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Earlier versions of RCU used the scheduling-clock tick to detect idleness by checking for the idle task, but handled idleness differently for CONFIG_NO_HZ=y. But there are now a number of uses of RCU read-side critical sections in the idle task, for example, for tracing. A more fine-grained detection of idleness is therefore required. This commit presses the old dyntick-idle code into full-time service, so that rcu_idle_enter(), previously known as rcu_enter_nohz(), is always invoked at the beginning of an idle loop iteration. Similarly, rcu_idle_exit(), previously known as rcu_exit_nohz(), is always invoked at the end of an idle-loop iteration. This allows the idle task to use RCU everywhere except between consecutive rcu_idle_enter() and rcu_idle_exit() calls, in turn allowing architecture maintainers to specify exactly where in the idle loop that RCU may be used. Because some of the userspace upcall uses can result in what looks to RCU like half of an interrupt, it is not possible to expect that the irq_enter() and irq_exit() hooks will give exact counts. This patch therefore expands the ->dynticks_nesting counter to 64 bits and uses two separate bitfields to count process/idle transitions and interrupt entry/exit transitions. It is presumed that userspace upcalls do not happen in the idle loop or from usermode execution (though usermode might do a system call that results in an upcall). The counter is hard-reset on each process/idle transition, which avoids the interrupt entry/exit error from accumulating. Overflow is avoided by the 64-bitness of the ->dyntick_nesting counter. This commit also adds warnings if a non-idle task asks RCU to enter idle state (and these checks will need some adjustment before applying Frederic's OS-jitter patches (http://lkml.org/lkml/2011/10/7/246). In addition, validation of ->dynticks and ->dynticks_nesting is added. Signed-off-by: Paul E. McKenney <paul.mckenney@linaro.org> Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com> Reviewed-by: Josh Triplett <josh@joshtriplett.org>
618 lines
27 KiB
Plaintext
618 lines
27 KiB
Plaintext
CONFIG_RCU_TRACE debugfs Files and Formats
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The rcutree and rcutiny implementations of RCU provide debugfs trace
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output that summarizes counters and state. This information is useful for
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debugging RCU itself, and can sometimes also help to debug abuses of RCU.
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The following sections describe the debugfs files and formats, first
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for rcutree and next for rcutiny.
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CONFIG_TREE_RCU and CONFIG_TREE_PREEMPT_RCU debugfs Files and Formats
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These implementations of RCU provides several debugfs files under the
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top-level directory "rcu":
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rcu/rcudata:
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Displays fields in struct rcu_data.
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rcu/rcudata.csv:
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Comma-separated values spreadsheet version of rcudata.
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rcu/rcugp:
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Displays grace-period counters.
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rcu/rcuhier:
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Displays the struct rcu_node hierarchy.
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rcu/rcu_pending:
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Displays counts of the reasons rcu_pending() decided that RCU had
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work to do.
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rcu/rcutorture:
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Displays rcutorture test progress.
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rcu/rcuboost:
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Displays RCU boosting statistics. Only present if
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CONFIG_RCU_BOOST=y.
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The output of "cat rcu/rcudata" looks as follows:
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rcu_