the plan is to apply them there as well, but this commit is already
bigger than would be good.)

Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
2017-04-18 11:38:18 -07:00
Paul E. McKenney
9226b10d78 rcu: Place guard on rcu_all_qs() and rcu_note_context_switch() actions
The rcu_all_qs() and rcu_note_context_switch() do a series of checks,
taking various actions to supply RCU with quiescent states, depending
on the outcomes of the various checks.  This is a bit much for scheduling
fastpaths, so this commit creates a separate ->rcu_urgent_qs field in
the rcu_dynticks structure that acts as a global guard for these checks.
Thus, in the common case, rcu_all_qs() and rcu_note_context_switch()
check the ->rcu_urgent_qs field, find it false, and simply return.

Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Cc: Peter Zijlstra <peterz@infradead.org>
2017-04-18 11:38:18 -07:00
Paul E. McKenney
0f9be8cabb rcu: Eliminate flavor scan in rcu_momentary_dyntick_idle()
The rcu_momentary_dyntick_idle() function scans the RCU flavors, checking
that one of them still needs a quiescent state before doing an expensive
atomic operation on the ->dynticks counter.  However, this check reduces
overhead only after a rare race condition, and increases complexity.  This
commit therefore removes the scan and the mechanism enabling the scan.

Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
2017-04-18 11:38:18 -07:00
Paul E. McKenney
9577df9a31 rcu: Pull rcu_qs_ctr into rcu_dynticks structure
The rcu_qs_ctr variable is yet another isolated per-CPU variable,
so this commit pulls it into the pre-existing rcu_dynticks per-CPU
structure.

Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
2017-04-18 11:38:17 -07:00
Paul E. McKenney
abb06b9948 rcu: Pull rcu_sched_qs_mask into rcu_dynticks structure
The rcu_sched_qs_mask variable is yet another isolated per-CPU variable,
so this commit pulls it into the pre-existing rcu_dynticks per-CPU
structure.

Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
2017-04-18 11:38:17 -07:00
Paul E. McKenney
88a4976d0e rcu: Semicolon inside RCU_TRACE() for tree.c
The current use of "RCU_TRACE(statement);" can cause odd bugs, especially
where "statement" is a local-variable declaration, as it can leave a
misplaced ";" in the source code.  This commit therefore converts these
to "RCU_TRACE(statement;)", which avoids the misplaced ";".

Reported-by: Josh Triplett <josh@joshtriplett.org>
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
2017-04-18 11:38:17 -07:00
Paul E. McKenney
b8c17e6664 rcu: Maintain special bits at bottom of ->dynticks counter
Currently, IPIs are used to force other CPUs to invalidate their TLBs
in response to a kernel virtual-memory mapping change.  This works, but
degrades both battery lifetime (for idle CPUs) and real-time response
(for nohz_full CPUs), and in addition results in unnecessary IPIs due to
the fact that CPUs executing in usermode are unaffected by stale kernel
mappings.  It would be better to cause a CPU executing in usermode to
wait until it is entering kernel mode to do the flush, first to avoid
interrupting usemode tasks and second to handle multiple flush requests
with a single flush in the case of a long-running user task.

This commit therefore reserves a bit at the bottom of the ->dynticks
counter, which is checked upon exit from extended quiescent states.
If it is set, it is cleared and then a new rcu_eqs_special_exit() macro is
invoked, which, if not supplied, is an empty single-pass do-while loop.
If this bottom bit is set on -entry- to an extended quiescent state,
then a WARN_ON_ONCE() triggers.

This bottom bit may be set using a new rcu_eqs_special_set() function,
which returns true if the bit was set, or false if the CPU turned
out to not be in an extended quiescent state.  Please note that this
function refuses to set the bit for a non-nohz_full CPU when that CPU
is executing in usermode because usermode execution is tracked by RCU
as a dyntick-idle extended quiescent state only for nohz_full CPUs.

Reported-by: Andy Lutomirski <luto@amacapital.net>
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Reviewed-by: Josh Triplett <josh@joshtriplett.org>
2017-04-18 11:19:22 -07:00
Steven Rostedt (VMware)
03ecd3f48e rcu/tracing: Add rcu_disabled to denote when rcu_irq_enter() will not work
Tracing uses rcu_irq_enter() as a way to make sure that RCU is watching when
it needs to use rcu_read_lock() and friends. This is because tracing can
happen as RCU is about to enter user space, or about to go idle, and RCU
does not watch for RCU read side critical sections as it makes the
transition.

There is a small location within the RCU infrastructure that rcu_irq_enter()
itself will not work. If tracing were to occur in that section it will break
if it tries to use rcu_irq_enter().

Originally, this happens with the stack_tracer, because it will call
save_stack_trace when it encounters stack usage that is greater than any
stack usage it had encountered previously. There was a case where that
happened in the RCU section where rcu_irq_enter() did not work, and lockdep
complained loudly about it. To fix it, stack tracing added a call to be
disabled and RCU would disable stack tracing during the critical section
that rcu_irq_enter() was inoperable. This solution worked, but there are
other cases that use rcu_irq_enter() and it would be a good idea to let RCU
give a way to let others know that rcu_irq_enter() will not work. For
example, in trace events.

Another helpful aspect of this change is that it also moves the per cpu
variable called in the RCU critical section into a cache locale along with
other RCU per cpu variables used in that same location.

I'm keeping the stack_trace_disable() code, as that still could be used in
the future by places that really need to disable it. And since it's only a
static inline, it wont take up any kernel text if it is not used.

Link: http://lkml.kernel.org/r/20170405093207.404f8deb@gandalf.local.home

Acked-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Signed-off-by: Steven Rostedt (VMware) <rostedt@goodmis.org>
2017-04-10 15:22:03 -04:00
Paul E. McKenney
a278d47189 rcu: Fix dyntick-idle tracing
The tracing subsystem started using rcu_irq_entry() and rcu_irq_exit()
(with my blessing) to allow the current _rcuidle alternative tracepoint
name to be dispensed with while still maintaining good performance.
Unfortunately, this causes RCU's dyntick-idle entry code's tracing to
appear to RCU like an interrupt that occurs where RCU is not designed
to handle interrupts.

This commit fixes this problem by moving the zeroing of ->dynticks_nesting
after the offending trace_rcu_dyntick() statement, which narrows the
window of vulnerability to a pair of adjacent statements that are now
marked with comments to that effect.

Link: http://lkml.kernel.org/r/20170405093207.404f8deb@gandalf.local.home
Link: http://lkml.kernel.org/r/20170405193928.GM1600@linux.vnet.ibm.com

Reported-by: Steven Rostedt <rostedt@goodmis.org>
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Signed-off-by: Steven Rostedt (VMware) <rostedt@goodmis.org>
2017-04-10 15:21:57 -04:00
Ingo Molnar
b17b01533b sched/headers: Prepare for new header dependencies before moving code to <linux/sched/debug.h>
We are going to split <linux/sched/debug.h> out of <linux/sched.h>, which
will have to be picked up from other headers and a couple of .c files.

Create a trivial placeholder <linux/sched/debug.h> file that just
maps to <linux/sched.h> to make this patch obviously correct and
bisectable.

Include the new header in the files that are going to need it.

Acked-by: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Mike Galbraith <efault@gmx.de>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: linux-kernel@vger.kernel.org
Signed-off-by: Ingo Molnar <mingo@kernel.org>
2017-03-02 08:42:34 +01:00
Ingo Molnar
ae7e81c077 sched/headers: Prepare for new header dependencies before moving code to <uapi/linux/sched/types.h>
We are going to move scheduler ABI details to <uapi/linux/sched/types.h>,
which will be used from a number of .c files.

Create empty placeholder header that maps to <linux/types.h>.

Include the new header in the files that are going to need it.

Acked-by: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Mike Galbraith <efault@gmx.de>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: linux-kernel@vger.kernel.org
Signed-off-by: Ingo Molnar <mingo@kernel.org>
2017-03-02 08:42:27 +01:00
Ingo Molnar
f9411ebe3d rcu: Separate the RCU synchronization types and APIs into <linux/rcupdate_wait.h>
So rcupdate.h is a pretty complex header, in particular it includes
<linux/completion.h> which includes <linux/wait.h> - creating a
dependency that includes <linux/wait.h> in <linux/sched.h>,
which prevents the isolation of <linux/sched.h> from the derived
<linux/wait.h> header.

Solve part of the problem by decoupling rcupdate.h from completions:
this can be done by separating out the rcu_synchronize types and APIs,
and updating their usage sites.

Since this is a mostly RCU-internal types this will not just simplify
<linux/sched.h>'s dependencies, but will make all the hundreds of
.c files that include rcupdate.h but not completions or wait.h build
faster.

( For rcutiny this means that two dependent APIs have to be uninlined,
  but that shouldn't be much of a problem as they are rare variants. )

Acked-by: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Mike Galbraith <efault@gmx.de>
Cc: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: linux-kernel@vger.kernel.org
Signed-off-by: Ingo Molnar <mingo@kernel.org>
2017-03-02 08:42:24 +01:00
Paul E. McKenney
31945aa9f1 Merge branches 'doc.2017.01.15b', 'dyntick.2017.01.23a', 'fixes.2017.01.23a', 'srcu.2017.01.25a' and 'torture.2017.01.15b' into HEAD
doc.2017.01.15b: Documentation updates
dyntick.2017.01.23a: Dyntick tracking consolidation
fixes.2017.01.23a: Miscellaneous fixes
srcu.2017.01.25a: SRCU rewrite, fixes, and verification
torture.2017.01.15b: Torture-test updates
2017-01-25 12:56:05 -08:00
Paul E. McKenney
38d30b336c rcu: Adjust FQS offline checks for exact online-CPU detection
Commit 7ec99de36f ("rcu: Provide exact CPU-online tracking for RCU"),
as its title suggests, got rid of RCU's remaining CPU-hotplug timing
guesswork.  This commit therefore removes the one-jiffy kludge that was
used to paper over this guesswork.

Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Reviewed-by: Josh Triplett <josh@joshtriplett.org>
2017-01-23 11:44:18 -08:00
Paul E. McKenney
3a19b46a5c rcu: Check cond_resched_rcu_qs() state less often to reduce GP overhead
Commit 4a81e8328d ("rcu: Reduce overhead of cond_resched() checks
for RCU") moved quiescent-state generation out of cond_resched()
and commit bde6c3aa99 ("rcu: Provide cond_resched_rcu_qs() to force
quiescent states in long loops") introduced cond_resched_rcu_qs(), and
commit 5cd37193ce ("rcu: Make cond_resched_rcu_qs() apply to normal RCU
flavors") introduced the per-CPU rcu_qs_ctr variable, which is frequently
polled by the RCU core state machine.

This frequent polling can increase grace-period rate, which in turn
increases grace-period overhead, which is visible in some benchmarks
(for example, the "open1" benchmark in Anton Blanchard's "will it scale"
suite).  This commit therefore reduces the rate at which rcu_qs_ctr
is polled by moving that polling into the force-quiescent-state (FQS)
machinery, and by further polling it only after the grace period has
been in effect for at least jiffies_till_sched_qs jiffies.

Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Reviewed-by: Josh Triplett <josh@joshtriplett.org>
2017-01-23 11:44:18 -08:00
Paul E. McKenney
02a5c550b2 rcu: Abstract extended quiescent state determination
This commit is the fourth step towards full abstraction of all accesses
to the ->dynticks counter, implementing previously open-coded checks and
comparisons in new rcu_dynticks_in_eqs() and rcu_dynticks_in_eqs_since()
functions.  This abstraction will ease changes to the ->dynticks counter
operation.

Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Reviewed-by: Josh Triplett <josh@joshtriplett.org>
2017-01-23 11:44:18 -08:00
Paul E. McKenney
2625d469ba rcu: Abstract dynticks extended quiescent state enter/exit operations
This commit is the third step towards full abstraction of all accesses
to the ->dynticks counter, implementing the previously open-coded atomic
add of 1 and entry checks in a new rcu_dynticks_eqs_enter() function, and
the same but with exit checks in a new rcu_dynticks_eqs_exit() function.
This abstraction will ease changes to the ->dynticks counter operation.

Note that this commit gets rid of the smp_mb__before_atomic() and the
smp_mb__after_atomic() calls that were previously present.  The reason
that this is OK from a memory-ordering perspective is that the atomic
operation is now atomic_add_return(), which, as a value-returning atomic,
guarantees full ordering.

Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
[ paulmck: Fixed RCU_TRACE() statements added by this commit. ]
Reviewed-by: Josh Triplett <josh@joshtriplett.org>
2017-01-23 11:42:43 -08:00
Paul E. McKenney
fdbb9b315c rcu: Make rcu_cpu_starting() use its "cpu" argument
The rcu_cpu_starting() function uses this_cpu_ptr() to locate the
incoming CPU's rcu_data structure.  This works for the boot CPU and for
all CPUs onlined after rcu_init() executes (during very early boot).
Currently, this is the full set of CPUs, so all is well.  But if
anyone ever parallelizes boot before rcu_init() time, it will fail.
This commit therefore substitutes the rcu_cpu_starting() function's
this_cpu_pointer() for per_cpu_ptr(), future-proofing the code and
(arguably) improving readability.

This commit inadvertently fixes a latent bug: If there ever had been
more than just the boot CPU online at rcu_init() time, the old code
would not initialize the non-boot CPUs, but rather would repeatedly
initialize the boot CPU.

Reported-by: Boqun Feng <boqun.feng@gmail.com>
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Reviewed-by: Josh Triplett <josh@joshtriplett.org>
2017-01-23 11:37:13 -08:00
Paul E. McKenney
630c7ed9ca rcu: Don't wake rcuc/X kthreads on NOCB CPUs
Chris Friesen notice that rcuc/X kthreads were consuming CPU even on
NOCB CPUs.  This makes no sense because the only purpose or these
kthreads is to invoke normal (non-offloaded) callbacks, of which there
will never be any on NOCB CPUs.  This problem was due to a bug in
cpu_has_callbacks_ready_to_invoke(), which should have been checking
->nxttail[RCU_NEXT_TAIL] for NULL, but which was instead (incorrectly)
checking ->nxttail[RCU_DONE_TAIL].  Because ->nxttail[RCU_DONE_TAIL] is
never NULL, the only effect is to cause the rcuc/X kthread to execute
when it should not do so.

This commit therefore checks ->nxttail[RCU_NEXT_TAIL], which is NULL
for NOCB CPUs.

Reported-by: Chris Friesen <chris.friesen@windriver.com>
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Reviewed-by: Josh Triplett <josh@joshtriplett.org>
2017-01-23 11:37:13 -08:00
Paul E. McKenney
7aa92230c9 rcu: Once again use NMI-based stack traces in stall warnings
This commit is for all intents and purposes a revert of bc1dce514e
("rcu: Don't use NMIs to dump other CPUs' stacks").  The reason to suppose
that this can now safely be reverted is the presence of 42a0bb3f71
("printk/nmi: generic solution for safe printk in NMI"), which is said
to have made NMI-based stack dumps safe.

However, this reversion keeps one nice property of bc1dce514e
("rcu: Don't use NMIs to dump other CPUs' stacks"), namely that
only those CPUs blocking the grace period are dumped.  The new
trigger_single_cpu_backtrace() is used to make this happen, as
suggested by Josh Poimboeuf.

Reported-by: Vince Weaver <vincent.weaver@maine.edu>
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Cc: Petr Mladek <pmladek@suse.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Reviewed-by: Josh Poimboeuf <jpoimboe@redhat.com>
Reviewed-by: Petr Mladek <pmladek@suse.com>
Reviewed-by: Josh Triplett <josh@joshtriplett.org>
2017-01-23 11:37:12 -08:00
Paul E. McKenney
b201fa6737 rcu: Remove short-term CPU kicking
Commit 4914950aaa ("rcu: Stop treating in-kernel CPU-bound workloads
as errors") added a (relatively) short-timeout call to resched_cpu().
This was inspired by as issue that was fixed by b7e7ade34e ("sched/core:
Fix remote wakeups").  But given that this issue was fixed, it is time
for the current commit to remove this call to resched_cpu().

Reported-by: Byungchul Park <byungchul.park@lge.com>
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Reviewed-by: Josh Triplett <josh@joshtriplett.org>
2017-01-23 11:37:12 -08:00
Paul E. McKenney
28053bc72c rcu: Add long-term CPU kicking
This commit prepares for the removal of short-term CPU kicking (in a
subsequent commit).  It does so by starting to invoke resched_cpu()
for each holdout at each force-quiescent-state interval that is more
than halfway through the stall-warning interval.

Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Reviewed-by: Josh Triplett <josh@joshtriplett.org>
2017-01-23 11:33:02 -08:00
Tobias Klauser
94060d2235 rcu: Remove unused but set variable
Since commit 7ec99de36f ("rcu: Provide exact CPU-online tracking for
RCU"), the variable mask in rcu_init_percpu_data is set but no longer
used. Remove it to fix the following warning when building with 'W=1':

  kernel/rcu/tree.c: In function ‘rcu_init_percpu_data’:
  kernel/rcu/tree.c:3765:16: warning: variable ‘mask’ set but not used [-Wunused-but-set-variable]

Signed-off-by: Tobias Klauser <tklauser@distanz.ch>
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Reviewed-by: Josh Triplett <josh@joshtriplett.org>
2017-01-23 11:32:35 -08:00
Byungchul Park
c4402b27f1 rcu: Only dump stalled-tasks stacks if there was a real stall
The print_other_cpu_stall() function currently unconditionally invokes
rcu_print_detail_task_stall().  This is OK because if there was a stall
sufficient to cause print_other_cpu_stall() to be invoked, that stall
is very likely to persist through the entire print_other_cpu_stall()
execution.  However, if the stall did not persist, the variable ndetected
will be zero, and that variable is already tested in an "if" statement.
Therefore, this commit moves the call to rcu_print_detail_task_stall()
under that pre-existing "if" to improve readability, with a very rare
reduction in overhead.

Signed-off-by: Byungchul Park <byungchul.park@lge.com>
[ paulmck: Reworked commit log. ]
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Reviewed-by: Josh Triplett <josh@joshtriplett.org>
2017-01-23 11:32:22 -08:00
Paul E. McKenney
8b2f63ab05 rcu: Abstract the dynticks snapshot operation
This commit is the second step towards full abstraction of all accesses to
the ->dynticks counter, implementing the previously open-coded atomic
add of zero in a new rcu_dynticks_snap() function.  This abstraction will
ease changes o the ->dynticks counter operation.

Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Reviewed-by: Josh Triplett <josh@joshtriplett.org>
2017-01-16 15:47:53 -08:00
Paul E. McKenney
6563de9d6f rcu: Abstract the dynticks momentary-idle operation
This commit is the first step towards full abstraction of all accesses to
the ->dynticks counter, implementing the previously open-coded atomic add
of two in a new rcu_dynticks_momentary_idle() function.  This abstraction
will ease changes to the ->dynticks counter operation.

Note that this commit gets rid of the smp_mb__before_atomic() and the
smp_mb__after_atomic() calls that were previously present.  The reason
that this is OK from a memory-ordering perspective is that the atomic
operation is now atomic_add_return(), which, as a value-returning atomic,
guarantees full ordering.

Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Reviewed-by: Josh Triplett <josh@joshtriplett.org>
2017-01-16 14:59:54 -08:00
Paul E. McKenney
52d7e48b86 rcu: Narrow early boot window of illegal synchronous grace periods
The current preemptible RCU implementation goes through three phases
during bootup.  In the first phase, there is only one CPU that is running
with preemption disabled, so that a no-op is a synchronous grace period.
In the second mid-boot phase, the scheduler is running, but RCU has
not yet gotten its kthreads spawned (and, for expedited grace periods,
workqueues are not yet running.  During this time, any attempt to do
a synchronous grace period will hang the system (or complain bitterly,
depending).  In the third and final phase, RCU is fully operational and
everything works normally.

This has been OK for some time, but there has recently been some
synchronous grace periods showing up during the second mid-boot phase.
This code worked "by accident" for awhile, but started failing as soon
as expedited RCU grace periods switched over to workqueues in commit
8b355e3bc1 ("rcu: Drive expedited grace periods from workqueue").
Note that the code was buggy even before this commit, as it was subject
to failure on real-time systems that forced all expedited grace periods
to run as normal grace periods (for example, using the rcu_normal ksysfs
parameter).  The callchain from the failure case is as follows:

early_amd_iommu_init()
|-> acpi_put_table(ivrs_base);
|-> acpi_tb_put_table(table_desc);
|-> acpi_tb_invalidate_table(table_desc);
|-> acpi_tb_release_table(...)
|-> acpi_os_unmap_memory
|-> acpi_os_unmap_iomem
|-> acpi_os_map_cleanup
|-> synchronize_rcu_expedited

The kernel showing this callchain was built with CONFIG_PREEMPT_RCU=y,
which caused the code to try using workqueues before they were
initialized, which did not go well.

This commit therefore reworks RCU to permit synchronous grace periods
to proceed during this mid-boot phase.  This commit is therefore a
fix to a regression introduced in v4.9, and is therefore being put
forward post-merge-window in v4.10.

This commit sets a flag from the existing rcu_scheduler_starting()
function which causes all synchronous grace periods to take the expedited
path.  The expedited path now checks this flag, using the requesting task
to drive the expedited grace period forward during the mid-boot phase.
Finally, this flag is updated by a core_initcall() function named
rcu_exp_runtime_mode(), which causes the runtime codepaths to be used.

Note that this arrangement assumes that tasks are not sent POSIX signals
(or anything similar) from the time that the first task is spawned
through core_initcall() time.

Fixes: 8b355e3bc1 ("rcu: Drive expedited grace periods from workqueue")
Reported-by: "Zheng, Lv" <lv.zheng@intel.com>
Reported-by: Borislav Petkov <bp@alien8.de>
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Tested-by: Stan Kain <stan.kain@gmail.com>
Tested-by: Ivan <waffolz@hotmail.com>
Tested-by: Emanuel Castelo <emanuel.castelo@gmail.com>
Tested-by: Bruno Pesavento <bpesavento@infinito.it>
Tested-by: Borislav Petkov <bp@suse.de>
Tested-by: Frederic Bezies <fredbezies@gmail.com>
Cc: <stable@vger.kernel.org> # 4.9.0-
2017-01-14 21:23:48 -08:00
Paul E. McKenney
aa3e0bf1aa rcu: Don't kick unless grace period or request
The current code can result in spurious kicks when there are no grace
periods in progress and no grace-period-related requests.  This is
sort of OK for a diagnostic aid, but the resulting ftrace-dump messages
in dmesg are annoying.  This commit therefore avoids spurious kicks
in the common case.

Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Reviewed-by: Josh Triplett <josh@joshtriplett.org>
2016-11-14 10:46:31 -08:00
Paul E. McKenney
1f21b50b77 rcu: Remove obsolete comment from __call_rcu()
The __call_rcu() comment about opportunistically noting grace period
beginnings and endings is obsolete.  RCU still does such opportunistic
noting, but in __call_rcu_core() rather than __call_rcu(), and there
already is an appropriate comment in __call_rcu_core().  This commit
therefore removes the obsolete comment.

Reported-by: Michalis Kokologiannakis <mixaskok@gmail.com>
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Reviewed-by: Josh Triplett <josh@joshtriplett.org>
2016-11-14 10:46:19 -08:00
Paul E. McKenney
5403d367a7 rcu: Remove obsolete rcu_check_callbacks() header comment
In the deep past, rcu_check_callbacks() was only invoked if rcu_pending()
returned true.  Which was fine, but these days rcu_check_callbacks()
is invoked unconditionally.  This commit therefore removes the obsolete
sentence from the header comment.

Reported-by: Michalis Kokologiannakis <mixaskok@gmail.com>
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Reviewed-by: Josh Triplett <josh@joshtriplett.org>
2016-11-14 10:46:14 -08:00
Paul E. McKenney
b8f2ed5384 rcu: Tighten up __call_rcu() rcu_head alignment check
Commit 720abae3d6 ("rcu: force alignment on struct
callback_head/rcu_head") forced the rcu_head (AKA callback_head)
structure's alignment to pointer size, that is, to 4-byte boundaries on
32-bit systems and to 8-byte boundaries on 64-bit systems.  This
commit therefore checks for this same alignment in __call_rcu(),
which used to contain a looser check for two-byte alignment.

Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Tested-by: Geert Uytterhoeven <geert@linux-m68k.org>
Acked-by: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>
Reviewed-by: Josh Triplett <josh@joshtriplett.org>
2016-11-14 10:46:08 -08:00
Linus Torvalds
9ffc66941d This adds a new gcc plugin named "latent_entropy". It is designed to
extract as much possible uncertainty from a running system at boot time as
 possible, hoping to capitalize on any possible variation in CPU operation
 (due to runtime data differences, hardware differences, SMP ordering,
 thermal timing variation, cache behavior, etc).
 
 At the very least, this plugin is a much more comprehensive example for
 how to manipulate kernel code using the gcc plugin internals.
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Merge tag 'gcc-plugins-v4.9-rc1' of git://git.kernel.org/pub/scm/linux/kernel/git/kees/linux

Pull gcc plugins update from Kees Cook:
 "This adds a new gcc plugin named "latent_entropy". It is designed to
  extract as much possible uncertainty from a running system at boot
  time as possible, hoping to capitalize on any possible variation in
  CPU operation (due to runtime data differences, hardware differences,
  SMP ordering, thermal timing variation, cache behavior, etc).

  At the very least, this plugin is a much more comprehensive example
  for how to manipulate kernel code using the gcc plugin internals"

* tag 'gcc-plugins-v4.9-rc1' of git://git.kernel.org/pub/scm/linux/kernel/git/kees/linux:
  latent_entropy: Mark functions with __latent_entropy
  gcc-plugins: Add latent_entropy plugin
2016-10-15 10:03:15 -07:00
Emese Revfy
0766f788eb latent_entropy: Mark functions with __latent_entropy
The __latent_entropy gcc attribute can be used only on functions and
variables.  If it is on a function then the plugin will instrument it for
gathering control-flow entropy. If the attribute is on a variable then
the plugin will initialize it with random contents.  The variable must
be an integer, an integer array type or a structure with integer fields.

These specific functions have been selected because they are init
functions (to help gather boot-time entropy), are called at unpredictable
times, or they have variable loops, each of which provide some level of
latent entropy.

Signed-off-by: Emese Revfy <re.emese@gmail.com>
[kees: expanded commit message]
Signed-off-by: Kees Cook <keescook@chromium.org>
2016-10-10 14:51:45 -07:00
Paul E. McKenney
d74b62bc32 Merge branches 'doc.2016.08.22c', 'exp.2016.08.22c', 'fixes.2016.09.14a', 'hotplug.2016.08.22c' and 'torture.2016.08.22c' into HEAD
doc.2016.08.22c: Documentation updates
exp.2016.08.22c: Expedited grace-period updates
fixes.2016.09.14a: Miscellaneous fixes
hotplug.2016.08.22c: CPU-hotplug changes
torture.2016.08.22c: Torture-test changes
2016-09-14 12:58:49 -07:00
Paul E. McKenney
7ec99de36f rcu: Provide exact CPU-online tracking for RCU
Up to now, RCU has assumed that the CPU-online process makes it from
CPU_UP_PREPARE to set_cpu_online() within one jiffy.  Given the recent
rise of virtualized environments, this assumption is very clearly
obsolete.  Failing to meet this deadline can result in RCU paying
attention to an incoming CPU for one jiffy, then ignoring it until the
grace period following the one in which that CPU sets itself online.
This situation might prove to be fatally disappointing to any RCU
read-side critical sections that had the misfortune to execute during
the time in which RCU was ignoring the slow-to-come-online CPU.

This commit therefore updates RCU's internal CPU state-tracking
information at notify_cpu_starting() time, thus providing RCU with
an exact transition of the CPU's state from offline to online.

Note that this means that incoming CPUs must not use RCU read-side
critical section (other than those of SRCU) until notify_cpu_starting()
time.  Note also that the CPU_STARTING notifiers -are- allowed to use
RCU read-side critical sections.  (Of course, CPU-hotplug notifiers are
rapidly becoming obsolete, so you need to act fast!)

If a given architecture or CPU family needs to use RCU read-side
critical sections earlier, the call to rcu_cpu_starting() from
notify_cpu_starting() will need to be architecture-specific, with
architectures that need early use being required to hand-place
the call to rcu_cpu_starting() at some point preceding the call to
notify_cpu_starting().

Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
2016-08-22 09:36:57 -07:00
Paul E. McKenney
3563a438f1 rcu: Avoid redundant quiescent-state chasing
Currently, __note_gp_changes() checks to see if the CPU has slept through
multiple grace periods.  If it has, it resynchronizes that CPU's view
of the grace-period state, which includes whether or not the current
grace period needs a quiescent state from this CPU.  The fact of this
need (or lack thereof) needs to be in two places, rdp->cpu_no_qs.b.norm
and rdp->core_needs_qs.  The former tells RCU's context-switch code to
go get a quiescent state and the latter says that it needs to be reported.
The current code unconditionally sets the former to true, but correctly
sets the latter.

This does not result in failures, but it does unnecessarily increase
the amount of work done on average at context-switch time.  This commit
therefore correctly sets both fields.

Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
2016-08-22 09:35:57 -07:00
Paul Gortmaker
e77b704125 rcu: Don't use modular infrastructure in non-modular code
The Kconfig currently controlling compilation of tree.c is:

init/Kconfig:config TREE_RCU
init/Kconfig:   bool

...and update.c and sync.c are "obj-y" meaning that none are ever
built as a module by anyone.

Since MODULE_ALIAS is a no-op for non-modular code, we can remove
them from these files.

We leave moduleparam.h behind since the files instantiate some boot
time configuration parameters with module_param() still.

Cc: "Paul E. McKenney" <paulmck@linux.vnet.ibm.com>
Cc: Josh Triplett <josh@joshtriplett.org>
Cc: Steven Rostedt <rostedt@goodmis.org>
Cc: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
Cc: Lai Jiangshan <jiangshanlai@gmail.com>
Signed-off-by: Paul Gortmaker <paul.gortmaker@windriver.com>
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
2016-08-22 09:35:27 -07:00
Jisheng Zhang
94d4477673 rcu: Use rcu_gp_kthread_wake() to wake up grace period kthreads
Commit abedf8e241 ("rcu: Use simple wait queues where possible in
rcutree") converts Tree RCU's wait queues to simple wait queues,
but it incorrectly reverts the commit 2aa792e6fa ("rcu: Use
rcu_gp_kthread_wake() to wake up grace period kthreads").  This can
result in redundant self-wakeups.

This commit therefore replaces the simple wait-queue wakeups with
rcu_gp_kthread_wake(), thus avoiding the redundant wakeups.

Signed-off-by: Jisheng Zhang <jszhang@marvell.com>
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
2016-08-22 09:33:46 -07:00
Thomas Gleixner
4df8374254 rcu: Convert rcutree to hotplug state machine
Straight forward conversion to the state machine. Though the question arises
whether this needs really all these state transitions to work.

Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Anna-Maria Gleixner <anna-maria@linutronix.de>
Reviewed-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: rt@linutronix.de
Link: http://lkml.kernel.org/r/20160713153337.982013161@linutronix.de
Signed-off-by: Ingo Molnar <mingo@kernel.org>
2016-07-15 10:41:44 +02:00
Mark Rutland
bc75e99983 rcu: Correctly handle sparse possible cpus
In many cases in the RCU tree code, we iterate over the set of cpus for
a leaf node described by rcu_node::grplo and rcu_node::grphi, checking
per-cpu data for each cpu in this range. However, if the set of possible
cpus is sparse, some cpus described in this range are not possible, and
thus no per-cpu region will have been allocated (or initialised) for
them by the generic percpu code.

Erroneous accesses to a per-cpu area for these !possible cpus may fault
or may hit other data depending on the addressed generated when the
erroneous per cpu offset is applied. In practice, both cases have been
observed on arm64 hardware (the former being silent, but detectable with
additional patches).

To avoid issues resulting from this, we must iterate over the set of
*possible* cpus for a given leaf node. This patch add a new helper,
for_each_leaf_node_possible_cpu, to enable this. As iteration is often
intertwined with rcu_node local bitmask manipulation, a new
leaf_node_cpu_bit helper is added to make this simpler and more
consistent. The RCU tree code is made to use both of these where
appropriate.

Without this patch, running reboot at a shell can result in an oops
like:

[ 3369.075979] Unable to handle kernel paging request at virtual address ffffff8008b21b4c
[ 3369.083881] pgd = ffffffc3ecdda000
[ 3369.087270] [ffffff8008b21b4c] *pgd=00000083eca48003, *pud=00000083eca48003, *pmd=0000000000000000
[ 3369.096222] Internal error: Oops: 96000007 [#1] PREEMPT SMP
[ 3369.101781] Modules linked in:
[ 3369.104825] CPU: 2 PID: 1817 Comm: NetworkManager Tainted: G        W       4.6.0+ #3
[ 3369.121239] task: ffffffc0fa13e000 ti: ffffffc3eb940000 task.ti: ffffffc3eb940000
[ 3369.128708] PC is at sync_rcu_exp_select_cpus+0x188/0x510
[ 3369.134094] LR is at sync_rcu_exp_select_cpus+0x104/0x510
[ 3369.139479] pc : [<ffffff80081109a8>] lr : [<ffffff8008110924>] pstate: 200001c5
[ 3369.146860] sp : ffffffc3eb9435a0
[ 3369.150162] x29: ffffffc3eb9435a0 x28: ffffff8008be4f88
[ 3369.155465] x27: ffffff8008b66c80 x26: ffffffc3eceb2600
[ 3369.160767] x25: 0000000000000001 x24: ffffff8008be4f88
[ 3369.166070] x23: ffffff8008b51c3c x22: ffffff8008b66c80
[ 3369.171371] x21: 0000000000000001 x20: ffffff8008b21b40
[ 3369.176673] x19: ffffff8008b66c80 x18: 0000000000000000
[ 3369.181975] x17: 0000007fa951a010 x16: ffffff80086a30f0
[ 3369.187278] x15: 0000007fa9505590 x14: 0000000000000000
[ 3369.192580] x13: ffffff8008b51000 x12: ffffffc3eb940000
[ 3369.197882] x11: 0000000000000006 x10: ffffff8008b51b78
[ 3369.203184] x9 : 0000000000000001 x8 : ffffff8008be4000
[ 3369.208486] x7 : ffffff8008b21b40 x6 : 0000000000001003
[ 3369.213788] x5 : 0000000000000000 x4 : ffffff8008b27280
[ 3369.219090] x3 : ffffff8008b21b4c x2 : 0000000000000001
[ 3369.224406] x1 : 0000000000000001 x0 : 0000000000000140
...
[ 3369.972257] [<ffffff80081109a8>] sync_rcu_exp_select_cpus+0x188/0x510
[ 3369.978685] [<ffffff80081128b4>] synchronize_rcu_expedited+0x64/0xa8
[ 3369.985026] [<ffffff80086b987c>] synchronize_net+0x24/0x30
[ 3369.990499] [<ffffff80086ddb54>] dev_deactivate_many+0x28c/0x298
[ 3369.996493] [<ffffff80086b6bb8>] __dev_close_many+0x60/0xd0
[ 3370.002052] [<ffffff80086b6d48>] __dev_close+0x28/0x40
[ 3370.007178] [<ffffff80086bf62c>] __dev_change_flags+0x8c/0x158
[ 3370.012999] [<ffffff80086bf718>] dev_change_flags+0x20/0x60
[ 3370.018558] [<ffffff80086cf7f0>] do_setlink+0x288/0x918
[ 3370.023771] [<ffffff80086d0798>] rtnl_newlink+0x398/0x6a8
[ 3370.029158] [<ffffff80086cee84>] rtnetlink_rcv_msg+0xe4/0x220
[ 3370.034891] [<ffffff80086e274c>] netlink_rcv_skb+0xc4/0xf8
[ 3370.040364] [<ffffff80086ced8c>] rtnetlink_rcv+0x2c/0x40
[ 3370.045663] [<ffffff80086e1fe8>] netlink_unicast+0x160/0x238
[ 3370.051309] [<ffffff80086e24b8>] netlink_sendmsg+0x2f0/0x358
[ 3370.056956] [<ffffff80086a0070>] sock_sendmsg+0x18/0x30
[ 3370.062168] [<ffffff80086a21cc>] ___sys_sendmsg+0x26c/0x280
[ 3370.067728] [<ffffff80086a30ac>] __sys_sendmsg+0x44/0x88
[ 3370.073027] [<ffffff80086a3100>] SyS_sendmsg+0x10/0x20
[ 3370.078153] [<ffffff8008085e70>] el0_svc_naked+0x24/0x28

Signed-off-by: Mark Rutland <mark.rutland@arm.com>
Reported-by: Dennis Chen <dennis.chen@arm.com>
Cc: Catalin Marinas <catalin.marinas@arm.com>
Cc: Josh Triplett <josh@joshtriplett.org>
Cc: Lai Jiangshan <jiangshanlai@gmail.com>
Cc: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
Cc: Steve Capper <steve.capper@arm.com>
Cc: Steven Rostedt <rostedt@goodmis.org>
Cc: Will Deacon <will.deacon@arm.com>
Cc: linux-kernel@vger.kernel.org
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
2016-06-15 16:00:05 -07:00
Daniel Bristot de Oliveira
088e9d253d rcu: sysctl: Panic on RCU Stall
It is not always easy to determine the cause of an RCU stall just by
analysing the RCU stall messages, mainly when the problem is caused
by the indirect starvation of rcu threads. For example, when preempt_rcu
is not awakened due to the starvation of a timer softirq.

We have been hard coding panic() in the RCU stall functions for
some time while testing the kernel-rt. But this is not possible in
some scenarios, like when supporting customers.

This patch implements the sysctl kernel.panic_on_rcu_stall. If
set to 1, the system will panic() when an RCU stall takes place,
enabling the capture of a vmcore. The vmcore provides a way to analyze
all kernel/tasks states, helping out to point to the culprit and the
solution for the stall.

The kernel.panic_on_rcu_stall sysctl is disabled by default.

Changes from v1:
- Fixed a typo in the git log
- The if(sysctl_panic_on_rcu_stall) panic() is in a static function
- Fixed the CONFIG_TINY_RCU compilation issue
- The var sysctl_panic_on_rcu_stall is now __read_mostly

Cc: Jonathan Corbet <corbet@lwn.net>
Cc: "Paul E. McKenney" <paulmck@linux.vnet.ibm.com>
Cc: Josh Triplett <josh@joshtriplett.org>
Cc: Steven Rostedt <rostedt@goodmis.org>
Cc: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
Cc: Lai Jiangshan <jiangshanlai@gmail.com>
Acked-by: Christian Borntraeger <borntraeger@de.ibm.com>
Reviewed-by: Josh Triplett <josh@joshtriplett.org>
Reviewed-by: Arnaldo Carvalho de Melo <acme@kernel.org>
Tested-by: "Luis Claudio R. Goncalves" <lgoncalv@redhat.com>
Signed-off-by: Daniel Bristot de Oliveira <bristot@redhat.com>
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
2016-06-15 16:00:05 -07:00
Paul E. McKenney
3549c2bc2c rcu: Move expedited code from tree.c to tree_exp.h
People have been having some difficulty finding their way around the
RCU code.  This commit therefore pulls some of the expedited grace-period
code from tree.c to a new tree_exp.h file.  This commit is strictly code
movement, with the exception of a forward declaration that was added
for the sync_sched_exp_online_cleanup() function.

A subsequent commit will move the remaining expedited grace-period code
from tree_plugin.h to tree_exp.h.

Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
2016-06-14 16:01:41 -07:00
Peter Zijlstra
d3acab65f2 rcu: Remove some superfluous lines
I think you'll find this condition is superfluous, as the whole function
is under #ifdef of that same.

Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
2016-06-14 16:01:41 -07:00
Paul E. McKenney
590d1757b9 rcu: Fix outdated hotplug-exclusion comment in rcu_gp_init()
In the past, RCU grace-period initialization excluded CPU-hotplug
operations, but this is no longer the case.  This commit therefore
removed an outdated comment in rcu_gp_init() claiming that these
are excluded.

Reported-by: Lihao Liang <lihao.liang@gmail.com>
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
2016-06-14 16:01:40 -07:00
Paul E. McKenney
0d95092ccb rcu: Fix outdated rcu_scheduler_active comment
The comment header for rcu_scheduler_active states that it is used
to optimize synchronize_sched() at early boot.  This is incorrect.
The synchronize_sched() function instead checks the number of online
CPUs.  This commit therefore replaces the comment's synchronize_sched()
with synchronize_rcu(), which really does use rcu_scheduler_active for
this purpose.

Reported-by: Lihao Liang <lihao.liang@gmail.com>
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
2016-06-14 16:01:39 -07:00
Paul E. McKenney
dcd36d01fb Merge branches 'doc.2016.04.19a', 'exp.2016.03.31d', 'fixes.2016.03.31d' and 'torture.2016.04.21a' into HEAD
doc.2016.04.19a: Documentation updates
exp.2016.03.31d: Expedited grace-period updates
fixes.2016.03.31d: Miscellaneous fixes
torture.2016.004.21a Torture-test updates
2016-04-21 13:48:20 -07:00
Paul E. McKenney
291783b8ad rcutorture: Expedited-GP batch progress access to torturing
This commit provides rcu_exp_batches_completed() and
rcu_exp_batches_completed_sched() functions to allow torture-test modules
to check how many expedited grace period batches have completed.
These are analogous to the existing rcu_batches_completed(),
rcu_batches_completed_bh(), and rcu_batches_completed_sched() functions.

Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
2016-03-31 13:37:37 -07:00
Paul E. McKenney
5dffed1e57 rcu: Dump ftrace buffer when kicking grace-period kthread
If it is necessary to kick the grace-period kthread, that is a good
time to dump the trace buffer in order to learn why kicking was needed.
This commit therefore does the dump.

Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
2016-03-31 13:36:37 -07:00
Paul E. McKenney
8c7c4829a8 rcu: Awaken grace-period kthread if too long since FQS
Recent kernels can fail to awaken the grace-period kthread for
quiescent-state forcing.  This commit is a crude hack that does
a wakeup if a scheduling-clock interrupt sees that it has been
too long since force-quiescent-state (FQS) processing.

Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
2016-03-31 13:34:50 -07:00
Paul E. McKenney
fcfd0a237b rcu: Make FQS schedule advance only if FQS happened
Currently, the force-quiescent-state (FQS) code in rcu_gp_kthread() can
advance the next FQS even if one was not executed last time.  This can
happen due timeout-duration uncertainty.  This commit therefore avoids
advancing the FQS schedule unless an FQS was just executed.  In the
corner case where an FQS was not executed, but is due now, the code does
a one-jiffy wait.

This change prepares for kthread kicking.

Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
2016-03-31 13:34:49 -07:00
Paul E. McKenney
86057b80ae rcu: Awaken grace-period kthread when stalled
Recent kernels can fail to awaken the grace-period kthread for
quiescent-state forcing.  This commit is a crude hack that does
a wakeup any time a stall is detected.

Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
2016-03-31 13:34:49 -07:00
Paul E. McKenney
3b5f668e71 rcu: Overlap wakeups with next expedited grace period
The current expedited grace-period implementation makes subsequent grace
periods wait on wakeups for the prior grace period.  This does not fit
the dictionary definition of "expedited", so this commit allows these two
phases to overlap.  Doing this requires four waitqueues rather than two
because tasks can now be waiting on the previous, current, and next grace
periods.  The fourth waitqueue makes the bit masking work out nicely.

Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
2016-03-31 13:34:11 -07:00
Paul E. McKenney
aff12cdf86 rcu: Consolidate expedited GP code into exp_funnel_lock()
This commit pulls the grace-period-start counter adjustment and tracing
from synchronize_rcu_expedited() and synchronize_sched_expedited()
into exp_funnel_lock(), thus eliminating some code duplication.

Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
2016-03-31 13:34:11 -07:00
Paul E. McKenney
179e5dcd1e rcu: Consolidate expedited GP tracing into rcu_exp_gp_seq_snap()
This commit moves some duplicate code from synchronize_rcu_expedited()
and synchronize_sched_expedited() into rcu_exp_gp_seq_snap().  This
doesn't save lines of code, but does eliminate a "tell me twice" issue.

Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
2016-03-31 13:34:10 -07:00
Paul E. McKenney
4ea3e85b11 rcu: Consolidate expedited GP code into rcu_exp_wait_wake()
Currently, synchronize_rcu_expedited() and rcu_sched_expedited() have
significant duplicate code.  This commit therefore consolidates some of
this code into rcu_exp_wake(), which is now renamed to rcu_exp_wait_wake()
in recognition of its added responsibilities.

Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
2016-03-31 13:34:10 -07:00
Paul E. McKenney
356051e1de rcu: Add exp_funnel_lock() fastpath
This commit speeds up the low-contention case, especially for systems
with large rcu_node trees, by attempting to directly acquire the
->exp_mutex.  This fastpath checks the leaves and root first in
order to avoid excessive memory contention on the mutex itself.

Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
2016-03-31 13:34:09 -07:00
Paul E. McKenney
f6a12f34a4 rcu: Enforce expedited-GP fairness via funnel wait queue
The current mutex-based funnel-locking approach used by expedited grace
periods is subject to severe unfairness.  The problem arises when a
few tasks, making a path from leaves to root, all wake up before other
tasks do.  A new task can then follow this path all the way to the root,
which needlessly delays tasks whose grace period is done, but who do
not happen to acquire the lock quickly enough.

This commit avoids this problem by maintaining per-rcu_node wait queues,
along with a per-rcu_node counter that tracks the latest grace period
sought by an earlier task to visit this node.  If that grace period
would satisfy the current task, instead of proceeding up the tree,
it waits on the current rcu_node structure using a pair of wait queues
provided for that purpose.  This decouples awakening of old tasks from
the arrival of new tasks.

If the wakeups prove to be a bottleneck, additional kthreads can be
brought to bear for that purpose.

Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
2016-03-31 13:34:08 -07:00
Paul E. McKenney
d40a4f09a4 rcu: Shorten expedited_workdone* to exp_workdone*
Just a name change to save a few lines and a bit of typing.

Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
2016-03-31 13:34:08 -07:00
Paul E. McKenney
ec3833ed02 rcu: Force boolean subscript for expedited stall warnings
The cpu_online() function can return values other than 0 and 1, which
can result in subscript overflow when applied to a two-element array.
This commit allows for this behavior by using "!!" on the return value
from cpu_online() when used as a subscript.

Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
2016-03-31 13:34:07 -07:00
Paul E. McKenney
e2fd9d3584 rcu: Remove expedited GP funnel-lock bypass
Commit #cdacbe1f91264 ("rcu: Add fastpath bypassing funnel locking")
turns out to be a pessimization at high load because it forces a tree
full of tasks to wait for an expedited grace period that they probably
do not need.  This commit therefore removes this optimization.

Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
2016-03-31 13:34:07 -07:00
Paul E. McKenney
4f41530245 rcu: Add expedited-grace-period event tracing
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
2016-03-31 13:34:06 -07:00
Paul E. McKenney
bea2de44ae rcu: Add funnel-locking tracing for expedited grace periods
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
2016-03-31 13:34:06 -07:00
Paul E. McKenney
a1e1224849 rcu: Make cond_resched_rcu_qs() supply RCU-sched expedited QS
Although cond_resched_rcu_qs() supplies quiescent states to all flavors
of normal RCU grace periods, it does nothing for expedited RCU-sched
grace periods.  This commit therefore adds a check for a need for a
quiescent state from the current CPU by an expedited RCU-sched grace
period, and invokes rcu_sched_qs() to supply that quiescent state if so.

Note that the check is racy in that we might be migrated to some other
CPU just after checking the per-CPU variable.  This is OK because the
act of migration will do a context switch, which will supply the needed
quiescent state.  The only downside is that we might do an unnecessary
call to rcu_sched_qs(), but the probability is low and the overhead
is small.

Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
2016-03-31 13:34:04 -07:00
Paul E. McKenney
251c617c75 rcu: Make expedited RCU-preempt stall warnings count accurately
Currently, synchronize_sched_expedited_wait() simply sets the ndetected
variable to the rcu_print_task_exp_stall() return value.  This means
that if the last rcu_node structure has no stalled tasks, record of
any stalled tasks in previous rcu_node structures is lost, which can
in turn result in failure to dump out the blocking rcu_node structures.
Or could, had the test been correct.

This commit therefore adds the return value of rcu_print_task_exp_stall()
to ndetected and corrects the later test for ndetected.

Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
2016-03-31 13:34:03 -07:00
Paul E. McKenney
28728dd310 rcu: Make expedited RCU-sched grace period immediately detect idle
Currently, sync_sched_exp_handler() will force a reschedule unless
this CPU has already checked in or unless a reschedule has already
been called for.  This is clearly wasteful if sync_sched_exp_handler()
interrupted an idle CPU, so this commit immediately reports the
quiescent state in that case.

Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
2016-03-31 13:34:03 -07:00
Paul E. McKenney
274529ba9b rcu: Consolidate dumping of ftrace buffer
This commit consolidates a couple definitions and several calls for
single-shot ftrace-buffer dumping.

Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
2016-03-31 13:29:08 -07:00
Linus Torvalds
710d60cbf1 Merge branch 'smp-hotplug-for-linus' of git://git.kernel.org/pub/scm/linux/kernel/git/tip/tip
Pull cpu hotplug updates from Thomas Gleixner:
 "This is the first part of the ongoing cpu hotplug rework:

   - Initial implementation of the state machine

   - Runs all online and prepare down callbacks on the plugged cpu and
     not on some random processor

   - Replaces busy loop waiting with completions

   - Adds tracepoints so the states can be followed"

More detailed commentary on this work from an earlier email:
 "What's wrong with the current cpu hotplug infrastructure?

   - Asymmetry

     The hotplug notifier mechanism is asymmetric versus the bringup and
     teardown.  This is mostly caused by the notifier mechanism.

   - Largely undocumented dependencies

     While some notifiers use explicitely defined notifier priorities,
     we have quite some notifiers which use numerical priorities to
     express dependencies without any documentation why.

   - Control processor driven

     Most of the bringup/teardown of a cpu is driven by a control
     processor.  While it is understandable, that preperatory steps,
     like idle thread creation, memory allocation for and initialization
     of essential facilities needs to be done before a cpu can boot,
     there is no reason why everything else must run on a control
     processor.  Before this patch series, bringup looks like this:

       Control CPU                     Booting CPU

       do preparatory steps
       kick cpu into life

                                       do low level init

       sync with booting cpu           sync with control cpu

       bring the rest up

   - All or nothing approach

     There is no way to do partial bringups.  That's something which is
     really desired because we waste e.g.  at boot substantial amount of
     time just busy waiting that the cpu comes to life.  That's stupid
     as we could very well do preparatory steps and the initial IPI for
     other cpus and then go back and do the necessary low level
     synchronization with the freshly booted cpu.

   - Minimal debuggability

     Due to the notifier based design, it's impossible to switch between
     two stages of the bringup/teardown back and forth in order to test
     the correctness.  So in many hotplug notifiers the cancel
     mechanisms are either not existant or completely untested.

   - Notifier [un]registering is tedious

     To [un]register notifiers we need to protect against hotplug at
     every callsite.  There is no mechanism that bringup/teardown
     callbacks are issued on the online cpus, so every caller needs to
     do it itself.  That also includes error rollback.

  What's the new design?

     The base of the new design is a symmetric state machine, where both
     the control processor and the booting/dying cpu execute a well
     defined set of states.  Each state is symmetric in the end, except
     for some well defined exceptions, and the bringup/teardown can be
     stopped and reversed at almost all states.

     So the bringup of a cpu will look like this in the future:

       Control CPU                     Booting CPU

       do preparatory steps
       kick cpu into life

                                       do low level init

       sync with booting cpu           sync with control cpu

                                       bring itself up

     The synchronization step does not require the control cpu to wait.
     That mechanism can be done asynchronously via a worker or some
     other mechanism.

     The teardown can be made very similar, so that the dying cpu cleans
     up and brings itself down.  Cleanups which need to be done after
     the cpu is gone, can be scheduled asynchronously as well.

  There is a long way to this, as we need to refactor the notion when a
  cpu is available.  Today we set the cpu online right after it comes
  out of the low level bringup, which is not really correct.

  The proper mechanism is to set it to available, i.e. cpu local
  threads, like softirqd, hotplug thread etc. can be scheduled on that
  cpu, and once it finished all booting steps, it's set to online, so
  general workloads can be scheduled on it.  The reverse happens on
  teardown.  First thing to do is to forbid scheduling of general
  workloads, then teardown all the per cpu resources and finally shut it
  off completely.

  This patch series implements the basic infrastructure for this at the
  core level.  This includes the following:

   - Basic state machine implementation with well defined states, so
     ordering and prioritization can be expressed.

   - Interfaces to [un]register state callbacks

     This invokes the bringup/teardown callback on all online cpus with
     the proper protection in place and [un]installs the callbacks in
     the state machine array.

     For callbacks which have no particular ordering requirement we have
     a dynamic state space, so that drivers don't have to register an
     explicit hotplug state.

     If a callback fails, the code automatically does a rollback to the
     previous state.

   - Sysfs interface to drive the state machine to a particular step.

     This is only partially functional today.  Full functionality and
     therefor testability will be achieved once we converted all
     existing hotplug notifiers over to the new scheme.

   - Run all CPU_ONLINE/DOWN_PREPARE notifiers on the booting/dying
     processor:

       Control CPU                     Booting CPU

       do preparatory steps
       kick cpu into life

                                       do low level init

       sync with booting cpu           sync with control cpu
       wait for boot
                                       bring itself up

                                       Signal completion to control cpu

     In a previous step of this work we've done a full tree mechanical
     conversion of all hotplug notifiers to the new scheme.  The balance
     is a net removal of about 4000 lines of code.

     This is not included in this series, as we decided to take a
     different approach.  Instead of mechanically converting everything
     over, we will do a proper overhaul of the usage sites one by one so
     they nicely fit into the symmetric callback scheme.

     I decided to do that after I looked at the ugliness of some of the
     converted sites and figured out that their hotplug mechanism is
     completely buggered anyway.  So there is no point to do a
     mechanical conversion first as we need to go through the usage
     sites one by one again in order to achieve a full symmetric and
     testable behaviour"

* 'smp-hotplug-for-linus' of git://git.kernel.org/pub/scm/linux/kernel/git/tip/tip: (23 commits)
  cpu/hotplug: Document states better
  cpu/hotplug: Fix smpboot thread ordering
  cpu/hotplug: Remove redundant state check
  cpu/hotplug: Plug death reporting race
  rcu: Make CPU_DYING_IDLE an explicit call
  cpu/hotplug: Make wait for dead cpu completion based
  cpu/hotplug: Let upcoming cpu bring itself fully up
  arch/hotplug: Call into idle with a proper state
  cpu/hotplug: Move online calls to hotplugged cpu
  cpu/hotplug: Create hotplug threads
  cpu/hotplug: Split out the state walk into functions
  cpu/hotplug: Unpark smpboot threads from the state machine
  cpu/hotplug: Move scheduler cpu_online notifier to hotplug core
  cpu/hotplug: Implement setup/removal interface
  cpu/hotplug: Make target state writeable
  cpu/hotplug: Add sysfs state interface
  cpu/hotplug: Hand in target state to _cpu_up/down
  cpu/hotplug: Convert the hotplugged cpu work to a state machine
  cpu/hotplug: Convert to a state machine for the control processor
  cpu/hotplug: Add tracepoints
  ...
2016-03-15 13:50:29 -07:00
Ingo Molnar
8bc6782fe2 Merge commit 'fixes.2015.02.23a' into core/rcu
Conflicts:
	kernel/rcu/tree.c

Signed-off-by: Ingo Molnar <mingo@kernel.org>
2016-03-15 09:01:06 +01:00
Thomas Gleixner
27d50c7eeb rcu: Make CPU_DYING_IDLE an explicit call
Make the RCU CPU_DYING_IDLE callback an explicit function call, so it gets
invoked at the proper place.

Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Cc: linux-arch@vger.kernel.org
Cc: Rik van Riel <riel@redhat.com>
Cc: Rafael Wysocki <rafael.j.wysocki@intel.com>
Cc: "Srivatsa S. Bhat" <srivatsa@mit.edu>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Arjan van de Ven <arjan@linux.intel.com>
Cc: Sebastian Siewior <bigeasy@linutronix.de>
Cc: Rusty Russell <rusty@rustcorp.com.au>
Cc: Steven Rostedt <rostedt@goodmis.org>
Cc: Oleg Nesterov <oleg@redhat.com>
Cc: Tejun Heo <tj@kernel.org>
Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: Paul McKenney <paulmck@linux.vnet.ibm.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Paul Turner <pjt@google.com>
Link: http://lkml.kernel.org/r/20160226182341.870167933@linutronix.de
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
2016-03-01 20:36:58 +01:00
Paul Gortmaker
abedf8e241 rcu: Use simple wait queues where possible in rcutree
As of commit dae6e64d2b ("rcu: Introduce proper blocking to no-CBs kthreads
GP waits") the RCU subsystem started making use of wait queues.

Here we convert all additions of RCU wait queues to use simple wait queues,
since they don't need the extra overhead of the full wait queue features.

Originally this was done for RT kernels[1], since we would get things like...

  BUG: sleeping function called from invalid context at kernel/rtmutex.c:659
  in_atomic(): 1, irqs_disabled(): 1, pid: 8, name: rcu_preempt
  Pid: 8, comm: rcu_preempt Not tainted
  Call Trace:
   [<ffffffff8106c8d0>] __might_sleep+0xd0/0xf0
   [<ffffffff817d77b4>] rt_spin_lock+0x24/0x50
   [<ffffffff8106fcf6>] __wake_up+0x36/0x70
   [<ffffffff810c4542>] rcu_gp_kthread+0x4d2/0x680
   [<ffffffff8105f910>] ? __init_waitqueue_head+0x50/0x50
   [<ffffffff810c4070>] ? rcu_gp_fqs+0x80/0x80
   [<ffffffff8105eabb>] kthread+0xdb/0xe0
   [<ffffffff8106b912>] ? finish_task_switch+0x52/0x100
   [<ffffffff817e0754>] kernel_thread_helper+0x4/0x10
   [<ffffffff8105e9e0>] ? __init_kthread_worker+0x60/0x60
   [<ffffffff817e0750>] ? gs_change+0xb/0xb

...and hence simple wait queues were deployed on RT out of necessity
(as simple wait uses a raw lock), but mainline might as well take
advantage of the more streamline support as well.

[1] This is a carry forward of work from v3.10-rt; the original conversion
was by Thomas on an earlier -rt version, and Sebastian extended it to
additional post-3.10 added RCU waiters; here I've added a commit log and
unified the RCU changes into one, and uprev'd it to match mainline RCU.

Signed-off-by: Daniel Wagner <daniel.wagner@bmw-carit.de>
Acked-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Cc: linux-rt-users@vger.kernel.org
Cc: Boqun Feng <boqun.feng@gmail.com>
Cc: Marcelo Tosatti <mtosatti@redhat.com>
Cc: Steven Rostedt <rostedt@goodmis.org>
Cc: Paul Gortmaker <paul.gortmaker@windriver.com>
Cc: Paolo Bonzini <pbonzini@redhat.com>
Cc: "Paul E. McKenney" <paulmck@linux.vnet.ibm.com>
Link: http://lkml.kernel.org/r/1455871601-27484-6-git-send-email-wagi@monom.org
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
2016-02-25 11:27:16 +01:00
Daniel Wagner
065bb78c5b rcu: Do not call rcu_nocb_gp_cleanup() while holding rnp->lock
rcu_nocb_gp_cleanup() is called while holding rnp->lock. Currently,
this is okay because the wake_up_all() in rcu_nocb_gp_cleanup() will
not enable the IRQs. lockdep is happy.

By switching over using swait this is not true anymore. swake_up_all()
enables the IRQs while processing the waiters. __do_softirq() can now
run and will eventually call rcu_process_callbacks() which wants to
grap nrp->lock.

Let's move the rcu_nocb_gp_cleanup() call outside the lock before we
switch over to swait.

If we would hold the rnp->lock and use swait, lockdep reports
following:

 =================================
 [ INFO: inconsistent lock state ]
 4.2.0-rc5-00025-g9a73ba0 #136 Not tainted
 ---------------------------------
 inconsistent {IN-SOFTIRQ-W} -> {SOFTIRQ-ON-W} usage.
 rcu_preempt/8 [HC0[0]:SC0[0]:HE1:SE1] takes:
  (rcu_node_1){+.?...}, at: [<ffffffff811387c7>] rcu_gp_kthread+0xb97/0xeb0
 {IN-SOFTIRQ-W} state was registered at:
   [<ffffffff81109b9f>] __lock_acquire+0xd5f/0x21e0
   [<ffffffff8110be0f>] lock_acquire+0xdf/0x2b0
   [<ffffffff81841cc9>] _raw_spin_lock_irqsave+0x59/0xa0
   [<ffffffff81136991>] rcu_process_callbacks+0x141/0x3c0
   [<ffffffff810b1a9d>] __do_softirq+0x14d/0x670
   [<ffffffff810b2214>] irq_exit+0x104/0x110
   [<ffffffff81844e96>] smp_apic_timer_interrupt+0x46/0x60
   [<ffffffff81842e70>] apic_timer_interrupt+0x70/0x80
   [<ffffffff810dba66>] rq_attach_root+0xa6/0x100
   [<ffffffff810dbc2d>] cpu_attach_domain+0x16d/0x650
   [<ffffffff810e4b42>] build_sched_domains+0x942/0xb00
   [<ffffffff821777c2>] sched_init_smp+0x509/0x5c1
   [<ffffffff821551e3>] kernel_init_freeable+0x172/0x28f
   [<ffffffff8182cdce>] kernel_init+0xe/0xe0
   [<ffffffff8184231f>] ret_from_fork+0x3f/0x70
 irq event stamp: 76
 hardirqs last  enabled at (75): [<ffffffff81841330>] _raw_spin_unlock_irq+0x30/0x60
 hardirqs last disabled at (76): [<ffffffff8184116f>] _raw_spin_lock_irq+0x1f/0x90
 softirqs last  enabled at (0): [<ffffffff810a8df2>] copy_process.part.26+0x602/0x1cf0
 softirqs last disabled at (0): [<          (null)>]           (null)
 other info that might help us debug this:
  Possible unsafe locking scenario:
        CPU0
        ----
   lock(rcu_node_1);
   <Interrupt>
     lock(rcu_node_1);
  *** DEADLOCK ***
 1 lock held by rcu_preempt/8:
  #0:  (rcu_node_1){+.?...}, at: [<ffffffff811387c7>] rcu_gp_kthread+0xb97/0xeb0
 stack backtrace:
 CPU: 0 PID: 8 Comm: rcu_preempt Not tainted 4.2.0-rc5-00025-g9a73ba0 #136
 Hardware name: Dell Inc. PowerEdge R820/066N7P, BIOS 2.0.20 01/16/2014
  0000000000000000 000000006d7e67d8 ffff881fb081fbd8 ffffffff818379e0
  0000000000000000 ffff881fb0812a00 ffff881fb081fc38 ffffffff8110813b
  0000000000000000 0000000000000001 ffff881f00000001 ffffffff8102fa4f
 Call Trace:
  [<ffffffff818379e0>] dump_stack+0x4f/0x7b
  [<ffffffff8110813b>] print_usage_bug+0x1db/0x1e0
  [<ffffffff8102fa4f>] ? save_stack_trace+0x2f/0x50
  [<ffffffff811087ad>] mark_lock+0x66d/0x6e0
  [<ffffffff81107790>] ? check_usage_forwards+0x150/0x150
  [<ffffffff81108898>] mark_held_locks+0x78/0xa0
  [<ffffffff81841330>] ? _raw_spin_unlock_irq+0x30/0x60
  [<ffffffff81108a28>] trace_hardirqs_on_caller+0x168/0x220
  [<ffffffff81108aed>] trace_hardirqs_on+0xd/0x10
  [<ffffffff81841330>] _raw_spin_unlock_irq+0x30/0x60
  [<ffffffff810fd1c7>] swake_up_all+0xb7/0xe0
  [<ffffffff811386e1>] rcu_gp_kthread+0xab1/0xeb0
  [<ffffffff811089bf>] ? trace_hardirqs_on_caller+0xff/0x220
  [<ffffffff81841341>] ? _raw_spin_unlock_irq+0x41/0x60
  [<ffffffff81137c30>] ? rcu_barrier+0x20/0x20
  [<ffffffff810d2014>] kthread+0x104/0x120
  [<ffffffff81841330>] ? _raw_spin_unlock_irq+0x30/0x60
  [<ffffffff810d1f10>] ? kthread_create_on_node+0x260/0x260
  [<ffffffff8184231f>] ret_from_fork+0x3f/0x70
  [<ffffffff810d1f10>] ? kthread_create_on_node+0x260/0x260

Signed-off-by: Daniel Wagner <daniel.wagner@bmw-carit.de>
Acked-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Cc: linux-rt-users@vger.kernel.org
Cc: Boqun Feng <boqun.feng@gmail.com>
Cc: Marcelo Tosatti <mtosatti@redhat.com>
Cc: Steven Rostedt <rostedt@goodmis.org>
Cc: Paul Gortmaker <paul.gortmaker@windriver.com>
Cc: Paolo Bonzini <pbonzini@redhat.com>
Cc: "Paul E. McKenney" <paulmck@linux.vnet.ibm.com>
Link: http://lkml.kernel.org/r/1455871601-27484-5-git-send-email-wagi@monom.org
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
2016-02-25 11:27:16 +01:00
Paul E. McKenney
4b455dc3e1 rcu: Catch up rcu_report_qs_rdp() comment with reality
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
2016-02-23 19:59:56 -08:00
Boqun Feng
67c583a7de RCU: Privatize rcu_node::lock
In patch:

"rcu: Add transitivity to remaining rcu_node ->lock acquisitions"

All locking operations on rcu_node::lock are replaced with the wrappers
because of the need of transitivity, which indicates we should never
write code using LOCK primitives alone(i.e. without a proper barrier
following) on rcu_node::lock outside those wrappers. We could detect
this kind of misuses on rcu_node::lock in the future by adding __private
modifier on rcu_node::lock.

To privatize rcu_node::lock, unlock wrappers are also needed. Replacing
spinlock unlocks with these wrappers not only privatizes rcu_node::lock
but also makes it easier to figure out critical sections of rcu_node.

This patch adds __private modifier to rcu_node::lock and makes every
access to it wrapped by ACCESS_PRIVATE(). Besides, unlock wrappers are
added and raw_spin_unlock(&rnp->lock) and its friends are replaced with
those wrappers.

Signed-off-by: Boqun Feng <boqun.feng@gmail.com>
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
2016-02-23 19:59:54 -08:00
Chen Gang
1914aab543 rcu: Remove useless rcu_data_p when !PREEMPT_RCU
The related warning from gcc 6.0:

  In file included from kernel/rcu/tree.c:4630:0:
  kernel/rcu/tree_plugin.h:810:40: warning: ‘rcu_data_p’ defined but not used [-Wunused-const-variable]
   static struct rcu_data __percpu *const rcu_data_p = &rcu_sched_data;
                                          ^~~~~~~~~~

Also remove always redundant rcu_data_p in tree.c.

Signed-off-by: Chen Gang <gang.chen.5i5j@gmail.com>
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
2016-02-23 19:59:53 -08:00
Paul E. McKenney
23a9bacd35 rcu: Set rdp->gpwrap when CPU is idle
Commit #e3663b1024d1 ("rcu: Handle gpnum/completed wrap while dyntick
idle") sets rdp->gpwrap on the wrong side of the "if" statement in
dyntick_save_progress_counter(), that is, it sets it when the CPU is
not idle instead of when it is idle.  Of course, if the CPU is not idle,
its rdp->gpnum won't be lagging beind the global rsp->gpnum, which means
that rdp->gpwrap will never be set.

This commit therefore moves this code to the proper leg of that "if"
statement.  This change means that the "else" cause is just "return 0"
and the "then" clause ends with "return 1", so also move the "return 0"
to follow the "if", dropping the "else" clause.

Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
2016-02-23 19:59:52 -08:00
Paul E. McKenney
4914950aaa rcu: Stop treating in-kernel CPU-bound workloads as errors
Commit 4a81e8328d ("Reduce overhead of cond_resched() checks for RCU")
handles the error case where a nohz_full loops indefinitely in the kernel
with the scheduling-clock interrupt disabled.  However, this handling
includes IPIing the CPU running the offending loop, which is not what
we want for real-time workloads.  And there are starting to be real-time
CPU-bound in-kernel workloads, and these must be handled without IPIing
the CPU, at least not in the common case.  Therefore, this situation can
no longer be dismissed as an error case.

This commit therefore splits the handling out, so that the setting of
bits in the per-CPU rcu_sched_qs_mask variable is done relatively early,
but if the problem persists, resched_cpu() is eventually used to IPI the
CPU containing the offending loop.  Assuming that in-kernel CPU-bound
loops used by real-time tasks contain frequent calls cond_resched_rcu_qs()
(as in more than once per few tens of milliseconds), the real-time tasks
will never be IPIed.

Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Cc: Steven Rostedt <rostedt@goodmis.org>
2016-02-23 19:59:51 -08:00
Paul E. McKenney
8994515cf0 rcu: Update rcu_report_qs_rsp() comment
The header comment for rcu_report_qs_rsp() was obsolete, dating well
before the advent of RCU grace-period kthreads.  This commit therefore
brings this comment back into alignment with current reality.

Reported-by: Lihao Liang <lihao.liang@cs.ox.ac.uk>
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
2016-02-23 19:59:51 -08:00
Paul E. McKenney
bb53e416e0 rcu: Assign false instead of 0 for ->core_needs_qs
A zero seems to have escaped earlier true/false substitution efforts,
so this commit changes 0 to false for the ->core_needs_qs boolean field.

Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
2016-02-23 19:59:51 -08:00
Paul E. McKenney
648c630c64 Merge branches 'doc.2015.12.05a', 'exp.2015.12.07a', 'fixes.2015.12.07a', 'list.2015.12.04b' and 'torture.2015.12.05a' into HEAD
doc.2015.12.05a:  Documentation updates
exp.2015.12.07a:  Expedited grace-period updates
fixes.2015.12.07a:  Miscellaneous fixes
list.2015.12.04b:  Linked-list updates
torture.2015.12.05a:  Torture-test updates
2015-12-07 17:02:54 -08:00
Paul E. McKenney
45fed3e7cf rcu: Make rcu_gp_init() be bool rather than int
The return value from rcu_gp_init() is always used as a bool, so
this commit makes it be a bool.

Reported-by: Iftekhar Ahmed <ahmedi@oregonstate.edu>
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
2015-12-07 17:01:33 -08:00
Peter Zijlstra
e11f13355b rcu: Move wakeup out from under rnp->lock
This patch removes a potential deadlock hazard by moving the
wake_up_process() in rcu_spawn_gp_kthread() out from under rnp->lock.

Signed-off-by: Peter Zijlstra <peterz@infradead.org>
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
2015-12-07 17:01:32 -08:00
Paul E. McKenney
7c9906ca5e rcu: Don't redundantly disable irqs in rcu_irq_{enter,exit}()
This commit replaces a local_irq_save()/local_irq_restore() pair with
a lockdep assertion that interrupts are already disabled.  This should
remove the corresponding overhead from the interrupt entry/exit fastpaths.

This change was inspired by the fact that Iftekhar Ahmed's mutation
testing showed that removing rcu_irq_enter()'s call to local_ird_restore()
had no effect, which might indicate that interrupts were always enabled
anyway.

Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
2015-12-07 17:01:31 -08:00
Paul E. McKenney
d117c8aa1d rcu: Make cpu_needs_another_gp() be bool
The cpu_needs_another_gp() function is currently of type int, but only
returns zero or one.  Bow to reality and make it be of type bool.

Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
2015-12-07 17:01:31 -08:00
Paul E. McKenney
a87f203e27 rcu: Eliminate unused rcu_init_one() argument
Now that the rcu_state structure's ->rda field is compile-time initialized,
there is no need to pass the per-CPU rcu_data structure into rcu_init_one().
This commit therefore eliminates this now-unused parameter.

Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
2015-12-07 17:01:19 -08:00
Paul E. McKenney
6b50e119c4 rcutorture: Print symbolic name for ->gp_state
Currently, ->gp_state is printed as an integer, which slows debugging.
This commit therefore prints a symbolic name in addition to the integer.

Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
[ paulmck: Updated to fix relational operator called out by Dan Carpenter. ]
[ paulmck: More "const", as suggested by Josh Triplett. ]
Reviewed-by: Josh Triplett <josh@joshtriplett.org>
2015-12-05 17:58:26 -08:00
Paul E. McKenney
b1adb3e273 rcutorture: Dump stack when GP kthread stalls
This commit increases debug information in the case where the grace-period
kthread is being prevented from running by dumping that kthread's stack.

Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
[ paulmck: Split into prior commit and this commit, as suggested by
  Josh Triplett. ]
Reviewed-by: Josh Triplett <josh@joshtriplett.org>
2015-12-05 17:58:05 -08:00
Paul E. McKenney
a0e3a3aa28 rcutorture: Flag nonexistent RCU GP kthread
Currently, if the RCU grace-period kthread has not yet been created,
in which case the starvation-check code will print zero for the state,
which maps to TASK_RUNNING.  This could clearly be quite confusing, so
this commit prints ~0, which does not map to any legal ->state value.

Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Reviewed-by: Josh Triplett <josh@joshtriplett.org>
2015-12-05 17:58:00 -08:00
Paul E. McKenney
46a5d164db rcu: Stop disabling interrupts in scheduler fastpaths
We need the scheduler's fastpaths to be, well, fast, and unnecessarily
disabling and re-enabling interrupts is not necessarily consistent with
this goal.  Especially given that there are regions of the scheduler that
already have interrupts disabled.

This commit therefore moves the call to rcu_note_context_switch()
to one of the interrupts-disabled regions of the scheduler, and
removes the now-redundant disabling and re-enabling of interrupts from
rcu_note_context_switch() and the functions it calls.

Reported-by: Peter Zijlstra <peterz@infradead.org>
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
[ paulmck: Shift rcu_note_context_switch() to avoid deadlock, as suggested
  by Peter Zijlstra. ]
2015-12-04 12:27:31 -08:00
Paul E. McKenney
fecbf6f01f rcu: Simplify rcu_sched_qs() control flow
This commit applies an early-exit approach to rcu_sched_qs(), reducing
the nesting level and saving a line of code.

Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
2015-12-04 12:27:29 -08:00
Paul E. McKenney
3dc5dbe9a1 rcu: Move lock_class_key to local scope
Currently, the rcu_node_class[], rcu_fqs_class[], and rcu_exp_class[]
arrays needlessly pollute the global namespace within tree.c.  This
commit therefore converts them to static local variables within
rcu_init_one().

Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
2015-12-04 12:27:29 -08:00
Paul E. McKenney
5a9be7c628 rcu: Add rcu_normal kernel parameter to suppress expediting
Although expedited grace periods can be quite useful, and although their
OS jitter has been greatly reduced, they can still pose problems for
extreme real-time workloads.  This commit therefore adds a rcu_normal
kernel boot parameter (which can also be manipulated via sysfs)
to suppress expedited grace periods, that is, to treat requests for
expedited grace periods as if they were requests for normal grace periods.
If both rcu_expedited and rcu_normal are specified, rcu_normal wins.
This means that if you are relying on expedited grace periods to speed up
boot, you will want to specify rcu_expedited on the kernel command line,
and then specify rcu_normal via sysfs once boot completes.

Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
2015-12-04 12:26:53 -08:00
Paul E. McKenney
72611ab9f5 rcu: Add more diagnostics to expedited stall warning messages.
This commit adds print statements that check the rcu_node structure to
find which ->expmask bits and which ->exp_tasks structures are blocking
the current expedited grace period.

Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
2015-12-04 12:26:53 -08:00
Paul E. McKenney
73f36f9de8 rcu: Make expedited grace periods resolve stall-warning ties
Currently, if a grace period ends just as the stall-warning timeout
fires, an empty stall warning will be printed.  This is not helpful,
so this commit avoids these useless warnings by rechecking completion
after awakening in synchronize_sched_expedited_wait().

Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
2015-12-04 12:26:52 -08:00
Paul E. McKenney
df5bd5144a rcu: Reduce expedited GP memory contention via per-CPU variables
Currently, the piggybacked-work checks carried out by sync_exp_work_done()
atomically increment a small set of variables (the ->expedited_workdone0,
->expedited_workdone1, ->expedited_workdone2, ->expedited_workdone3
fields in the rcu_state structure), which will form a memory-contention
bottleneck given a sufficiently large number of CPUs concurrently invoking
either synchronize_rcu_expedited() or synchronize_sched_expedited().

This commit therefore moves these for fields to the per-CPU rcu_data
structure, eliminating the memory contention.  The show_rcuexp() function
also changes to sum up each field in the rcu_data structures.

Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
2015-12-04 12:26:52 -08:00
Paul E. McKenney
1307f21487 rcu: Invert sync_rcu_exp_select_cpus() "if" statement
This commit saves a couple lines of code and reduces indentation
by inverting the sense of an "if" statement in the function
sync_rcu_exp_select_cpus().

Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
2015-12-04 12:26:51 -08:00
Paul E. McKenney
886ef5a18a rcu: Move smp_mb() from rcu_seq_snap() to rcu_exp_gp_seq_snap()
The memory barrier in rcu_seq_snap() is needed only for grace periods,
so this commit moves it to the grace-period-oriented wrapper
rcu_exp_gp_seq_snap().

Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
2015-12-04 12:26:51 -08:00
Paul E. McKenney
06f60de19d rcu: Short-circuit synchronize_sched_expedited() if only one CPU
If there is only one CPU, then invoking synchronize_sched_expedited()
is by definition a grace period.  This commit checks for this condition
and does a short-circuit return in that case.

Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
2015-12-04 12:26:50 -08:00
Paul E. McKenney
6cf1008122 rcu: Add transitivity to remaining rcu_node ->lock acquisitions
The rule is that all acquisitions of the rcu_node structure's ->lock
must provide transitivity:  The lock is not acquired that frequently,
and sorting out exactly which required it and which did not would be
a maintenance nightmare.  This commit therefore supplies the needed
transitivity to the remaining ->lock acquisitions.

Reported-by: Peter Zijlstra <peterz@infradead.org>
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
2015-11-23 10:37:35 -08:00
Peter Zijlstra
2a67e741bb rcu: Create transitive rnp->lock acquisition functions
Providing RCU's memory-ordering guarantees requires that the rcu_node
tree's locking provide transitive memory ordering, which the Linux kernel's
spinlocks currently do not provide unless smp_mb__after_unlock_lock()
is used.  Having a separate smp_mb__after_unlock_lock() after each and
every lock acquisition is error-prone, hard to read, and a bit annoying,
so this commit provides wrapper functions that pull in the
smp_mb__after_unlock_lock() invocations.

Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
2015-11-23 10:37:35 -08:00
Paul E. McKenney
d2856b046d Merge branches 'fixes.2015.10.06a' and 'exp.2015.10.07a' into HEAD
exp.2015.10.07a:  Reduce OS jitter of RCU-sched expedited grace periods.
fixes.2015.10.06a:  Miscellaneous fixes.
2015-10-07 16:05:21 -07:00
Paul E. McKenney
338b0f760e rcu: Better hotplug handling for synchronize_sched_expedited()
Earlier versions of synchronize_sched_expedited() can prematurely end
grace periods due to the fact that a CPU marked as cpu_is_offline()
can still be using RCU read-side critical sections during the time that
CPU makes its last pass through the scheduler and into the idle loop
and during the time that a given CPU is in the process of coming online.
This commit therefore eliminates this window by adding additional
interaction with the CPU-hotplug operations.

Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
2015-10-07 16:02:50 -07:00
Paul E. McKenney
c58656382e rcu: Add tasks to expedited stall-warning messages
This commit adds task-print ability to the expedited RCU CPU stall
warning messages in preparation for adding stall warnings to
synchornize_rcu_expedited().

Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
2015-10-07 16:02:50 -07:00
Paul E. McKenney
74611ecb0f rcu: Add online/offline info to expedited stall warning message
This commit makes the RCU CPU stall warning message print online/offline
indications immediately after the CPU number.  A "O" indicates global
offline, a "." global online, and a "o" indicates RCU believes that the
CPU is offline for the current grace period and "." otherwise, and an
"N" indicates that RCU believes that the CPU will be offline for the
next grace period, and "." otherwise, all right after the CPU number.
So for CPU 10, you would normally see "10-...:" indicating that everything
believes that the CPU is online.

Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
2015-10-07 16:02:50 -07:00
Paul E. McKenney
dcdb8807ba rcu: Consolidate expedited CPU selection
Now that sync_sched_exp_select_cpus() and sync_rcu_exp_select_cpus()
are identical aside from the the argument to smp_call_function_single(),
this commit consolidates them with a functional argument.

Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
2015-10-07 16:02:50 -07:00
Paul E. McKenney
66fe6cbee4 rcu: Prepare for consolidating expedited CPU selection
This commit brings sync_sched_exp_select_cpus() into alignment with
sync_rcu_exp_select_cpus(), as a first step towards consolidating them
into one function.

Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
2015-10-07 16:02:50 -07:00
Paul E. McKenney
807226e2fb rcu: Stop excluding CPU hotplug in synchronize_sched_expedited()
Now that synchronize_sched_expedited() uses IPIs, a hook in
rcu_sched_qs(), and the ->expmask field in the rcu_node combining
tree, it is no longer necessary to exclude CPU hotplug.  Any
races with CPU hotplug will be detected when attempting to send
the IPI.  This commit therefore removes the code excluding
CPU hotplug operations.

Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
2015-10-07 16:02:49 -07:00
Paul E. McKenney
83c2c735e7 rcu: Stop silencing lockdep false positive for expedited grace periods
This reverts commit af859beaab (rcu: Silence lockdep false positive
for expedited grace periods).  Because synchronize_rcu_expedited()
no longer invokes synchronize_sched_expedited(), ->exp_funnel_mutex
acquisition is no longer nested, so the false positive no longer happens.
This commit therefore removes the extra lockdep data structures, as they
are no longer needed.
2015-10-07 16:02:49 -07:00
Paul E. McKenney
6587a23b6b rcu: Switch synchronize_sched_expedited() to IPI
This commit switches synchronize_sched_expedited() from stop_one_cpu_nowait()
to smp_call_function_single(), thus moving from an IPI and a pair of
context switches to an IPI and a single pass through the scheduler.
Of course, if the scheduler actually does decide to switch to a different
task, there will still be a pair of context switches, but there would
likely have been a pair of context switches anyway, just a bit later.

Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
2015-10-07 16:01:12 -07:00
Petr Mladek
77f81fe08e rcu: Finish folding ->fqs_state into ->gp_state
Commit commit 4cdfc175c2 ("rcu: Move quiescent-state forcing
into kthread") started the process of folding the old ->fqs_state into
->gp_state, but did not complete it.  This situation does not cause
any malfunction, but can result in extremely confusing trace output.
This commit completes this task of eliminating ->fqs_state in favor
of ->gp_state.

The old ->fqs_state was also used to decide when to collect dyntick-idle
snapshots.  For this purpose, we add a boolean variable into the kthread,
which is set on the first call to rcu_gp_fqs() for a given grace period
and clear otherwise.

Signed-off-by: Petr Mladek <pmladek@suse.com>
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Reviewed-by: Josh Triplett <josh@joshtriplett.org>
2015-10-06 11:15:59 -07:00
Paul E. McKenney
ee968ac61d rcu: Eliminate panic when silly boot-time fanout specified
This commit loosens rcutree.rcu_fanout_leaf range checks
and replaces a panic() with a fallback to compile-time values.
This fallback is accompanied by a WARN_ON(), and both occur when the
rcutree.rcu_fanout_leaf value is too small to accommodate the number of
CPUs.  For example, given the current four-level limit for the rcu_node
tree, a system with more than 16 CPUs built with CONFIG_FANOUT=2 must
have rcutree.rcu_fanout_leaf larger than 2.

Reported-by: Peter Zijlstra <peterz@infradead.org>
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Reviewed-by: Josh Triplett <josh@joshtriplett.org>
2015-10-06 11:09:41 -07:00
Boqun Feng
bb73c52bad rcu: Don't disable preemption for Tiny and Tree RCU readers
Because preempt_disable() maps to barrier() for non-debug builds,
it forces the compiler to spill and reload registers.  Because Tree
RCU and Tiny RCU now only appear in CONFIG_PREEMPT=n builds, these
barrier() instances generate needless extra code for each instance of
rcu_read_lock() and rcu_read_unlock().  This extra code slows down Tree
RCU and bloats Tiny RCU.

This commit therefore removes the preempt_disable() and preempt_enable()
from the non-preemptible implementations of __rcu_read_lock() and
__rcu_read_unlock(), respectively.  However, for debug purposes,
preempt_disable() and preempt_enable() are still invoked if
CONFIG_PREEMPT_COUNT=y, because this allows detection of sleeping inside
atomic sections in non-preemptible kernels.

However, Tiny and Tree RCU operates by coalescing all RCU read-side
critical sections on a given CPU that lie between successive quiescent
states.  It is therefore necessary to compensate for removing barriers
from __rcu_read_lock() and __rcu_read_unlock() by adding them to a
couple of the RCU functions invoked during quiescent states, namely to
rcu_all_qs() and rcu_note_context_switch().  However, note that the latter
is more paranoia than necessity, at least until link-time optimizations
become more aggressive.

This is based on an earlier patch by Paul E. McKenney, fixing
a bug encountered in kernels built with CONFIG_PREEMPT=n and
CONFIG_PREEMPT_COUNT=y.

Signed-off-by: Boqun Feng <boqun.feng@gmail.com>
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
2015-10-06 11:08:23 -07:00
Boqun Feng
b6a4ae766e rcu: Use rcu_callback_t in call_rcu*() and friends
As we now have rcu_callback_t typedefs as the type of rcu callbacks, we
should use it in call_rcu*() and friends as the type of parameters. This
could save us a few lines of code and make it clear which function
requires an rcu callbacks rather than other callbacks as its argument.

Besides, this can also help cscope to generate a better database for
code reading.

Signed-off-by: Boqun Feng <boqun.feng@gmail.com>
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Reviewed-by: Josh Triplett <josh@joshtriplett.org>
2015-10-06 11:08:05 -07:00
Paul E. McKenney
5b74c45890 rcu: Make ->cpu_no_qs be a union for aggregate OR
This commit converts the rcu_data structure's ->cpu_no_qs field
to a union.  The bytewise side of this union allows individual access
to indications as to whether this CPU needs to find a quiescent state
for a normal (.norm) and/or expedited (.exp) grace period.  The setwise
side of the union allows testing whether or not a quiescent state is
needed at all, for either type of grace period.

For now, only .norm is used.  A later commit will introduce the expedited
usage.

Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
2015-09-20 21:16:21 -07:00
Paul E. McKenney
0d43eb34f9 rcu: Invert passed_quiesce and rename to cpu_no_qs
This commit inverts the sense of the rcu_data structure's ->passed_quiesce
field and renames it to ->cpu_no_qs.  This will allow a later commit to
use an "aggregate OR" operation to test expedited as well as normal grace
periods without added overhead.

Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
2015-09-20 21:16:21 -07:00
Paul E. McKenney
97c668b8e9 rcu: Rename qs_pending to core_needs_qs
An upcoming commit needs to invert the sense of the ->passed_quiesce
rcu_data structure field, so this commit is taking this opportunity
to clarify things a bit by renaming ->qs_pending to ->core_needs_qs.

So if !rdp->core_needs_qs, then this CPU need not concern itself with
quiescent states, in particular, it need not acquire its leaf rcu_node
structure's ->lock to check.  Otherwise, it needs to report the next
quiescent state.

Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
2015-09-20 21:16:20 -07:00
Paul E. McKenney
bce5fa12aa rcu: Move synchronize_sched_expedited() to combining tree
Currently, synchronize_sched_expedited() uses a single global counter
to track the number of remaining context switches that the current
expedited grace period must wait on.  This is problematic on large
systems, where the resulting memory contention can be pathological.
This commit therefore makes synchronize_sched_expedited() instead use
the combining tree in the same manner as synchronize_rcu_expedited(),
keeping memory contention down to a dull roar.

This commit creates a temporary function sync_sched_exp_select_cpus()
that is very similar to sync_rcu_exp_select_cpus().  A later commit
will consolidate these two functions, which becomes possible when
synchronize_sched_expedited() switches from stop_one_cpu_nowait() to
smp_call_function_single().

Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
2015-09-20 21:16:20 -07:00
Paul E. McKenney
8203d6d0ee rcu: Use single-stage IPI algorithm for RCU expedited grace period
The current preemptible-RCU expedited grace-period algorithm invokes
synchronize_sched_expedited() to enqueue all tasks currently running
in a preemptible-RCU read-side critical section, then waits for all the
->blkd_tasks lists to drain.  This works, but results in both an IPI and
a double context switch even on CPUs that do not happen to be running
in a preemptible RCU read-side critical section.

This commit implements a new algorithm that causes less OS jitter.
This new algorithm IPIs all online CPUs that are not idle (from an
RCU perspective), but refrains from self-IPIs.  If a CPU receiving
this IPI is not in a preemptible RCU read-side critical section (or
is just now exiting one), it pushes quiescence up the rcu_node tree,
otherwise, it sets a flag that will be handled by the upcoming outermost
rcu_read_unlock(), which will then push quiescence up the tree.

The expedited grace period must of course wait on any pre-existing blocked
readers, and newly blocked readers must be queued carefully based on
the state of both the normal and the expedited grace periods.  This
new queueing approach also avoids the need to update boost state,
courtesy of the fact that blocked tasks are no longer ever migrated to
the root rcu_node structure.

Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
2015-09-20 21:16:19 -07:00
Paul E. McKenney
b9585e940a rcu: Consolidate tree setup for synchronize_rcu_expedited()
This commit replaces sync_rcu_preempt_exp_init1(() and
sync_rcu_preempt_exp_init2() with sync_exp_reset_tree_hotplug()
and sync_exp_reset_tree(), which will also be used by
synchronize_sched_expedited(), and sync_rcu_exp_select_nodes(), which
contains code specific to synchronize_rcu_expedited().

Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
2015-09-20 21:16:18 -07:00
Paul E. McKenney
7922cd0e56 rcu: Move rcu_report_exp_rnp() to allow consolidation
This is a nearly pure code-movement commit, moving rcu_report_exp_rnp(),
sync_rcu_preempt_exp_done(), and rcu_preempted_readers_exp() so
that later commits can make synchronize_sched_expedited() use them.
The non-code-movement portion of this commit tags rcu_report_exp_rnp()
as __maybe_unused to avoid build errors when CONFIG_PREEMPT=n.

Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
2015-09-20 21:16:18 -07:00
Paul E. McKenney
f4ecea309d rcu: Use rsp->expedited_wq instead of sync_rcu_preempt_exp_wq
Now that there is an ->expedited_wq waitqueue in each rcu_state structure,
there is no need for the sync_rcu_preempt_exp_wq global variable.  This
commit therefore substitutes ->expedited_wq for sync_rcu_preempt_exp_wq.
It also initializes ->expedited_wq only once at boot instead of at the
start of each expedited grace period.

Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
2015-09-20 21:16:17 -07:00
Paul E. McKenney
19a5ecde08 rcu: Suppress lockdep false positive for rcp->exp_funnel_mutex
In kernels built with CONFIG_PREEMPT=y, synchronize_rcu_expedited()
invokes synchronize_sched_expedited() while holding RCU-preempt's
root rcu_node structure's ->exp_funnel_mutex, which is acquired after
the rcu_data structure's ->exp_funnel_mutex.  The first thing that
synchronize_sched_expedited() will do is acquire RCU-sched's rcu_data
structure's ->exp_funnel_mutex.   There is no danger of an actual deadlock
because the locking order is always from RCU-preempt's expedited mutexes
to those of RCU-sched.  Unfortunately, lockdep considers both rcu_data
structures' ->exp_funnel_mutex to be in the same lock class and therefore
reports a deadlock cycle.

This commit silences this false positive by placing RCU-sched's rcu_data
structures' ->exp_funnel_mutex locks into their own lock class.

Reported-by: Sasha Levin <sasha.levin@oracle.com>
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
2015-09-20 21:01:22 -07:00
Paul E. McKenney
8ff4fbfd69 Merge branches 'fixes.2015.07.22a' and 'initexp.2015.08.04a' into HEAD
fixes.2015.07.22a: Miscellaneous fixes.
initexp.2015.08.04a: Initialization and expedited updates.
	(Single branch due to conflicts.)
2015-08-04 08:40:58 -07:00
Paul E. McKenney
af859beaab rcu: Silence lockdep false positive for expedited grace periods
In a CONFIG_PREEMPT=y kernel, synchronize_rcu_expedited()
acquires the ->exp_funnel_mutex in rcu_preempt_state, then invokes
synchronize_sched_expedited, which acquires the ->exp_funnel_mutex in
rcu_sched_state.  There can be no deadlock because rcu_preempt_state
->exp_funnel_mutex acquisition always precedes that of rcu_sched_state.
But lockdep does not know that, so it gives false-positive splats.

This commit therefore associates a separate lock_class_key structure
with the rcu_sched_state structure's ->exp_funnel_mutex, allowing
lockdep to see the lock ordering, avoiding the false positives.

Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
2015-08-04 08:39:21 -07:00
Paul E. McKenney
f78f5b90c4 rcu: Rename rcu_lockdep_assert() to RCU_LOCKDEP_WARN()
This commit renames rcu_lockdep_assert() to RCU_LOCKDEP_WARN() for
consistency with the WARN() series of macros.  This also requires
inverting the sense of the conditional, which this commit also does.

Reported-by: Ingo Molnar <mingo@kernel.org>
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Reviewed-by: Ingo Molnar <mingo@kernel.org>
2015-07-22 15:27:32 -07:00
Alexei Starovoitov
46f00d18fc rcu: Make rcu_is_watching() really notrace
Although rcu_is_watching() is marked notrace, it invokes preempt_disable()
and preempt_enable(), both of which can be traced.  This defeats the
purpose of the notrace on rcu_is_watching(), so this commit substitutes
preempt_disable_notrace() and preempt_enable_notrace().

Signed-off-by: Alexei Starovoitov <ast@plumgrid.com>
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Acked-by: Steven Rostedt <rostedt@goodmis.org>
2015-07-22 15:27:31 -07:00
Paul E. McKenney
24560056de rcu: Add RCU-sched flavors of get-state and cond-sync
The get_state_synchronize_rcu() and cond_synchronize_rcu() functions
allow polling for grace-period completion, with an actual wait for a
grace period occurring only when cond_synchronize_rcu() is called too
soon after the corresponding get_state_synchronize_rcu().  However,
these functions work only for vanilla RCU.  This commit adds the
get_state_synchronize_sched() and cond_synchronize_sched(), which provide
the same capability for RCU-sched.

Reported-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
2015-07-22 15:26:58 -07:00
Paul E. McKenney
cdacbe1f91 rcu: Add fastpath bypassing funnel locking
In the common case, there will be only one expedited grace period in
the system at a given time, in which case it is not helpful to use
funnel locking.  This commit therefore adds a fastpath that bypasses
funnel locking when the root ->exp_funnel_mutex is not held.

Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
2015-07-17 14:59:06 -07:00
Paul E. McKenney
32bb1c7999 rcu: Rename RCU_GP_DONE_FQS to RCU_GP_DOING_FQS
The grace-period kthread sleeps waiting to do a force-quiescent-state
scan, and when awakened sets rsp->gp_state to RCU_GP_DONE_FQS.
However, this is confusing because the kthread has not done the
force-quiescent-state, but is instead just starting to do it.  This commit
therefore renames RCU_GP_DONE_FQS to RCU_GP_DOING_FQS in order to make
things a bit easier on reviewers.

Reported-by: Peter Zijlstra <peterz@infradead.org>
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
2015-07-17 14:59:05 -07:00
Paul E. McKenney
b9a425cfcb rcu: Pull out wait_event*() condition into helper function
The condition for the wait_event_interruptible_timeout() that waits
to do the next force-quiescent-state scan is a bit ornate:

	((gf = READ_ONCE(rsp->gp_flags)) &
	 RCU_GP_FLAG_FQS) ||
	(!READ_ONCE(rnp->qsmask) &&
	 !rcu_preempt_blocked_readers_cgp(rnp))

This commit therefore pulls this condition out into a helper function
and comments its component conditions.

Reported-by: Peter Zijlstra <peterz@infradead.org>
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
2015-07-17 14:59:04 -07:00
Paul E. McKenney
cf3620a6c7 rcu: Add stall warnings to synchronize_sched_expedited()
Although synchronize_sched_expedited() historically has no RCU CPU stall
warnings, the availability of the rcupdate.rcu_expedited boot parameter
invalidates the old assumption that synchronize_sched()'s stall warnings
would suffice.  This commit therefore adds RCU CPU stall warnings to
synchronize_sched_expedited().

Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
2015-07-17 14:59:01 -07:00
Paul E. McKenney
2cd6ffafec rcu: Extend expedited funnel locking to rcu_data structure
The strictly rcu_node based funnel-locking scheme works well in many
cases, but systems with CONFIG_RCU_FANOUT_LEAF=64 won't necessarily get
all that much concurrency.  This commit therefore extends the funnel
locking into the per-CPU rcu_data structure, providing concurrency equal
to the number of CPUs.

Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
2015-07-17 14:59:00 -07:00
Paul E. McKenney
704dd435ac rcu: Consolidate last open-coded expedited memory barrier
One of the requirements on RCU grace periods is that if there is a
causal chain of operations that starts after one grace period and
ends before another grace period, then the two grace periods must
be serialized.  There has been (and might still be) code that relies
on this, for example, certain types of reference-counting code that
does a call_rcu() within an RCU callback function.

This requirement is why there is an smp_mb() at the end of both
synchronize_sched_expedited() and synchronize_rcu_expedited().
However, this is the only smp_mb() in these functions, so it would
be nicer to consolidate it into rcu_exp_gp_seq_end().  This commit
does just that.

Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
2015-07-17 14:58:59 -07:00
Paul E. McKenney
4f525a528b rcu: Apply rcu_seq operations to _rcu_barrier()
The rcu_seq operations were open-coded in _rcu_barrier(), so this commit
replaces the open-coding with the shiny new rcu_seq operations.

Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
2015-07-17 14:58:57 -07:00
Paul E. McKenney
29fd930940 rcu: Use funnel locking for synchronize_rcu_expedited()'s polling loop
This commit gets rid of synchronize_rcu_expedited()'s mutex_trylock()
polling loop in favor of the funnel-locking scheme that was abstracted
from synchronize_sched_expedited().

Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
2015-07-17 14:58:56 -07:00
Paul E. McKenney
7fd0ddc5bf rcu: Fix synchronize_sched_expedited() type error for "s"
The type of "s" has been "long" rather than the correct "unsigned long"
for quite some time.  This commit fixes this type error.

Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
2015-07-17 14:58:55 -07:00
Paul E. McKenney
b09e5f8601 rcu: Abstract funnel locking from synchronize_sched_expedited()
This commit abstracts funnel locking from synchronize_sched_expedited()
so that it may be used by synchronize_rcu_expedited().

Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
2015-07-17 14:58:53 -07:00
Paul E. McKenney
28f00767e3 rcu: Abstract sequence counting from synchronize_sched_expedited()
This commit creates rcu_exp_gp_seq_start() and rcu_exp_gp_seq_end() to
bracket an expedited grace period, rcu_exp_gp_seq_snap() to snapshot the
sequence counter, and rcu_exp_gp_seq_done() to check to see if a full
expedited grace period has elapsed since the snapshot.  These will be
applied to synchronize_rcu_expedited().  These are defined in terms of
underlying rcu_seq_start(), rcu_seq_end(), rcu_seq_snap(), rcu_seq_done(),
which will be applied to _rcu_barrier().

One reason that this commit doesn't use the seqcount primitives themselves
is that the smp_wmb() in those primitive is insufficient due to the fact
that expedited grace periods do reads as well as writes.  In addition,
the read-side seqcount primitives detect a potentially partial change,
where the expedited primitives instead need a guaranteed full change.

Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
2015-07-17 14:58:51 -07:00
Peter Zijlstra
3a6d7c64d7 rcu: Make expedited GP CPU stoppage asynchronous
Sequentially stopping the CPUs slows down expedited grace periods by
at least a factor of two, based on rcutorture's grace-period-per-second
rate.  This is a conservative measure because rcutorture uses unusually
long RCU read-side critical sections and because rcutorture periodically
quiesces the system in order to test RCU's ability to ramp down to and
up from the idle state.  This commit therefore replaces the stop_one_cpu()
with stop_one_cpu_nowait(), using an atomic-counter scheme to determine
when all CPUs have passed through the stopped state.

Signed-off-by: Peter Zijlstra <peterz@infradead.org>
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
2015-07-17 14:58:50 -07:00
Paul E. McKenney
385b73c06f rcu: Get rid of synchronize_sched_expedited()'s polling loop
This commit gets rid of synchronize_sched_expedited()'s mutex_trylock()
polling loop in favor of a funnel-locking scheme based on the rcu_node
tree.  The work-done check is done at each level of the tree, allowing
high-contention situations to be resolved quickly with reasonable levels
of mutex contention.

Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
2015-07-17 14:58:48 -07:00
Paul E. McKenney
d6ada2cf2f rcu: Rework synchronize_sched_expedited() counter handling
Now that synchronize_sched_expedited() have a mutex, it can use simpler
work-already-done detection scheme.  This commit simplifies this scheme
by using something similar to the sequence-locking counter scheme.
A counter is incremented before and after each grace period, so that
the counter is odd in the midst of the grace period and even otherwise.
So if the counter has advanced to the second even number that is
greater than or equal to the snapshot, the required grace period has
already happened.

Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
2015-07-17 14:58:47 -07:00
Peter Zijlstra
c190c3b16c rcu: Switch synchronize_sched_expedited() to stop_one_cpu()
The synchronize_sched_expedited() currently invokes try_stop_cpus(),
which schedules the stopper kthreads on each online non-idle CPU,
and waits until all those kthreads are running before letting any
of them stop.  This is disastrous for real-time workloads, which
get hit with a preemption that is as long as the longest scheduling
latency on any CPU, including any non-realtime housekeeping CPUs.
This commit therefore switches to using stop_one_cpu() on each CPU
in turn.  This avoids inflicting the worst-case scheduling latency
on the worst-case CPU onto all other CPUs, and also simplifies the
code a little bit.

Follow-up commits will simplify the counter-snapshotting algorithm
and convert a number of the counters that are now protected by the
new ->expedited_mutex to non-atomic.

Signed-off-by: Peter Zijlstra <peterz@infradead.org>
[ paulmck: Kept stop_one_cpu(), dropped disabling of "guardrails". ]
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
2015-07-17 14:58:45 -07:00
Paul E. McKenney
13bd64947f rcu: Reset rcu_fanout_leaf if out of bounds
Currently if the rcu_fanout_leaf boot parameter is out of bounds (that
is, less than RCU_FANOUT_LEAF or greater than the number of bits in an
unsigned long), a warning is issued and execution continues with the
out-of-bounds value.  This can result in all manner of failures, so this
patch resets rcu_fanout_leaf to RCU_FANOUT_LEAF when an out-of-bounds
condition is detected.

Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
2015-07-17 14:58:41 -07:00
Alexander Gordeev
cb00710239 rcu: Limit count of static data to the number of RCU levels
Although a number of RCU levels may be less than the current
maximum of four, some static data associated with each level
are allocated for all four levels. As result, the extra data
never get accessed and just wast memory. This update limits
count of allocated items to the number of used RCU levels.

Cc: "Paul E. McKenney" <paulmck@linux.vnet.ibm.com>
Cc: Steven Rostedt <rostedt@goodmis.org>
Signed-off-by: Alexander Gordeev <agordeev@redhat.com>
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
2015-07-15 14:45:20 -07:00
Alexander Gordeev
199977bff9 rcu: Remove unnecessary fields from rcu_state structure
Members rcu_state::levelcnt[] and rcu_state::levelspread[]
are only used at init. There is no reason to keep them
afterwards.

Cc: "Paul E. McKenney" <paulmck@linux.vnet.ibm.com>
Cc: Steven Rostedt <rostedt@goodmis.org>
Signed-off-by: Alexander Gordeev <agordeev@redhat.com>
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
2015-07-15 14:45:19 -07:00
Alexander Gordeev
05b84aec46 rcu: Limit rcu_capacity[] size to RCU_NUM_LVLS items
Number of items in rcu_capacity[] array is defined by macro
MAX_RCU_LVLS. However, that array is never accessed beyond
RCU_NUM_LVLS index. Therefore, we can limit the array to
RCU_NUM_LVLS items and eliminate MAX_RCU_LVLS. As result,
in most cases the memory is conserved.

Cc: "Paul E. McKenney" <paulmck@linux.vnet.ibm.com>
Cc: Steven Rostedt <rostedt@goodmis.org>
Signed-off-by: Alexander Gordeev <agordeev@redhat.com>
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
2015-07-15 14:45:18 -07:00
Alexander Gordeev
9618138b09 rcu: Simplify rcu_init_geometry() capacity arithmetics
Current code suggests that introducing the extra level to
rcu_capacity[] array makes some of the arithmetic easier.
Well, in fact it appears rather confusing and unnecessary.

Cc: "Paul E. McKenney" <paulmck@linux.vnet.ibm.com>
Cc: Steven Rostedt <rostedt@goodmis.org>
Signed-off-by: Alexander Gordeev <agordeev@redhat.com>
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
2015-07-15 14:45:15 -07:00
Alexander Gordeev
679f9858b1 rcu: Cleanup rcu_init_geometry() code and arithmetics
This update simplifies rcu_init_geometry() code flow
and makes calculation of the total number of rcu_node
structures more easy to read.

The update relies on the fact num_rcu_lvl[] is never
accessed beyond rcu_num_lvls index by the rest of the
code. Therefore, there is no need initialize the whole
num_rcu_lvl[].

Cc: "Paul E. McKenney" <paulmck@linux.vnet.ibm.com>
Cc: Steven Rostedt <rostedt@goodmis.org>
Signed-off-by: Alexander Gordeev <agordeev@redhat.com>
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
2015-07-15 14:45:14 -07:00
Alexander Gordeev
372b0ec24f rcu: Remove superfluous local variable in rcu_init_geometry()
Local variable 'n' mimics 'nr_cpu_ids' while the both are
used within one function. There is no reason for 'n' to
exist whatsoever.

Cc: "Paul E. McKenney" <paulmck@linux.vnet.ibm.com>
Cc: Steven Rostedt <rostedt@goodmis.org>
Signed-off-by: Alexander Gordeev <agordeev@redhat.com>
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
2015-07-15 14:45:13 -07:00
Alexander Gordeev
75cf15a4c0 rcu: Panic if RCU tree can not accommodate all CPUs
Currently a condition when RCU tree is unable to accommodate
the configured number of CPUs is not permitted and causes
a fall back to compile-time values. However, the code has no
means to exceed the RCU tree capacity neither at compile-time
nor in run-time. Therefore, if the condition is met in run-
time then it indicates a serios problem elsewhere and should
be handled with a panic.

Cc: "Paul E. McKenney" <paulmck@linux.vnet.ibm.com>
Cc: Steven Rostedt <rostedt@goodmis.org>
Signed-off-by: Alexander Gordeev <agordeev@redhat.com>
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
2015-07-15 14:45:12 -07:00
Paul E. McKenney
319362c90f rcu: Provide more diagnostics for stalled GP kthread
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
2015-07-15 14:45:10 -07:00
Paul E. McKenney
d1ec4c34c7 rcu: Drop RCU_USER_QS in favor of NO_HZ_FULL
The RCU_USER_QS Kconfig parameter is now just a synonym for NO_HZ_FULL,
so this commit eliminates RCU_USER_QS, replacing all uses with NO_HZ_FULL.

Reported-by: Frederic Weisbecker <fweisbec@gmail.com>
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Acked-by: Frederic Weisbecker <fweisbec@gmail.com>
2015-07-06 13:52:18 -07:00
Linus Torvalds
e382608254 This patch series contains several clean ups and even a new trace clock
"monitonic raw". Also some enhancements to make the ring buffer even
 faster. But the biggest and most noticeable change is the renaming of
 the ftrace* files, structures and variables that have to deal with
 trace events.
 
 Over the years I've had several developers tell me about their confusion
 with what ftrace is compared to events. Technically, "ftrace" is the
 infrastructure to do the function hooks, which include tracing and also
 helps with live kernel patching. But the trace events are a separate
 entity altogether, and the files that affect the trace events should
 not be named "ftrace". These include:
 
   include/trace/ftrace.h	->	include/trace/trace_events.h
   include/linux/ftrace_event.h	->	include/linux/trace_events.h
 
 Also, functions that are specific for trace events have also been renamed:
 
   ftrace_print_*()		->	trace_print_*()
   (un)register_ftrace_event()	->	(un)register_trace_event()
   ftrace_event_name()		->	trace_event_name()
   ftrace_trigger_soft_disabled()->	trace_trigger_soft_disabled()
   ftrace_define_fields_##call() ->	trace_define_fields_##call()
   ftrace_get_offsets_##call()	->	trace_get_offsets_##call()
 
 Structures have been renamed:
 
   ftrace_event_file		->	trace_event_file
   ftrace_event_{call,class}	->	trace_event_{call,class}
   ftrace_event_buffer		->	trace_event_buffer
   ftrace_subsystem_dir		->	trace_subsystem_dir
   ftrace_event_raw_##call	->	trace_event_raw_##call
   ftrace_event_data_offset_##call->	trace_event_data_offset_##call
   ftrace_event_type_funcs_##call ->	trace_event_type_funcs_##call
 
 And a few various variables and flags have also been updated.
 
 This has been sitting in linux-next for some time, and I have not heard
 a single complaint about this rename breaking anything. Mostly because
 these functions, variables and structures are mostly internal to the
 tracing system and are seldom (if ever) used by anything external to that.
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Merge tag 'trace-v4.2' of git://git.kernel.org/pub/scm/linux/kernel/git/rostedt/linux-trace

Pull tracing updates from Steven Rostedt:
 "This patch series contains several clean ups and even a new trace
  clock "monitonic raw".  Also some enhancements to make the ring buffer
  even faster.  But the biggest and most noticeable change is the
  renaming of the ftrace* files, structures and variables that have to
  deal with trace events.

  Over the years I've had several developers tell me about their
  confusion with what ftrace is compared to events.  Technically,
  "ftrace" is the infrastructure to do the function hooks, which include
  tracing and also helps with live kernel patching.  But the trace
  events are a separate entity altogether, and the files that affect the
  trace events should not be named "ftrace".  These include:

    include/trace/ftrace.h         ->    include/trace/trace_events.h
    include/linux/ftrace_event.h   ->    include/linux/trace_events.h

  Also, functions that are specific for trace events have also been renamed:

    ftrace_print_*()               ->    trace_print_*()
    (un)register_ftrace_event()    ->    (un)register_trace_event()
    ftrace_event_name()            ->    trace_event_name()
    ftrace_trigger_soft_disabled() ->    trace_trigger_soft_disabled()
    ftrace_define_fields_##call()  ->    trace_define_fields_##call()
    ftrace_get_offsets_##call()    ->    trace_get_offsets_##call()

  Structures have been renamed:

    ftrace_event_file              ->    trace_event_file
    ftrace_event_{call,class}      ->    trace_event_{call,class}
    ftrace_event_buffer            ->    trace_event_buffer
    ftrace_subsystem_dir           ->    trace_subsystem_dir
    ftrace_event_raw_##call        ->    trace_event_raw_##call
    ftrace_event_data_offset_##call->    trace_event_data_offset_##call
    ftrace_event_type_funcs_##call ->    trace_event_type_funcs_##call

  And a few various variables and flags have also been updated.

  This has been sitting in linux-next for some time, and I have not
  heard a single complaint about this rename breaking anything.  Mostly
  because these functions, variables and structures are mostly internal
  to the tracing system and are seldom (if ever) used by anything
  external to that"

* tag 'trace-v4.2' of git://git.kernel.org/pub/scm/linux/kernel/git/rostedt/linux-trace: (33 commits)
  ring_buffer: Allow to exit the ring buffer benchmark immediately
  ring-buffer-benchmark: Fix the wrong type
  ring-buffer-benchmark: Fix the wrong param in module_param
  ring-buffer: Add enum names for the context levels
  ring-buffer: Remove useless unused tracing_off_permanent()
  ring-buffer: Give NMIs a chance to lock the reader_lock
  ring-buffer: Add trace_recursive checks to ring_buffer_write()
  ring-buffer: Allways do the trace_recursive checks
  ring-buffer: Move recursive check to per_cpu descriptor
  ring-buffer: Add unlikelys to make fast path the default
  tracing: Rename ftrace_get_offsets_##call() to trace_event_get_offsets_##call()
  tracing: Rename ftrace_define_fields_##call() to trace_event_define_fields_##call()
  tracing: Rename ftrace_event_type_funcs_##call to trace_event_type_funcs_##call
  tracing: Rename ftrace_data_offset_##call to trace_event_data_offset_##call
  tracing: Rename ftrace_raw_##call event structures to trace_event_raw_##call
  tracing: Rename ftrace_trigger_soft_disabled() to trace_trigger_soft_disabled()
  tracing: Rename FTRACE_EVENT_FL_* flags to EVENT_FILE_FL_*
  tracing: Rename struct ftrace_subsystem_dir to trace_subsystem_dir
  tracing: Rename ftrace_event_name() to trace_event_name()
  tracing: Rename FTRACE_MAX_EVENT to TRACE_EVENT_TYPE_MAX
  ...
2015-06-26 14:02:43 -07:00
Paul E. McKenney
0868aa2216 Merge branches 'array.2015.05.27a', 'doc.2015.05.27a', 'fixes.2015.05.27a', 'hotplug.2015.05.27a', 'init.2015.05.27a', 'tiny.2015.05.27a' and 'torture.2015.05.27a' into HEAD
array.2015.05.27a:  Remove all uses of RCU-protected array indexes.
doc.2015.05.27a:  Docuemntation updates.
fixes.2015.05.27a:  Miscellaneous fixes.
hotplug.2015.05.27a:  CPU-hotplug updates.
init.2015.05.27a:  Initialization/Kconfig updates.
tiny.2015.05.27a:  Updates to Tiny RCU.
torture.2015.05.27a:  Torture-testing updates.
2015-05-27 13:00:49 -07:00
Paul E. McKenney
1ce46ee597 rcu: Conditionally compile RCU's eqs warnings
This commit applies some warning-omission micro-optimizations to RCU's
various extended-quiescent-state functions, which are on the kernel/user
hotpath for CONFIG_NO_HZ_FULL=y.

Reported-by: Rik van Riel <riel@redhat.com>
Reported by: Mike Galbraith <umgwanakikbuti@gmail.com>
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
2015-05-27 12:59:07 -07:00
Paul E. McKenney
26730f55c2 rcu: Make RCU able to tolerate undefined CONFIG_RCU_KTHREAD_PRIO
This commit updates the initialization of the kthread_prio boot parameter
so that RCU will build even when CONFIG_RCU_KTHREAD_PRIO is undefined.
The kthread_prio boot parameter is set to CONFIG_RCU_KTHREAD_PRIO if
that is defined, otherwise to 1 if CONFIG_RCU_BOOST is defined and
to zero otherwise.  This commit then makes CONFIG_RCU_KTHREAD_PRIO
depend on CONFIG_RCU_EXPERT, so that Kconfig users won't be asked about
CONFIG_RCU_KTHREAD_PRIO unless they want to be.

Reported-by: Linus Torvalds <torvalds@linux-foundation.org>
Reported-by: Ingo Molnar <mingo@kernel.org>
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Reviewed-by: Pranith Kumar <bobby.prani@gmail.com>
2015-05-27 12:59:06 -07:00
Paul E. McKenney
47d631af58 rcu: Make RCU able to tolerate undefined CONFIG_RCU_FANOUT_LEAF
This commit introduces an RCU_FANOUT_LEAF C-preprocessor macro so
that RCU will build even when CONFIG_RCU_FANOUT_LEAF is undefined.
The RCU_FANOUT_LEAF macro is set to the value of CONFIG_RCU_FANOUT_LEAF
when defined, otherwise it is set to 32 for 32-bit systems and 64 for
64-bit systems.  This commit then makes CONFIG_RCU_FANOUT_LEAF depend
on CONFIG_RCU_EXPERT, so that Kconfig users won't be asked about
CONFIG_RCU_FANOUT_LEAF unless they want to be.

Reported-by: Ingo Molnar <mingo@kernel.org>
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Reviewed-by: Pranith Kumar <bobby.prani@gmail.com>
2015-05-27 12:59:05 -07:00
Paul E. McKenney
05c5df31af rcu: Make RCU able to tolerate undefined CONFIG_RCU_FANOUT
This commit introduces an RCU_FANOUT C-preprocessor macro so that RCU will
build even when CONFIG_RCU_FANOUT is undefined.  The RCU_FANOUT macro is
set to the value of CONFIG_RCU_FANOUT when defined, otherwise it is set
to 32 for 32-bit systems and 64 for 64-bit systems.  This commit then
makes CONFIG_RCU_FANOUT depend on CONFIG_RCU_EXPERT, so that Kconfig
users won't be asked about CONFIG_RCU_FANOUT unless they want to be.

Reported-by: Ingo Molnar <mingo@kernel.org>
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Reviewed-by: Pranith Kumar <bobby.prani@gmail.com>
2015-05-27 12:59:05 -07:00
Paul E. McKenney
a3dc2948ce rcu: Enable diagnostic dump of rcu_node combining tree
The purpose of this commit is to make it easier to verify that RCU's
combining tree is set up correctly, which is useful to have when making
changes in how that tree is initialized.

Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Reviewed-by: Pranith Kumar <bobby.prani@gmail.com>
[ paulmck: Fold fix found by Fengguang's 0-day test robot. ]
2015-05-27 12:59:04 -07:00
Paul E. McKenney
7fa270010e rcu: Convert CONFIG_RCU_FANOUT_EXACT to boot parameter
The CONFIG_RCU_FANOUT_EXACT Kconfig parameter is used primarily (and
perhaps only) by rcutorture to verify that RCU works correctly in specific
rcu_node combining-tree configurations.  It therefore does not make
much sense have this as a question to people attempting to configure
their kernels.  So this commit creates an rcutree.rcu_fanout_exact=
boot parameter that rcutorture can use, and eliminates the original
CONFIG_RCU_FANOUT_EXACT Kconfig parameter.

Reported-by: Ingo Molnar <mingo@kernel.org>
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Reviewed-by: Pranith Kumar <bobby.prani@gmail.com>
2015-05-27 12:59:04 -07:00
Paul E. McKenney
0f41c0ddad rcu: Provide diagnostic option to slow down grace-period scans
Grace-period scans of the rcu_node combining tree normally
proceed quite quickly, so that it is very difficult to reproduce
races against them.  This commit therefore allows grace-period
pre-initialization and cleanup to be artificially slowed down,
increasing race-reproduction probability.  A pair of pairs of new
Kconfig parameters are provided, RCU_TORTURE_TEST_SLOW_PREINIT to
enable the slowing down of propagating CPU-hotplug changes up the
combining tree along with RCU_TORTURE_TEST_SLOW_PREINIT_DELAY to
specify the delay in jiffies, and RCU_TORTURE_TEST_SLOW_CLEANUP
to enable the slowing down of the end-of-grace-period cleanup scan
along with RCU_TORTURE_TEST_SLOW_CLEANUP_DELAY to specify the delay
in jiffies.  Boot-time parameters named rcutree.gp_preinit_delay and
rcutree.gp_cleanup_delay allow these delays to be specified at boot time.

Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
2015-05-27 12:59:02 -07:00
Paul E. McKenney
3eaaaf6cd6 rcu: Shut up spurious gcc uninitialized-variable warning
Because gcc doesn't realize that rcu_num_lvls must be strictly greater
than zero, some versions give a spurious warning about levelcnt[0] being
uninitialized in rcu_init_one().  This commit updates the condition on
the pre-existing panic() in order to educate gcc on this point.

Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
2015-05-27 12:59:02 -07:00
Paul E. McKenney
eab128e830 rcu: Modulate grace-period slow init to normalize delay
Currently, the larger the gp_init_delay boot parameter, the slower
rcutorture will sequence through grace periods.  This commit avoids this
issue by decreasing the probability of slowing initialization of a given
grace period as the degree of slowness increases.

Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
2015-05-27 12:59:01 -07:00
Paul E. McKenney
a738eec6c6 rcu: Correctly initialize ->rcu_qs_ctr_snap at online time
The rcu_data structure's ->rcu_qs_ctr_snap field is initialized at
CPU-online time from the current CPU's element of the per-CPU rcu_qs_ctr
variable.  Unfortunately, this is at CPU_UP_PREPARE time, so has nothing
to do with the CPU being onlined.  This commit therefore initializes
this variable from the incoming CPU's element of rcu_qs_ctr.

Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
2015-05-27 12:58:38 -07:00
Paul E. McKenney
cce7f1fc01 rcu: Remove redundant offline check
Because offline CPUs are propagated up the rcu_node tree's ->qsmaskinit
bits just before each grace period starts, the ->qsmaskinit bit cannot
be clear when the corresponding ->qsmask bit is set.  Furthermore, this
condition used to correspond to a CPU that was on its way offline, and
making RCU's notion of an offline CPU more precise has eliminated this
situation.  This commit therefore removes the now-redundant offline
check from force_qs_rnp().

Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
2015-05-27 12:58:38 -07:00
Paul E. McKenney
c5b5539506 rcu: Remove dead code from force_qs_rnp()
Because force_qs_rnp() is invoked only from the force-quiescent-state
code which runs only in the context of the grace-period kthread, a grace
period must always be in progress throughout force_qs_rnp()'s execution.
This commit therefore removes the rcu_gp_in_progress() check and the
associated dead code.

Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
2015-05-27 12:58:37 -07:00
Paul E. McKenney
ea46351cea rcu: Eliminate HOTPLUG_CPU #ifdef in favor of IS_ENABLED()
This commit removes a HOTPLUG_CPU #ifdef, replacing it with
IS_ENABLED()-protected return statements.  This relies on the
optimizer to remove any resulting dead code.

Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
2015-05-27 12:58:37 -07:00
Nicholas Mc Guire
82072c4fcf rcu: Change function declaration to bool
rcu_cpu_has_callbacks() is declared int. The current declaration was introduced
in commit c0f4dfd4f9 (rcu: Make RCU_FAST_NO_HZ take advantage of numbered
callbacks). But it is actually returning bool and as the function description
states " * Return true if the specified CPU has any callback....", this probably
should be a bool as all (3) call-sites currently treat it as bool.

Type-checking coccinelle spatches are being used to locate type mismatches
between function signatures and return values in this case this produced:
./kernel/rcu/tree.c:3538 WARNING: return of wrong type
                    int != bool,

Patch was compile tested with x86_64_defconfig (implies CONFIG_TREE_RCU=y)

Patch is against 4.1-rc3 (localversion-next is -next-20150511) and fixes

Signed-off-by: Nicholas Mc Guire <hofrat@osadl.org>
Reviewed-by: Josh Triplett <josh@joshtriplett.org>
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
2015-05-27 12:58:04 -07:00
Nicolas Iooss
c92fb05795 rcu: Make rcu_*_data variables static
rcu_bh_data, rcu_sched_data and rcu_preempt_data are never used outside
kernel/rcu/tree.c and thus can be made static.

Doing so fixes a section mismatch warning reported by clang when
building LLVMLinux with -Wsection, because these variables were declared
in .data..percpu and defined in .data..percpu..shared_aligned since
commit 11bbb235c2 ("rcu: Use DEFINE_PER_CPU_SHARED_ALIGNED for
rcu_data").

Signed-off-by: Nicolas Iooss <nicolas.iooss_linux@m4x.org>
Reviewed-by: Josh Triplett <josh@joshtriplett.org>
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
2015-05-27 12:58:03 -07:00
Paul E. McKenney
30ff1533b8 rcu: Make synchronize_sched_expedited() call wait_rcu_gp()
Currently, synchronize_sched_expedited() will call synchronize_sched()
if there is danger of counter wrap.  But if configuration says to
always do expedited grace periods, synchronize_sched() will just
call synchronize_sched_expedited() right back again.  In theory,
the old expedited operations will complete, the counters will
get back in synch, and the recursion will end.  But we could
easily run out of stack long before that time.  This commit
therefore makes synchronize_sched_expedited() invoke the underlying
wait_rcu_gp(call_rcu_sched) instead of synchronize_sched(), the same as
all the other calls out from synchronize_sched_expedited().

This bug was introduced by commit 1924bcb025 (Avoid counter wrap in
synchronize_sched_expedited()).

Reported-by: Rik van Riel <riel@redhat.com>
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
2015-05-27 12:58:03 -07:00
Paul E. McKenney
81e701e437 rcu: Add more debug info on "kthread starved" RCU CPU stall warnings
This commit adds grace number and command-flags information to the
"kthread starved" message that is sometimes printed out as part of
RCU CPU stall warnings.  This message is caused by the corresponding
RCU grace-period kthread not having run for at least two seconds, and
this added information can be helpful when debugging.

Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
2015-05-27 12:58:02 -07:00
Paul E. McKenney
cd73ca21cd rcu: Force wakeup of rcu_gp_kthread at grace-period end
The rcu_gp_kthread_wake() refuses to do a wakeup unless at least
one of the ->gp_flags bits are set, which normally will not be the
case when the last quiescent state is reported.  This results in
up to a 3-jiffy delay given default Kconfig settings.  This commit
therefore has rcu_report_qs_rsp() set RCU_GP_FLAG_FQS before invoking
rcu_gp_kthread_wake() in order to force a more immediate wakeup at
grace-period end, thus reducing grace-period latencies.

Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
2015-05-27 12:58:01 -07:00
Paul E. McKenney
2927a689e8 rcu: Create an immutable rcu_data_p pointer to default rcu_data structure
This commit creates an immutable rcu_data_p pointer that references
rcu_preempt_data for TREE_PREEMPT_RCU builds and that references
rcu_sched_data for TREE_RCU builds.  This rcu_data_p pointer will enable
more code to move from #ifdef to IS_ENABLED().

Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
2015-05-27 12:58:00 -07:00
Paul E. McKenney
b28a7c0166 rcu: Tell the compiler that rcu_state_p is immutable
This commit adds a "const" tag to the declarations of rcu_state_p,
which should allow the compiler to generate better code and also to
catch erroneous assignments to this variable.

Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
2015-05-27 12:57:59 -07:00
Paul E. McKenney
7d0ae8086b rcu: Convert ACCESS_ONCE() to READ_ONCE() and WRITE_ONCE()
This commit moves from the old ACCESS_ONCE() API to the new READ_ONCE()
and WRITE_ONCE() APIs.

Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
[ paulmck:  Updated to include kernel/torture.c as suggested by Jason Low. ]
2015-05-27 12:56:15 -07:00
Steven Rostedt (Red Hat)
af658dca22 tracing: Rename ftrace_event.h to trace_events.h
The term "ftrace" is really the infrastructure of the function hooks,
and not the trace events. Rename ftrace_event.h to trace_events.h to
represent the trace_event infrastructure and decouple the term ftrace
from it.

Signed-off-by: Steven Rostedt <rostedt@goodmis.org>
2015-05-13 14:05:12 -04:00
Paul E. McKenney
8d7dc9283f rcu: Control grace-period delays directly from value
In a misguided attempt to avoid an #ifdef, the use of the
gp_init_delay module parameter was conditioned on the corresponding
RCU_TORTURE_TEST_SLOW_INIT Kconfig variable, using IS_ENABLED() at
the point of use in the code.  This meant that the compiler always saw
the delay, which meant that RCU_TORTURE_TEST_SLOW_INIT_DELAY had to be
unconditionally defined.  This in turn caused "make oldconfig" to ask
pointless questions about the value of RCU_TORTURE_TEST_SLOW_INIT_DELAY
in cases where it was not even used.

This commit avoids these pointless questions by defining gp_init_delay
under #ifdef.  In one branch, gp_init_delay is initialized to
RCU_TORTURE_TEST_SLOW_INIT_DELAY and is also a module parameter (thus
allowing boot-time modification), and in the other branch gp_init_delay
is a const variable initialized by default to zero.

This approach also simplifies the code at the delay point by eliminating
the IS_DEFINED().  Because gp_init_delay is constant zero in the no-delay
case intended for production use, the "gp_init_delay > 0" check causes
the delay to become dead code, as desired in this case.  In addition,
this commit replaces magic constant "10" with the preprocessor variable
PER_RCU_NODE_PERIOD, which controls the number of grace periods that
are allowed to elapse at full speed before a delay is inserted.

Reported-by: Linus Torvalds <torvalds@linux-foundation.org> Signed-off-by:
Paul E. McKenney <paulmck@linux.vnet.ibm.com>
2015-04-14 19:33:59 -07:00
Paul E. McKenney
42528795ac Merge branches 'doc.2015.02.26a', 'earlycb.2015.03.03a', 'fixes.2015.03.03a', 'gpexp.2015.02.26a', 'hotplug.2015.03.20a', 'sysidle.2015.02.26b' and 'tiny.2015.02.26a' into HEAD
doc.2015.02.26a:  Documentation changes
earlycb.2015.03.03a:  Permit early-boot RCU callbacks
fixes.2015.03.03a:  Miscellaneous fixes
gpexp.2015.02.26a:  In-kernel expediting of normal grace periods
hotplug.2015.03.20a:  CPU hotplug fixes
sysidle.2015.02.26b:  NO_HZ_FULL_SYSIDLE fixes
tiny.2015.02.26a:  TINY_RCU fixes
2015-03-20 08:31:01 -07:00
Paul E. McKenney
654e953340 rcu: Associate quiescent-state reports with grace period
As noted in earlier commit logs, CPU hotplug operations running
concurrently with grace-period initialization can result in a given
leaf rcu_node structure having all CPUs offline and no blocked readers,
but with this rcu_node structure nevertheless blocking the current
grace period.  Therefore, the quiescent-state forcing code now checks
for this situation and repairs it.

Unfortunately, this checking can result in false positives, for example,
when the last task has just removed itself from this leaf rcu_node
structure, but has not yet started clearing the ->qsmask bits further
up the structure.  This means that the grace-period kthread (which
forces quiescent states) and some other task might be attempting to
concurrently clear these ->qsmask bits.  This is usually not a problem:
One of these tasks will be the first to acquire the upper-level rcu_node
structure's lock and with therefore clear the bit, and the other task,
seeing the bit already cleared, will stop trying to clear bits.

Sadly, this means that the following unusual sequence of events -can-
result in a problem:

1.	The grace-period kthread wins, and clears the ->qsmask bits.

2.	This is the last thing blocking the current grace period, so
	that the grace-period kthread clears ->qsmask bits all the way
	to the root and finds that the root ->qsmask field is now zero.

3.	Another grace period is required, so that the grace period kthread
	initializes it, including setting all the needed qsmask bits.

4.	The leaf rcu_node structure (the one that started this whole
	mess) is blocking this new grace period, either because it
	has at least one online CPU or because there is at least one
	task that had blocked within an RCU read-side critical section
	while running on one of this leaf rcu_node structure's CPUs.
	(And yes, that CPU might well have gone offline before the
	grace period in step (3) above started, which can mean that
	there is a task on the leaf rcu_node structure's ->blkd_tasks
	list, but ->qsmask equal to zero.)

5.	The other kthread didn't get around to trying to clear the upper
	level ->qsmask bits until all the above had happened.  This means
	that it now sees bits set in the upper-level ->qsmask field, so it
	proceeds to clear them.  Too bad that it is doing so on behalf of
	a quiescent state that does not apply to the current grace period!

This sequence of events can result in the new grace period being too
short.  It can also result in the new grace period ending before the
leaf rcu_node structure's ->qsmask bits have been cleared, which will
result in splats during initialization of the next grace period.  In
addition, it can result in tasks blocking the new grace period still
being queued at the start of the next grace period, which will result
in other splats.  Sasha's testing turned up another of these splats,
as did rcutorture testing.  (And yes, rcutorture is being adjusted to
make these splats show up more quickly.  Which probably is having the
undesirable side effect of making other problems show up less quickly.
Can't have everything!)

Reported-by: Sasha Levin <sasha.levin@oracle.com>
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Cc: <stable@vger.kernel.org> # 4.0.x
Tested-by: Sasha Levin <sasha.levin@oracle.com>
2015-03-20 08:28:25 -07:00
Paul E. McKenney
a77da14ce9 rcu: Yet another fix for preemption and CPU hotplug
As noted earlier, the following sequence of events can occur when
running PREEMPT_RCU and HOTPLUG_CPU on a system with a multi-level
rcu_node combining tree:

1.	A group of tasks block on CPUs corresponding to a given leaf
	rcu_node structure while within RCU read-side critical sections.
2.	All CPUs corrsponding to that rcu_node structure go offline.
3.	The next grace period starts, but because there are still tasks
	blocked, the upper-level bits corresponding to this leaf rcu_node
	structure remain set.
4.	All the tasks exit their RCU read-side critical sections and
	remove themselves from the leaf rcu_node structure's list,
	leaving it empty.
5.	But because there now is code to check for this condition at
	force-quiescent-state time, the upper bits are cleared and the
	grace period completes.

However, there is another complication that can occur following step 4 above:

4a.	The grace period starts, and the leaf rcu_node structure's
	gp_tasks pointer is set to NULL because there are no tasks
	blocked on this structure.
4b.	One of the CPUs corresponding to the leaf rcu_node structure
	comes back online.
4b.	An endless stream of tasks are preempted within RCU read-side
	critical sections on this CPU, such that the ->blkd_tasks
	list is always non-empty.

The grace period will never end.

This commit therefore makes the force-quiescent-state processing check only
for absence of tasks blocking the current grace period rather than absence
of tasks altogether.  This will cause a quiescent state to be reported if
the current leaf rcu_node structure is not blocking the current grace period
and its parent thinks that it is, regardless of how RCU managed to get
itself into this state.

Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Cc: <stable@vger.kernel.org> # 4.0.x
Tested-by: Sasha Levin <sasha.levin@oracle.com>
2015-03-20 08:27:33 -07:00
Paul E. McKenney
5c60d25fa1 rcu: Add diagnostics to grace-period cleanup
At grace-period initialization time, RCU checks that all quiescent
states were really reported for the previous grace period.  Now that
grace-period cleanup has been split out of grace-period initialization,
this commit also performs those checks at grace-period cleanup time.

Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
2015-03-12 15:19:38 -07:00
Paul E. McKenney
88428cc5c2 rcu: Handle outgoing CPUs on exit from idle loop
This commit informs RCU of an outgoing CPU just before that CPU invokes
arch_cpu_idle_dead() during its last pass through the idle loop (via a
new CPU_DYING_IDLE notifier value).  This change means that RCU need not
deal with outgoing CPUs passing through the scheduler after informing
RCU that they are no longer online.  Note that removing the CPU from
the rcu_node ->qsmaskinit bit masks is done at CPU_DYING_IDLE time,
and orphaning callbacks is still done at CPU_DEAD time, the reason being
that at CPU_DEAD time we have another CPU that can adopt them.

Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
2015-03-12 15:19:38 -07:00
Paul E. McKenney
c199068913 rcu: Eliminate ->onoff_mutex from rcu_node structure
Because that RCU grace-period initialization need no longer exclude
CPU-hotplug operations, this commit eliminates the ->onoff_mutex and
its uses.

Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
2015-03-12 15:19:37 -07:00
Paul E. McKenney
0aa04b055e rcu: Process offlining and onlining only at grace-period start
Races between CPU hotplug and grace periods can be difficult to resolve,
so the ->onoff_mutex is used to exclude the two events.  Unfortunately,
this means that it is impossible for an outgoing CPU to perform the
last bits of its offlining from its last pass through the idle loop,
because sleeplocks cannot be acquired in that context.

This commit avoids these problems by buffering online and offline events
in a new ->qsmaskinitnext field in the leaf rcu_node structures.  When a
grace period starts, the events accumulated in this mask are applied to
the ->qsmaskinit field, and, if needed, up the rcu_node tree.  The special
case of all CPUs corresponding to a given leaf rcu_node structure being
offline while there are still elements in that structure's ->blkd_tasks
list is handled using a new ->wait_blkd_tasks field.  In this case,
propagating the offline bits up the tree is deferred until the beginning
of the grace period after all of the tasks have exited their RCU read-side
critical sections and removed themselves from the list, at which point
the ->wait_blkd_tasks flag is cleared.  If one of that leaf rcu_node
structure's CPUs comes back online before the list empties, then the
->wait_blkd_tasks flag is simply cleared.

This of course means that RCU's notion of which CPUs are offline can be
out of date.  This is OK because RCU need only wait on CPUs that were
online at the time that the grace period started.  In addition, RCU's
force-quiescent-state actions will handle the case where a CPU goes
offline after the grace period starts.

Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
2015-03-12 15:19:37 -07:00
Paul E. McKenney
cc99a310ca rcu: Move rcu_report_unblock_qs_rnp() to common code
The rcu_report_unblock_qs_rnp() function is invoked when the
last task blocking the current grace period exits its outermost
RCU read-side critical section.  Previously, this was called only
from rcu_read_unlock_special(), and was therefore defined only when
CONFIG_RCU_PREEMPT=y.  However, this function will be invoked even when
CONFIG_RCU_PREEMPT=n once CPU-hotplug operations are processed only at
the beginnings of RCU grace periods.  The reason for this change is that
the last task on a given leaf rcu_node structure's ->blkd_tasks list
might well exit its RCU read-side critical section between the time that
recent CPU-hotplug operations were applied and when the new grace period
was initialized.  This situation could result in RCU waiting forever on
that leaf rcu_node structure, because if all that structure's CPUs were
already offline, there would be no quiescent-state events to drive that
structure's part of the grace period.

This commit therefore moves rcu_report_unblock_qs_rnp() to common code
that is built unconditionally so that the quiescent-state-forcing code
can clean up after this situation, avoiding the grace-period stall.

Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
2015-03-12 15:19:36 -07:00
Paul E. McKenney
999c286347 rcu: Remove event tracing from rcu_cpu_notify(), used by offline CPUs
Offline CPUs cannot safely invoke trace events, but such CPUs do execute
within rcu_cpu_notify().  Therefore, this commit removes the trace events
from rcu_cpu_notify().  These trace events are for utilization, against
which rcu_cpu_notify() execution time should be negligible.

Reported-by: Fengguang Wu <fengguang.wu@intel.com>
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
2015-03-11 13:22:39 -07:00
Paul E. McKenney
37745d2810 rcu: Provide diagnostic option to slow down grace-period initialization
Grace-period initialization normally proceeds quite quickly, so
that it is very difficult to reproduce races against grace-period
initialization.  This commit therefore allows grace-period
initialization to be artificially slowed down, increasing
race-reproduction probability.  A pair of new Kconfig parameters are
provided, CONFIG_RCU_TORTURE_TEST_SLOW_INIT to enable the slowdowns, and
CONFIG_RCU_TORTURE_TEST_SLOW_INIT_DELAY to specify the number of jiffies
of slowdown to apply.  A boot-time parameter named rcutree.gp_init_delay
allows boot-time delay to be specified.  By default, no delay will be
applied even if CONFIG_RCU_TORTURE_TEST_SLOW_INIT is set.

Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
2015-03-11 13:22:38 -07:00
Paul E. McKenney
237a0f2193 rcu: Detect stalls caused by failure to propagate up rcu_node tree
If all CPUs have passed through quiescent states, then stalls might be
due to starvation of the grace-period kthread or to failure to propagate
the quiescent states up the rcu_node combining tree.  The current stall
warning messages do not differentiate, so this commit adds a printout
of the root rcu_node structure's ->qsmask field.

Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
2015-03-11 13:22:38 -07:00
Paul E. McKenney
78043c467a rcu: Put all orphan-callback-related code under same comment
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
2015-03-11 13:22:37 -07:00
Paul E. McKenney
b33078b609 rcu: Consolidate offline-CPU callback initialization
Currently, both rcu_cleanup_dead_cpu() and rcu_send_cbs_to_orphanage()
initialize the outgoing CPU's callback list.  However, only
rcu_cleanup_dead_cpu() invokes rcu_send_cbs_to_orphanage(), and
it does so unconditionally, which means that only one of these
initializations is required.  This commit therefore consolidates the
callback-list initialization with the rest of the callback handling in
rcu_send_cbs_to_orphanage().

Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
2015-03-11 13:22:36 -07:00
Yao Dongdong
9910affa89 rcu: Remove redundant check of cpu_online()
Because invoke_cpu_core() checks whether the current CPU is online,
there is no need for __call_rcu_core() to redundantly check it.
There should not be any performance degradation because the called
function is visible to the compiler.  This commit therefore removes
the redundant check.

Signed-off-by: Yao Dongdong <yaodongdong@huawei.com>
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
2015-03-03 11:17:34 -08:00
Paul E. McKenney
e7580f3388 rcu: Get rcu_sched_force_quiescent_state() where it belongs
The very similar functions rcu_force_quiescent_state(),
rcu_bh_force_quiescent_state(), and rcu_sched_force_quiescent_state()
are supposed to be together, but have drifted apart.  This commit
restores rcu_sched_force_quiescent_state() to its rightful place.

Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
2015-03-03 11:17:19 -08:00
Paul E. McKenney
6629240575 rcu: Use IS_ENABLED() to CONFIG_RCU_FANOUT_EXACT #ifdef
This commit uses IS_ENABLED() to remove the #ifdef from the
rcu_init_levelspread() functions.  No effect on executable code.

Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
2015-03-03 11:14:08 -08:00
Paul E. McKenney
4762767810 rcu: Move early boot callback tests earlier
Because callbacks can now be posted quite early in boot, move the
early boot callback tests to precede RCU initialization.

Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
2015-03-03 11:06:22 -08:00
Paul E. McKenney
34404ca8fb rcu: Move early-boot callbacks to no-CBs lists for no-CBs CPUs
When a CPU is first determined to be a no-CBs CPUs, this commit causes
any early boot callbacks to be moved to the no-CBs callback list,
allowing them to be invoked.

Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
2015-03-03 11:06:02 -08:00
Paul E. McKenney
5871968d53 rcu: Tighten up affinity and check for sysidle
If the RCU grace-period kthread invoking rcu_sysidle_check_cpu()
happens to be running on the tick_do_timer_cpu initially,
then rcu_bind_gp_kthread() won't bind it.  This kthread might
then migrate before invoking rcu_gp_fqs(), which will trigger the
WARN_ON_ONCE() in rcu_sysidle_check_cpu().  This commit therefore makes
rcu_bind_gp_kthread() do the binding even if the kthread is currently
on the same CPU.  Because this incurs added overhead, this commit also
causes each RCU grace-period kthread to invoke rcu_bind_gp_kthread()
once at boot rather than at the beginning of each grace period.
And as long as rcu_bind_gp_kthread() is being modified, this commit
eliminates its #ifdef.

Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
2015-02-26 16:04:37 -08:00
Paul E. McKenney
675da67f24 rcu: Fixes to NO_HZ_FULL sysidle accounting
On second and subsequent passes through quiescent-state forcing, the
isidle variable was initialized to false, which would prevent full sysidle
state from being reached if a grace period needed more than one round
of quiescent-state forcing (which most should not).  However, the check
for offline CPUs in the quiescent-state forcing main loop had the wrong
sense, which could prevent CPUs from ever entering full sysidle state.

This commit fixes both of these bugs.  Given that sysidle is not yet
wired up, this has no effect in old kernels, but might have proven
frustrating had anyone attempted to wire it up.

Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
2015-02-26 12:11:03 -08:00
Paul E. McKenney
5afff48bdf rcu: Update from rcu_expedited variable to rcu_gp_is_expedited()
This commit updates open-coded tests of the rcu_expedited variable
to instead use rcu_gp_is_expedited().

Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
2015-02-26 12:03:01 -08:00
Paul E. McKenney
1925d1967c rcu: Fix a couple of typos in rcu_all_qs() comment header
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
2015-02-26 12:02:10 -08:00
Paul E. McKenney
39c8d313c3 rcu: Avoid clobbering early boot callbacks
When a CPU comes online, it initializes its callback list.  This
is a bad thing if this is the first time that the CPU has come
online and if that CPU has early boot callbacks.  This commit therefore
avoid initializing the callback list if there are callbacks present,
in which case the initial call_rcu() did the initialization for us.

Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
2015-02-26 12:01:30 -08:00
Paul E. McKenney
143da9c2fc rcu: Prevent early-boot RCU callbacks from splatting
Currently, a call_rcu() that precedes rcu_init() will splat due to the
callback lists not having yet been initialized.  This commit causes the
first such callback to initialize the boot CPU's RCU callback list.

Note that this commit does not change rcu_init()-time initialization,
which means that the callback will be discarded at rcu_init() time.
Fixing this is the job of later commits.

Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
2015-02-26 12:01:28 -08:00
Paul E. McKenney
2723249a31 rcu: Wire ->rda pointers at compile time
This commit wires up the rcu_state structures' ->rda pointers to the
per-CPU rcu_data structures at compile time, thus ensuring that this
linkage is present at early boot, in turn allowing posting of callbacks
before rcu_init() is executed.

Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
2015-02-26 12:01:27 -08:00
Paul E. McKenney
d3f3f3f25b rcu: Abstract default callback-list initialization from init_callback_list()
In preparation for early-boot posting of callbacks, this commit abstracts
initialization of the default (non-no-CB) callbacks list from the
init_callback_list() function into a new init_default_callback_list()
function.

Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
2015-02-26 12:01:25 -08:00
Lai Jiangshan
3f47da0f32 rcu_tree: Avoid touching rnp->completed when a new GP is started
In rcu_gp_init(), rnp->completed equals to rsp->completed in THEORY,
we don't need to touch it normally.  If something goes wrong,
it will complain and fixup rnp->completed and avoid oops.
This commit thus avoids the normal needless store to rnp->completed.

Signed-off-by: Lai Jiangshan <laijs@cn.fujitsu.com>
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
2015-02-25 17:03:05 -08:00
Paul E. McKenney
78e691f4ae Merge branches 'doc.2015.01.07a', 'fixes.2015.01.15a', 'preempt.2015.01.06a', 'srcu.2015.01.06a', 'stall.2015.01.16a' and 'torture.2015.01.11a' into HEAD
doc.2015.01.07a: Documentation updates.
fixes.2015.01.15a: Miscellaneous fixes.
preempt.2015.01.06a: Changes to handling of lists of preempted tasks.
srcu.2015.01.06a: SRCU updates.
stall.2015.01.16a: RCU CPU stall-warning updates and fixes.
torture.2015.01.11a: RCU torture-test updates and fixes.
2015-01-15 23:34:34 -08:00
Paul E. McKenney
fb81a44b88 rcu: Add GP-kthread-starvation checks to CPU stall warnings
This commit adds a message that is printed if the relevant grace-period
kthread has not been able to run for the two seconds preceding the
stall warning.  (The two seconds is double the maximum interval between
successive bouts of quiescent-state forcing.)

Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
2015-01-15 23:33:15 -08:00
Paul E. McKenney
5cd37193ce rcu: Make cond_resched_rcu_qs() apply to normal RCU flavors
Although cond_resched_rcu_qs() only applies to TASKS_RCU, it is used
in places where it would be useful for it to apply to the normal RCU
flavors, rcu_preempt, rcu_sched, and rcu_bh.  This is especially the
case for workloads that aggressively overload the system, particularly
those that generate large numbers of RCU updates on systems running
NO_HZ_FULL CPUs.  This commit therefore communicates quiescent states
from cond_resched_rcu_qs() to the normal RCU flavors.

Note that it is unfortunately necessary to leave the old ->passed_quiesce
mechanism in place to allow quiescent states that apply to only one
flavor to be recorded.  (Yes, we could decrement ->rcu_qs_ctr_snap in
that case, but that is not so good for debugging of RCU internals.)
In addition, if one of the RCU flavor's grace period has stalled, this
will invoke rcu_momentary_dyntick_idle(), resulting in a heavy-weight
quiescent state visible from other CPUs.

Reported-by: Sasha Levin <sasha.levin@oracle.com>
Reported-by: Dave Jones <davej@redhat.com>
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
[ paulmck: Merge commit from Sasha Levin fixing a bug where __this_cpu()
  was used in preemptible code. ]
2015-01-15 23:33:14 -08:00
Paul E. McKenney
a94844b22a rcu: Optionally run grace-period kthreads at real-time priority
Recent testing has shown that under heavy load, running RCU's grace-period
kthreads at real-time priority can improve performance (according to 0day
test robot) and reduce the incidence of RCU CPU stall warnings.  However,
most systems do just fine with the default non-realtime priorities for
these kthreads, and it does not make sense to expose the entire user
base to any risk stemming from this change, given that this change is
of use only to a few users running extremely heavy workloads.

Therefore, this commit allows users to specify realtime priorities
for the grace-period kthreads, but leaves them running SCHED_OTHER
by default.  The realtime priority may be specified at build time
via the RCU_KTHREAD_PRIO Kconfig parameter, or at boot time via the
rcutree.kthread_prio parameter.  Either way, 0 says to continue the
default SCHED_OTHER behavior and values from 1-99 specify that priority
of SCHED_FIFO behavior.  Note that a value of 0 is not permitted when
the RCU_BOOST Kconfig parameter is specified.

Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
2015-01-15 23:25:04 -08:00
Paul E. McKenney
917963d0b3 rcutorture: Check from beginning to end of grace period
Currently, rcutorture's Reader Batch checks measure from the end of
the previous grace period to the end of the current one.  This commit
tightens up these checks by measuring from the start and end of the same
grace period.  This involves adding rcu_batches_started() and friends
corresponding to the existing rcu_batches_completed() and friends.

We leave SRCU alone for the moment, as it does not yet have a way of
tracking both ends of its grace periods.

Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
2015-01-10 19:08:02 -08:00
Paul E. McKenney
9733e4f0a9 rcu: Make _batches_completed() functions return unsigned long
Long ago, the various ->completed fields were of type long, but now are
unsigned long due to signed-integer-overflow concerns.  However, the
various _batches_completed() functions remained of type long, even though
their only purpose in life is to return the corresponding ->completed
field.  This patch cleans this up by changing these functions' return
types to unsigned long.

Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
2015-01-10 19:07:56 -08:00
Paul E. McKenney
e3663b1024 rcu: Handle gpnum/completed wrap while dyntick idle
Subtle race conditions can result if a CPU stays in dyntick-idle mode
long enough for the ->gpnum and ->completed fields to wrap.  For
example, consider the following sequence of events:

o	CPU 1 encounters a quiescent state while waiting for grace period
	5 to complete, but then enters dyntick-idle mode.

o	While CPU 1 is in dyntick-idle mode, the grace-period counters
	wrap around so that the grace period number is now 4.

o	Just as CPU 1 exits dyntick-idle mode, grace period 4 completes
	and grace period 5 begins.

o	The quiescent state that CPU 1 passed through during the old
	grace period 5 looks like it applies to the new grace period
	5.  Therefore, the new grace period 5 completes without CPU 1
	having passed through a quiescent state.

This could clearly be a fatal surprise to any long-running RCU read-side
critical section that happened to be running on CPU 1 at the time.  At one
time, this was not a problem, given that it takes significant time for
the grace-period counters to overflow even on 32-bit systems.  However,
with the advent of NO_HZ_FULL and SMP embedded systems, arbitrarily long
idle periods are now becoming quite feasible.  It is therefore time to
close this race.

This commit therefore avoids this race condition by having the
quiescent-state forcing code detect when a CPU is falling too far
behind, and setting a new rcu_data field ->gpwrap when this happens.
Whenever this new ->gpwrap field is set, the CPU's ->gpnum and ->completed
fields are known to be untrustworthy, and can be ignored, along with
any associated quiescent states.

Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
2015-01-06 11:05:28 -08:00
Paul E. McKenney
6ccd2ecd42 rcu: Improve diagnostics for spurious RCU CPU stall warnings
The current RCU CPU stall warning code will print "Stall ended before
state dump start" any time that the stall-warning code is triggered on
a CPU that has already reported a quiescent state for the current grace
period and if all quiescent states have been reported for the current
grace period.  However, a true stall can result in these symptoms, for
example, by preventing RCU's grace-period kthreads from ever running

This commit therefore checks for this condition, reporting the end of
the stall only if one of the grace-period counters has actually advanced.
Otherwise, it reports the last time that the grace-period kthread made
meaningful progress.  (In normal situations, the grace-period kthread
should make meaningful progress at least every jiffies_till_next_fqs
jiffies.)

Reported-by: Miroslav Benes <mbenes@suse.cz>
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Tested-by: Miroslav Benes <mbenes@suse.cz>
2015-01-06 11:05:27 -08:00
Paul E. McKenney
fc908ed33e rcu: Make RCU_CPU_STALL_INFO include number of fqs attempts
One way that an RCU CPU stall warning can happen is if the grace-period
kthread is not allowed to execute.  One proxy for this kthread's
forward progress is the number of force-quiescent-state (fqs) scans.
This commit therefore adds the number of fqs scans to the RCU CPU stall
warning printouts when CONFIG_RCU_CPU_STALL_INFO=y.

Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
2015-01-06 11:05:25 -08:00
Paul E. McKenney
ab954c167e rcu: Remove redundant callback-list initialization
The RCU callback lists are initialized in both rcu_boot_init_percpu_data()
and rcu_init_percpu_data().  The former is intended for initializing
immutable data, so this commit removes the initialization from
rcu_boot_init_percpu_data() and leaves it in rcu_init_percpu_data().
This change prepares for permitting callbacks to be queued very early
in boot.

Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
2015-01-06 11:02:54 -08:00
Paul E. McKenney
6cd534ef8b rcu: Don't scan root rcu_node structure for stalled tasks
Now that blocked tasks are no longer migrated to the root rcu_node
structure, there is no need to scan the root rcu_node structure for
blocked tasks stalling the current grace period.  This commit therefore
removes this scan.

Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
2015-01-06 11:02:53 -08:00
Paul E. McKenney
3ba4d0e09b rcu: Note quiescent state when CPU goes offline
The rcu_cleanup_dead_cpu() function (called after a CPU has gone
completely offline) has not reported a quiescent state because there
was probably at least one synchronize_rcu() between the time the CPU
went offline and the CPU_DEAD notifier, and this would have detected
the CPU's offline state via quiescent-state forcing.  However, the plan
is for CPUs to take themselves offline, at which point it makes sense
for them to report their own quiescent state.  This commit makes this
change in preparation for the new CPU-hotplug setup.

Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
2015-01-06 11:02:51 -08:00
Paul E. McKenney
1be0085b51 rcu: Don't initiate RCU priority boosting on root rcu_node
Because there is no longer any preempted tasks on the root rcu_node, and
because there is no longer ever an rcub kthread for the root rcu_node,
this commit drops the code in force_qs_rnp() that attempts to awaken
the non-existent root rcub kthread.  This is strictly a performance
enhancement, removing a root rcu_node ->lock acquisition and release
along with some tests in rcu_initiate_boost(), ending with the test that
notes that there is no rcub kthread.

Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
2015-01-06 11:02:48 -08:00
Paul E. McKenney
a8f4cbadfb rcu: Shorten irq-disable region in rcu_cleanup_dead_cpu()
Now that we are not migrating callbacks, there is no need to hold the
->orphan_lock across the the ->qsmaskinit bit-clearing process.
This commit therefore releases ->orphan_lock immediately after adopting
the orphaned RCU callbacks.

Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
2015-01-06 11:02:45 -08:00
Paul E. McKenney
d19fb8d1f3 rcu: Don't migrate blocked tasks even if all corresponding CPUs offline
When the last CPU associated with a given leaf rcu_node structure
goes offline, something must be done about the tasks queued on that
rcu_node structure.  Each of these tasks has been preempted on one of
the leaf rcu_node structure's CPUs while in an RCU read-side critical
section that it have not yet exited.  Handling these tasks is the job of
rcu_preempt_offline_tasks(), which migrates them from the leaf rcu_node
structure to the root rcu_node structure.

Unfortunately, this migration has to be done one task at a time because
each tasks allegiance must be shifted from the original leaf rcu_node to
the root, so that future attempts to deal with these tasks will acquire
the root rcu_node structure's ->lock rather than that of the leaf.
Worse yet, this migration must be done with interrupts disabled, which
is not so good for realtime response, especially given that there is
no bound on the number of tasks on a given rcu_node structure's list.
(OK, OK, there is a bound, it is just that it is unreasonably large,
especially on 64-bit systems.)  This was not considered a problem back
when rcu_preempt_offline_tasks() was first written because realtime
systems were assumed not to do CPU-hotplug operations while real-time
applications were running.  This assumption has proved of dubious validity
given that people are starting to run multiple realtime applications
on a single SMP system and that it is common practice to offline then
online a CPU before starting its real-time application in order to clear
extraneous processing off of that CPU.  So we now need CPU hotplug
operations to avoid undue latencies.

This commit therefore avoids migrating these tasks, instead letting
them be dequeued one by one from the original leaf rcu_node structure
by rcu_read_unlock_special().  This means that the clearing of bits
from the upper-level rcu_node structures must be deferred until the
last such task has been dequeued, because otherwise subsequent grace
periods won't wait on them.  This commit has the beneficial side effect
of simplifying the CPU-hotplug code for TREE_PREEMPT_RCU, especially in
CONFIG_RCU_BOOST builds.

Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
2015-01-06 11:02:44 -08:00
Paul E. McKenney
b6a932d1d9 rcu: Make rcu_read_unlock_special() propagate ->qsmaskinit bit clearing
This commit causes rcu_read_unlock_special() to propagate ->qsmaskinit
bit clearing up the rcu_node tree once a given rcu_node structure's
blkd_tasks list becomes empty.  This is the final commit in preparation
for the rework of RCU priority boosting:  It enables preempted tasks to
remain queued on their rcu_node structure even after all of that rcu_node
structure's CPUs have gone offline.

Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
2015-01-06 11:02:43 -08:00
Paul E. McKenney
8af3a5e78c rcu: Abstract rcu_cleanup_dead_rnp() from rcu_cleanup_dead_cpu()
This commit abstracts rcu_cleanup_dead_rnp() from rcu_cleanup_dead_cpu()
in preparation for the rework of RCU priority boosting.  This new function
will be invoked from rcu_read_unlock_special() in the reworked scheme,
which is why rcu_cleanup_dead_rnp() assumes that the leaf rcu_node
structure's ->qsmaskinit field has already been updated.

Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
2015-01-06 11:02:41 -08:00
Paul E. McKenney
41050a0096 rcu: Fix rcu_barrier() race that could result in too-short wait
The rcu_barrier() no-callbacks check for no-CBs CPUs has race conditions.
It checks a given CPU's lists of callbacks, and if all three no-CBs lists
are empty, ignores that CPU.  However, these three lists could potentially
be empty even when callbacks are present if the check executed just as
the callbacks were being moved from one list to another.  It turns out
that recent versions of rcutorture can spot this race.

This commit plugs this hole by consolidating the per-list counts of
no-CBs callbacks into a single count, which is incremented before
the corresponding callback is posted and after it is invoked.  Then
rcu_barrier() checks this single count to reliably determine whether
the corresponding CPU has no-CBs callbacks.

Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
2015-01-06 11:01:15 -08:00
Lai Jiangshan
5f6130fa52 tiny_rcu: Directly force QS when call_rcu_[bh|sched]() on idle_task
For RCU in UP, context-switch = QS = GP, thus we can force a
context-switch when any call_rcu_[bh|sched]() is happened on idle_task.
After doing so, rcu_idle/irq_enter/exit() are useless, so we can simply
make these functions empty.

More important, this change does not change the functionality logically.
Note: raise_softirq(RCU_SOFTIRQ)/rcu_sched_qs() in rcu_idle_enter() and
outmost rcu_irq_exit() will have to wake up the ksoftirqd
(due to in_interrupt() == 0).

Before this patch		After this patch:
call_rcu_sched() in idle;	call_rcu_sched() in idle
				  set resched
do other stuffs;		do other stuffs
outmost rcu_irq_exit()		outmost rcu_irq_exit() (empty function)
  (or rcu_idle_enter())		  (or rcu_idle_enter(), also empty function)
				start to resched. (see above)
  rcu_sched_qs()		rcu_sched_qs()
    QS,and GP and advance cb	  QS,and GP and advance cb
    wake up the ksoftirqd	    wake up the ksoftirqd
      set resched
resched to ksoftirqd (or other)	resched to ksoftirqd (or other)

These two code patches are almost the same.

Size changed after patched:

size kernel/rcu/tiny-old.o kernel/rcu/tiny-patched.o
   text	   data	    bss	    dec	    hex	filename
   3449	    206	      8	   3663	    e4f	kernel/rcu/tiny-old.o
   2406	    144	      8	   2558	    9fe	kernel/rcu/tiny-patched.o

Signed-off-by: Lai Jiangshan <laijs@cn.fujitsu.com>
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
2015-01-06 11:01:12 -08:00
Paul E. McKenney
924df8a011 rcu: Fix invoke_rcu_callbacks() comment
Despite what the comment says, it is only softirqs that are disabled,
not interrupts.  This commit therefore fixes the comment.

Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
2014-12-30 17:40:19 -08:00
Paul E. McKenney
734d168013 rcu: Make rcu_nmi_enter() handle nesting
The x86 architecture has multiple types of NMI-like interrupts: real
NMIs, machine checks, and, for some values of NMI-like, debugging
and breakpoint interrupts.  These interrupts can nest inside each
other.  Andy Lutomirski is adding RCU support to these interrupts,
so rcu_nmi_enter() and rcu_nmi_exit() must now correctly handle nesting.

This commit therefore introduces nesting, using a clever NMI-coordination
algorithm suggested by Andy.  The trick is to atomically increment
->dynticks (if needed) before manipulating ->dynticks_nmi_nesting on entry
(and, accordingly, after on exit).  In addition, ->dynticks_nmi_nesting
is incremented by one if ->dynticks was incremented and by two otherwise.
This means that when rcu_nmi_exit() sees ->dynticks_nmi_nesting equal
to one, it knows that ->dynticks must be atomically incremented.

This NMI-coordination algorithms has been validated by the following
Promela model:

------------------------------------------------------------------------

/*
 * Promela model for Andy Lutomirski's suggested change to rcu_nmi_enter()
 * that allows nesting.
 *
 * This program is free software; you can redistribute it and/or modify
 * it under the terms of the GNU General Public License as published by
 * the Free Software Foundation; either version 2 of the License, or
 * (at your option) any later version.
 *
 * This program is distributed in the hope that it will be useful,
 * but WITHOUT ANY WARRANTY; without even the implied warranty of
 * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
 * GNU General Public License for more details.
 *
 * You should have received a copy of the GNU General Public License
 * along with this program; if not, you can access it online at
 * http://www.gnu.org/licenses/gpl-2.0.html.
 *
 * Copyright IBM Corporation, 2014
 *
 * Author: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
 */

byte dynticks_nmi_nesting = 0;
byte dynticks = 0;

/*
 * Promela verision of rcu_nmi_enter().
 */
inline rcu_nmi_enter()
{
	byte incby;
	byte tmp;

	incby = BUSY_INCBY;
	assert(dynticks_nmi_nesting >= 0);
	if
	:: (dynticks & 1) == 0 ->
		atomic {
			dynticks = dynticks + 1;
		}
		assert((dynticks & 1) == 1);
		incby = 1;
	:: else ->
		skip;
	fi;
	tmp = dynticks_nmi_nesting;
	tmp = tmp + incby;
	dynticks_nmi_nesting = tmp;
	assert(dynticks_nmi_nesting >= 1);
}

/*
 * Promela verision of rcu_nmi_exit().
 */
inline rcu_nmi_exit()
{
	byte tmp;

	assert(dynticks_nmi_nesting > 0);
	assert((dynticks & 1) != 0);
	if
	:: dynticks_nmi_nesting != 1 ->
		tmp = dynticks_nmi_nesting;
		tmp = tmp - BUSY_INCBY;
		dynticks_nmi_nesting = tmp;
	:: else ->
		dynticks_nmi_nesting = 0;
		atomic {
			dynticks = dynticks + 1;
		}
		assert((dynticks & 1) == 0);
	fi;
}

/*
 * Base-level NMI runs non-atomically.  Crudely emulates process-level
 * dynticks-idle entry/exit.
 */
proctype base_NMI()
{
	byte busy;

	busy = 0;
	do
	::	/* Emulate base-level dynticks and not. */
		if
		:: 1 ->	atomic {
				dynticks = dynticks + 1;
			}
			busy = 1;
		:: 1 ->	skip;
		fi;

		/* Verify that we only sometimes have base-level dynticks. */
		if
		:: busy == 0 -> skip;
		:: busy == 1 -> skip;
		fi;

		/* Model RCU's NMI entry and exit actions. */
		rcu_nmi_enter();
		assert((dynticks & 1) == 1);
		rcu_nmi_exit();

		/* Emulated re-entering base-level dynticks and not. */
		if
		:: !busy -> skip;
		:: busy ->
			atomic {
				dynticks = dynticks + 1;
			}
			busy = 0;
		fi;

		/* We had better now be in dyntick-idle mode. */
		assert((dynticks & 1) == 0);
	od;
}

/*
 * Nested NMI runs atomically to emulate interrupting base_level().
 */
proctype nested_NMI()
{
	do
	::	/*
		 * Use an atomic section to model a nested NMI.  This is
		 * guaranteed to interleave into base_NMI() between a pair
		 * of base_NMI() statements, just as a nested NMI would.
		 */
		atomic {
			/* Verify that we only sometimes are in dynticks. */
			if
			:: (dynticks & 1) == 0 -> skip;
			:: (dynticks & 1) == 1 -> skip;
			fi;

			/* Model RCU's NMI entry and exit actions. */
			rcu_nmi_enter();
			assert((dynticks & 1) == 1);
			rcu_nmi_exit();
		}
	od;
}

init {
	run base_NMI();
	run nested_NMI();
}

------------------------------------------------------------------------

The following script can be used to run this model if placed in
rcu_nmi.spin:

------------------------------------------------------------------------

if ! spin -a rcu_nmi.spin
then
	echo Spin errors!!!
	exit 1
fi
if ! cc -DSAFETY -o pan pan.c
then
	echo Compilation errors!!!
	exit 1
fi
./pan -m100000

Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Reviewed-by: Lai Jiangshan <laijs@cn.fujitsu.com>
2014-12-30 17:40:16 -08:00
Paul E. McKenney
9ea6c58856 Merge branches 'torture.2014.11.03a', 'cpu.2014.11.03a', 'doc.2014.11.13a', 'fixes.2014.11.13a', 'signal.2014.10.29a' and 'rt.2014.10.29a' into HEAD
cpu.2014.11.03a: Changes for per-CPU variables.
doc.2014.11.13a: Documentation updates.
fixes.2014.11.13a: Miscellaneous fixes.
signal.2014.10.29a: Signal changes.
rt.2014.10.29a: Real-time changes.
torture.2014.11.03a: torture-test changes.
2014-11-13 10:39:04 -08:00
Pranith Kumar
aa23c6fbc5 rcutorture: Add early boot self tests
Add early boot self tests for RCU under CONFIG_PROVE_RCU.

Currently the only test is adding a dummy callback which increments a counter
which we then later verify after calling rcu_barrier*().

Signed-off-by: Pranith Kumar <bobby.prani@gmail.com>
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
2014-11-03 19:26:37 -08:00
Paul E. McKenney
8fa7845df5 rcu: Remove "cpu" argument to rcu_cleanup_after_idle()
The "cpu" argument to rcu_cleanup_after_idle() is always the current
CPU, so drop it.  This moves the smp_processor_id() from the caller to
rcu_cleanup_after_idle(), saving argument-passing overhead.  Again,
the anticipated cross-CPU uses of these functions has been replaced
by NO_HZ_FULL.

Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Reviewed-by: Pranith Kumar <bobby.prani@gmail.com>
2014-11-03 19:20:56 -08:00
Paul E. McKenney
198bbf8127 rcu: Remove "cpu" argument to rcu_prepare_for_idle()
The "cpu" argument to rcu_prepare_for_idle() is always the current
CPU, so drop it.  This in turn allows two of the uses of "cpu" in
this function to be replaced with a this_cpu_ptr() and the third by
smp_processor_id(), replacing that of the call to rcu_prepare_for_idle().
Again, the anticipated cross-CPU uses of these functions has been replaced
by NO_HZ_FULL.

Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Reviewed-by: Pranith Kumar <bobby.prani@gmail.com>
2014-11-03 19:20:49 -08:00
Paul E. McKenney
aa6da5140b rcu: Remove "cpu" argument to rcu_needs_cpu()
The "cpu" argument to rcu_needs_cpu() is always the current CPU, so drop
it.  This in turn allows the "cpu" argument to rcu_cpu_has_callbacks()
to be removed, which allows the uses of "cpu" in both functions to be
replaced with a this_cpu_ptr().  Again, the anticipated cross-CPU uses
of these functions has been replaced by NO_HZ_FULL.

Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Reviewed-by: Pranith Kumar <bobby.prani@gmail.com>
2014-11-03 19:20:43 -08:00
Paul E. McKenney
38200cf247 rcu: Remove "cpu" argument to rcu_note_context_switch()
The "cpu" argument to rcu_note_context_switch() is always the current
CPU, so drop it.  This in turn allows the "cpu" argument to
rcu_preempt_note_context_switch() to be removed, which allows the sole
use of "cpu" in both functions to be replaced with a this_cpu_ptr().
Again, the anticipated cross-CPU uses of these functions has been
replaced by NO_HZ_FULL.

Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Reviewed-by: Pranith Kumar <bobby.prani@gmail.com>
2014-11-03 19:20:34 -08:00
Paul E. McKenney
86aea0e6e7 rcu: Remove "cpu" argument to rcu_preempt_check_callbacks()
Because rcu_preempt_check_callbacks()'s argument is guaranteed to
always be the current CPU, drop the argument and replace per_cpu()
with __this_cpu_read().

Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Reviewed-by: Pranith Kumar <bobby.prani@gmail.com>
2014-11-03 19:20:26 -08:00
Paul E. McKenney
e3950ecd55 rcu: Remove "cpu" argument to rcu_pending()
Because rcu_pending()'s argument is guaranteed to always be the current
CPU, drop the argument and replace per_cpu_ptr() with this_cpu_ptr().

Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Reviewed-by: Pranith Kumar <bobby.prani@gmail.com>
2014-11-03 19:20:18 -08:00
Paul E. McKenney
c3377c2da6 rcu: Remove "cpu" argument to rcu_check_callbacks()
The "cpu" argument was kept around on the off-chance that RCU might
offload scheduler-clock interrupts.  However, this offload approach
has been replaced by NO_HZ_FULL, which offloads -all- RCU processing
from qualifying CPUs.  It is therefore time to remove the "cpu" argument
to rcu_check_callbacks(), which this commit does.

Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Reviewed-by: Pranith Kumar <bobby.prani@gmail.com>
2014-11-03 19:20:11 -08:00
Paul E. McKenney
11bbb235c2 rcu: Use DEFINE_PER_CPU_SHARED_ALIGNED for rcu_data
The rcu_data per-CPU variable has a number of fields that are atomically
manipulated, potentially by any CPU.  This situation can result in false
sharing with per-CPU variables that have the misfortune of being allocated
adjacent to rcu_data in memory.  This commit therefore changes the
DEFINE_PER_CPU() to DEFINE_PER_CPU_SHARED_ALIGNED() in order to avoid
this false sharing.

Reported-by: Christoph Lameter <cl@linux.com>
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Reviewed-by: Christoph Lameter <cl@linux.com>
Reviewed-by: Pranith Kumar <bobby.prani@gmail.com>
2014-11-03 19:20:03 -08:00
Christoph Lameter
28ced795cb rcu: Remove rcu_dynticks * parameters when they are always this_cpu_ptr(&rcu_dynticks)
For some functions in kernel/rcu/tree* the rdtp parameter is always
this_cpu_ptr(rdtp).  Remove the parameter if constant and calculate the
pointer in function.

This will have the advantage that it is obvious that the address are
all per cpu offsets and thus it will enable the use of this_cpu_ops in
the future.

Signed-off-by: Christoph Lameter <cl@linux.com>
[ paulmck: Forward-ported to rcu/dev, whitespace adjustment. ]
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Reviewed-by: Pranith Kumar <bobby.prani@gmail.com>
2014-11-03 19:19:26 -08:00
Paul E. McKenney
776d680711 rcu: Kick rcuo kthreads after their CPU goes offline
If a no-CBs CPU were to post an RCU callback with interrupts disabled
after it entered the idle loop for the last time, there might be no
deferred wakeup for the corresponding rcuo kthreads.  This commit
therefore adds a set of calls to do_nocb_deferred_wakeup() after the
CPU has gone completely offline.

Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
2014-10-29 10:20:07 -07:00
Paul E. McKenney
e0775cefb5 rcu: Avoid IPIing idle CPUs from synchronize_sched_expedited()
Currently, synchronize_sched_expedited() sends IPIs to all online CPUs,
even those that are idle or executing in nohz_full= userspace.  Because
idle CPUs and nohz_full= userspace CPUs are in extended quiescent states,
there is no need to IPI them in the first place.  This commit therefore
avoids IPIing CPUs that are already in extended quiescent states.

Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
2014-10-28 13:49:30 -07:00
Paul E. McKenney
61cfd0970e rcu: Move RCU_BOOST variable declarations, eliminating #ifdef
There are some RCU_BOOST-specific per-CPU variable declarations that
are needlessly defined under #ifdef in kernel/rcu/tree.c.  This commit
therefore moves these declarations into a pre-existing #ifdef in
kernel/rcu/tree_plugin.h.

Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
2014-10-28 13:49:28 -07:00
Paul E. McKenney
d7e2993396 rcu: Make rcu_barrier() understand about missing rcuo kthreads
Commit 35ce7f29a4 (rcu: Create rcuo kthreads only for onlined CPUs)
avoids creating rcuo kthreads for CPUs that never come online.  This
fixes a bug in many instances of firmware: Instead of lying about their
age, these systems instead lie about the number of CPUs that they have.
Before commit 35ce7f29a4, this could result in huge numbers of useless
rcuo kthreads being created.

It appears that experience indicates that I should have told the
people suffering from this problem to fix their broken firmware, but
I instead produced what turned out to be a partial fix.   The missing
piece supplied by this commit makes sure that rcu_barrier() knows not to
post callbacks for no-CBs CPUs that have not yet come online, because
otherwise rcu_barrier() will hang on systems having firmware that lies
about the number of CPUs.

It is tempting to simply have rcu_barrier() refuse to post a callback on
any no-CBs CPU that does not have an rcuo kthread.  This unfortunately
does not work because rcu_barrier() is required to wait for all pending
callbacks.  It is therefore required to wait even for those callbacks
that cannot possibly be invoked.  Even if doing so hangs the system.

Given that posting a callback to a no-CBs CPU that does not yet have an
rcuo kthread can hang rcu_barrier(), It is tempting to report an error
in this case.  Unfortunately, this will result in false positives at
boot time, when it is perfectly legal to post callbacks to the boot CPU
before the scheduler has started, in other words, before it is legal
to invoke rcu_barrier().

So this commit instead has rcu_barrier() avoid posting callbacks to
CPUs having neither rcuo kthread nor pending callbacks, and has it
complain bitterly if it finds CPUs having no rcuo kthread but some
pending callbacks.  And when rcu_barrier() does find CPUs having no rcuo
kthread but pending callbacks, as noted earlier, it has no choice but
to hang indefinitely.

Reported-by: Yanko Kaneti <yaneti@declera.com>
Reported-by: Jay Vosburgh <jay.vosburgh@canonical.com>
Reported-by: Meelis Roos <mroos@linux.ee>
Reported-by: Eric B Munson <emunson@akamai.com>
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Tested-by: Eric B Munson <emunson@akamai.com>
Tested-by: Jay Vosburgh <jay.vosburgh@canonical.com>
Tested-by: Yanko Kaneti <yaneti@declera.com>
Tested-by: Kevin Fenzi <kevin@scrye.com>
Tested-by: Meelis Roos <mroos@linux.ee>
2014-10-28 13:24:13 -07:00
Paul E. McKenney
dd56af42bd rcu: Eliminate deadlock between CPU hotplug and expedited grace periods
Currently, the expedited grace-period primitives do get_online_cpus().
This greatly simplifies their implementation, but means that calls
to them holding locks that are acquired by CPU-hotplug notifiers (to
say nothing of calls to these primitives from CPU-hotplug notifiers)
can deadlock.  But this is starting to become inconvenient, as can be
seen here: https://lkml.org/lkml/2014/8/5/754.  The problem in this
case is that some developers need to acquire a mutex from a CPU-hotplug
notifier, but also need to hold it across a synchronize_rcu_expedited().
As noted above, this currently results in deadlock.

This commit avoids the deadlock and retains the simplicity by creating
a try_get_online_cpus(), which returns false if the get_online_cpus()
reference count could not immediately be incremented.  If a call to
try_get_online_cpus() returns true, the expedited primitives operate as
before.  If a call returns false, the expedited primitives fall back to
normal grace-period operations.  This falling back of course results in
increased grace-period latency, but only during times when CPU hotplug
operations are actually in flight.  The effect should therefore be
negligible during normal operation.

Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Cc: Josh Triplett <josh@joshtriplett.org>
Cc: "Rafael J. Wysocki" <rjw@rjwysocki.net>
Tested-by: Lan Tianyu <tianyu.lan@intel.com>
2014-09-18 16:22:27 -07:00
Paul E. McKenney
96b4672703 Merge branch 'rcu-tasks.2014.09.10a' into HEAD
rcu-tasks.2014.09.10a: Add RCU-tasks flavor of RCU.
2014-09-16 10:10:44 -07:00
Paul E. McKenney
e98d06dd6c Merge branches 'doc.2014.09.07a', 'fixes.2014.09.10a', 'nocb-nohz.2014.09.16b' and 'torture.2014.09.07a' into HEAD
doc.2014.09.07a: Documentation updates.
fixes.2014.09.10a: Miscellaneous fixes.
nocb-nohz.2014.09.16b: No-CBs CPUs and NO_HZ_FULL updates.
torture.2014.09.07a: Torture-test updates.
2014-09-16 10:08:34 -07:00
Paul E. McKenney
35ce7f29a4 rcu: Create rcuo kthreads only for onlined CPUs
RCU currently uses for_each_possible_cpu() to spawn rcuo kthreads,
which can result in more rcuo kthreads than one would expect, for
example, derRichard reported 64 CPUs worth of rcuo kthreads on an
8-CPU image.  This commit therefore creates rcuo kthreads only for
those CPUs that actually come online.

This was reported by derRichard on the OFTC IRC network.

Reported-by: Richard Weinberger <richard@nod.at>
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Reviewed-by: Josh Triplett <josh@joshtriplett.org>
Tested-by: Paul Gortmaker <paul.gortmaker@windriver.com>
2014-09-16 10:08:02 -07:00
Paul E. McKenney
9386c0b75d rcu: Rationalize kthread spawning
Currently, RCU spawns kthreads from several different early_initcall()
functions.  Although this has served RCU well for quite some time,
as more kthreads are added a more deterministic approach is required.
This commit therefore causes all of RCU's early-boot kthreads to be
spawned from a single early_initcall() function.

Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Reviewed-by: Josh Triplett <josh@joshtriplett.org>
Tested-by: Paul Gortmaker <paul.gortmaker@windriver.com>
2014-09-16 10:08:01 -07:00
Paul E. McKenney
284a8c93af rcu: Per-CPU operation cleanups to rcu_*_qs() functions
The rcu_bh_qs(), rcu_preempt_qs(), and rcu_sched_qs() functions use
old-style per-CPU variable access and write to ->passed_quiesce even
if it is already set.  This commit therefore updates to use the new-style
per-CPU variable access functions and avoids the spurious writes.
This commit also eliminates the "cpu" argument to these functions because
they are always invoked on the indicated CPU.

Reported-by: Peter Zijlstra <peterz@infradead.org>
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
2014-09-07 16:27:35 -07:00
Paul E. McKenney
176f8f7a52 rcu: Make TASKS_RCU handle nohz_full= CPUs
Currently TASKS_RCU would ignore a CPU running a task in nohz_full=
usermode execution.  There would be neither a context switch nor a
scheduling-clock interrupt to tell TASKS_RCU that the task in question
had passed through a quiescent state.  The grace period would therefore
extend indefinitely.  This commit therefore makes RCU's dyntick-idle
subsystem record the task_struct structure of the task that is running
in dyntick-idle mode on each CPU.  The TASKS_RCU grace period can
then access this information and record a quiescent state on
behalf of any CPU running in dyntick-idle usermode.

Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
2014-09-07 16:27:30 -07:00
Paul E. McKenney
bde6c3aa99 rcu: Provide cond_resched_rcu_qs() to force quiescent states in long loops
RCU-tasks requires the occasional voluntary context switch
from CPU-bound in-kernel tasks.  In some cases, this requires
instrumenting cond_resched().  However, there is some reluctance
to countenance unconditionally instrumenting cond_resched() (see
http://lwn.net/Articles/603252/), so this commit creates a separate
cond_resched_rcu_qs() that may be used in place of cond_resched() in
locations prone to long-duration in-kernel looping.

This commit currently instruments only RCU-tasks.  Future possibilities
include also instrumenting RCU, RCU-bh, and RCU-sched in order to reduce
IPI usage.

Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
2014-09-07 16:27:20 -07:00
Paul E. McKenney
8315f42295 rcu: Add call_rcu_tasks()
This commit adds a new RCU-tasks flavor of RCU, which provides
call_rcu_tasks().  This RCU flavor's quiescent states are voluntary
context switch (not preemption!) and userspace execution (not the idle
loop -- use some sort of schedule_on_each_cpu() if you need to handle the
idle tasks.  Note that unlike other RCU flavors, these quiescent states
occur in tasks, not necessarily CPUs.  Includes fixes from Steven Rostedt.

This RCU flavor is assumed to have very infrequent latency-tolerant
updaters.  This assumption permits significant simplifications, including
a single global callback list protected by a single global lock, along
with a single task-private linked list containing all tasks that have not
yet passed through a quiescent state.  If experience shows this assumption
to be incorrect, the required additional complexity will be added.

Suggested-by: Steven Rostedt <rostedt@goodmis.org>
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
2014-09-07 16:27:19 -07:00
Paul E. McKenney
73a860cd58 rcu: Replace flush_signals() with WARN_ON(signal_pending())
Currently, when RCU awakens from a wait_event_interruptible() that
might have awakened prematurely, it does a flush_signals(). This is
done on the off-chance that someone figured out how to deliver a signal
to a kthread, which is supposed to be impossible.  Given that this
is supposed to be impossible, this commit changes the flush_signals()
calls into WARN_ON(signal_pending()).

Reported-by: Oleg Nesterov <oleg@redhat.com>
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
2014-09-07 16:18:20 -07:00
Pranith Kumar
2aa792e6fa rcu: Use rcu_gp_kthread_wake() to wake up grace period kthreads
The rcu_gp_kthread_wake() function checks for three conditions before
waking up grace period kthreads:

*  Is the thread we are trying to wake up the current thread?
*  Are the gp_flags zero? (all threads wait on non-zero gp_flags condition)
*  Is there no thread created for this flavour, hence nothing to wake up?

If any one of these condition is true, we do not call wake_up().
It was found that there are quite a few avoidable wake ups both during
idle time and under stress induced by rcutorture.

Idle:

Total:66000, unnecessary:66000, case1:61827, case2:66000, case3:0
Total:68000, unnecessary:68000, case1:63696, case2:68000, case3:0

rcutorture:

Total:254000, unnecessary:254000, case1:199913, case2:254000, case3:0
Total:256000, unnecessary:256000, case1:201784, case2:256000, case3:0

Here case{1-3} are the cases listed above. We can avoid these wake
ups by using rcu_gp_kthread_wake() to conditionally wake up the grace
period kthreads.

There is a comment about an implied barrier supplied by the wake_up()
logic.  This barrier is necessary for the awakened thread to see the
updated ->gp_flags.  This flag is always being updated with the root node
lock held. Also, the awakened thread tries to acquire the root node lock
before reading ->gp_flags because of which there is proper ordering.

Hence this commit tries to avoid calling wake_up() whenever we can by
using rcu_gp_kthread_wake() function.

Signed-off-by: Pranith Kumar <bobby.prani@gmail.com>
CC: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
2014-09-07 16:18:19 -07:00
Pranith Kumar
66d701ea7e rcu: Remove stale comment in tree.c
This commit removes a stale comment in rcu/tree.c which was left
out when some code was moved around previously in commit 2036d94a7b
("rcu:  Rework detection of use of RCU by offline CPUs") For reference,
the following updated comment exists a few lines below this which means
the same:

/* Remove the outgoing CPU from the masks in the rcu_node hierarchy. */

Signed-off-by: Pranith Kumar <bobby.prani@gmail.com>
Reviewed-by: Josh Triplett <josh@joshtriplett.org>
Reviewed-by: Lai Jiangshan <laijs@cn.fujitsu.com>
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
2014-09-07 16:18:16 -07:00
Ard Biesheuvel
a8a29b3b7b rcu: Define tracepoint strings only if CONFIG_TRACING is set
Commit f7f7bac9cb ("rcu: Have the RCU tracepoints use the tracepoint_string
infrastructure") unconditionally populates the __tracepoint_str input section,
but this section is not assigned an output section if CONFIG_TRACING is not set.
This results in the __tracepoint_str turning up in unexpected places, i.e.,
after _edata.

Signed-off-by: Ard Biesheuvel <ard.biesheuvel@linaro.org>
Reviewed-by: Steven Rostedt <rostedt@goodmis.org>
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
2014-09-07 16:18:14 -07:00
Pranith Kumar
e02b2edfa1 rcu: Use true/false instead of 1/0 for a bool type
This commit uses true/false instead of 1/0 for bool types in rcu_gp_fqs()
and force_qs_rnp().

Signed-off-by: Pranith Kumar <bobby.prani@gmail.com>
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
2014-09-07 16:18:12 -07:00
Pranith Kumar
f534ed1fd7 rcu: Use bool type for return value in rcu_is_watching()
Use a bool type for return in rcu_is_watching().

Signed-off-by: Pranith Kumar <bobby.prani@gmail.com>
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
2014-09-07 16:18:09 -07:00
Pranith Kumar
4de376a1b1 rcu: Remove remaining read-modify-write ACCESS_ONCE() calls
Change the remaining uses of ACCESS_ONCE() so that each ACCESS_ONCE() either does a load or a store, but not both.

Signed-off-by: Pranith Kumar <bobby.prani@gmail.com>
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
2014-09-07 16:18:07 -07:00
Paul E. McKenney
11992c703a rcu: Remove CONFIG_PROVE_RCU_DELAY
The CONFIG_PROVE_RCU_DELAY Kconfig parameter doesn't appear to be very
effective at finding race conditions, so this commit removes it.

Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Cc: Andi Kleen <ak@linux.intel.com>
[ paulmck: Remove definition and uses as noted by Paul Bolle. ]
2014-07-09 09:15:31 -07:00
Shan Wei
d860d40327 rcu: Use __this_cpu_read() instead of per_cpu_ptr()
The __this_cpu_read() function produces better code than does
per_cpu_ptr() on both ARM and x86.  For example, gcc (Ubuntu/Linaro
4.7.3-12ubuntu1) 4.7.3 produces the following:

ARMv7 per_cpu_ptr():

force_quiescent_state:
    mov    r3, sp    @,
    bic    r1, r3, #8128    @ tmp171,,
    ldr    r2, .L98    @ tmp169,
    bic    r1, r1, #63    @ tmp170, tmp171,
    ldr    r3, [r0, #220]    @ __ptr, rsp_6(D)->rda
    ldr    r1, [r1, #20]    @ D.35903_68->cpu, D.35903_68->cpu
    mov    r6, r0    @ rsp, rsp
    ldr    r2, [r2, r1, asl #2]    @ tmp173, __per_cpu_offset
    add    r3, r3, r2    @ tmp175, __ptr, tmp173
    ldr    r5, [r3, #12]    @ rnp_old, D.29162_13->mynode

ARMv7 __this_cpu_read():

force_quiescent_state:
    ldr    r3, [r0, #220]    @ rsp_7(D)->rda, rsp_7(D)->rda
    mov    r6, r0    @ rsp, rsp
    add    r3, r3, #12    @ __ptr, rsp_7(D)->rda,
    ldr    r5, [r2, r3]    @ rnp_old, *D.29176_13

Using gcc 4.8.2:

x86_64 per_cpu_ptr():

    movl %gs:cpu_number,%edx    # cpu_number, pscr_ret__
    movslq    %edx, %rdx    # pscr_ret__, pscr_ret__
    movq    __per_cpu_offset(,%rdx,8), %rdx    # __per_cpu_offset, tmp93
    movq    %rdi, %r13    # rsp, rsp
    movq    1000(%rdi), %rax    # rsp_9(D)->rda, __ptr
    movq    24(%rdx,%rax), %r12    # _15->mynode, rnp_old

x86_64 __this_cpu_read():

    movq    %rdi, %r13    # rsp, rsp
    movq    1000(%rdi), %rax    # rsp_9(D)->rda, rsp_9(D)->rda
    movq %gs:24(%rax),%r12    # _10->mynode, rnp_old

Because this change produces significant benefits for these two very
diverse architectures, this commit makes this change.

Signed-off-by: Shan Wei <davidshan@tencent.com>
Acked-by: Christoph Lameter <cl@linux.com>
Signed-off-by: Pranith Kumar <bobby.prani@gmail.com>
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Reviewed-by: Josh Triplett <josh@joshtriplett.org>
Reviewed-by: Lai Jiangshan <laijs@cn.fujitsu.com>
2014-07-09 09:15:21 -07:00
Paul E. McKenney
bc1dce514e rcu: Don't use NMIs to dump other CPUs' stacks
Although NMI-based stack dumps are in principle more accurate, they are
also more likely to trigger deadlocks.  This commit therefore replaces
all uses of trigger_all_cpu_backtrace() with rcu_dump_cpu_stacks(), so
that the CPU detecting an RCU CPU stall does the stack dumping.

Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Reviewed-by: Lai Jiangshan <laijs@cn.fujitsu.com>
2014-07-09 09:15:04 -07:00
Pranith Kumar
48bd8e9b82 rcu: Check both root and current rcu_node when setting up future grace period
The rcu_start_future_gp() function checks the current rcu_node's ->gpnum
and ->completed twice, once without ACCESS_ONCE() and once with it.
Which is pointless because we hold that rcu_node's ->lock at that point.
The intent was to check the current rcu_node structure and the root
rcu_node structure, the latter locklessly with ACCESS_ONCE().  This
commit therefore makes that change.

The reason that it is safe to locklessly check the root rcu_nodes's
->gpnum and ->completed fields is that we hold the current rcu_node's
->lock, which constrains the root rcu_node's ability to change its
->gpnum and ->completed fields.  Of course, if there is a single rcu_node
structure, then rnp_root==rnp, and holding the lock prevents all changes.
If there is more than one rcu_node structure, then the code updates the
fields in the following order:

1.	Increment rnp_root->gpnum to start new grace period.
2.	Increment rnp->gpnum to initialize the current rcu_node,
	continuing initialization for the new grace period.
3.	Increment rnp_root->completed to end the current grace period.
4.	Increment rnp->completed to continue cleaning up after the
	old grace period.

So there are four possible combinations of relative values of these
four fields:

N   N   N   N:  RCU idle, new grace period must be initiated.
		Although rnp_root->gpnum might be incremented immediately
		after we check, that will just result in unnecessary work.
		The grace period already started, and we try to start it.

N+1 N   N   N:  RCU grace period just started.  No further change is
		possible because we hold rnp->lock, so the checks of
		rnp_root->gpnum and rnp_root->completed are stable.
		We know that our request for a future grace period will
		be seen during grace-period cleanup.

N+1 N   N+1 N:  RCU grace period is ongoing.  Because rnp->gpnum is
		different than rnp->completed, we won't even look at
		rnp_root->gpnum and rnp_root->completed, so the possible
		concurrent change to rnp_root->completed does not matter.
		We know that our request for a future grace period will
		be seen during grace-period cleanup, which cannot pass
		this rcu_node because we hold its ->lock.

N+1 N+1 N+1 N:  RCU grace period has ended, but not yet been cleaned up.
		Because rnp->gpnum is different than rnp->completed, we
		won't look at rnp_root->gpnum and rnp_root->completed, so
		the possible concurrent change to rnp_root->completed does
		not matter.  We know that our request for a future grace
		period will be seen during grace-period cleanup, which
		cannot pass this rcu_node because we hold its ->lock.

Therefore, despite initial appearances, the lockless check is safe.

Signed-off-by: Pranith Kumar <bobby.prani@gmail.com>
[ paulmck: Update comment to say why the lockless check is safe. ]
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
2014-07-09 09:15:01 -07:00
Paul E. McKenney
1146edcbef rcu: Loosen __call_rcu()'s rcu_head alignment constraint
The m68k architecture aligns only to 16-bit boundaries, which can cause
the align-to-32-bits check in __call_rcu() to trigger.  Because there is
currently no known potential need for more than one low-order bit, this
commit loosens the check to 16-bit boundaries.

Reported-by: Greg Ungerer <gerg@uclinux.org>
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Reviewed-by: Lai Jiangshan <laijs@cn.fujitsu.com>
2014-07-09 09:14:50 -07:00
Paul E. McKenney
a792563bd4 rcu: Eliminate read-modify-write ACCESS_ONCE() calls
RCU contains code of the following forms:

	ACCESS_ONCE(x)++;
	ACCESS_ONCE(x) += y;
	ACCESS_ONCE(x) -= y;

Now these constructs do operate correctly, but they really result in a
pair of volatile accesses, one to do the load and another to do the store.
This can be confusing, as the casual reader might well assume that (for
example) gcc might generate a memory-to-memory add instruction for each
of these three cases.  In fact, gcc will do no such thing.  Also, there
is a good chance that the kernel will move to separate load and store
variants of ACCESS_ONCE(), and constructs like the above could easily
confuse both people and scripts attempting to make that sort of change.
Finally, most of RCU's read-modify-write uses of ACCESS_ONCE() really
only need the store to be volatile, so that the read-modify-write form
might be misleading.

This commit therefore changes the above forms in RCU so that each instance
of ACCESS_ONCE() either does a load or a store, but not both.  In a few
cases, ACCESS_ONCE() was not critical, for example, for maintaining
statisitics.  In these cases, ACCESS_ONCE() has been dispensed with
entirely.

Suggested-by: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
2014-07-09 09:14:49 -07:00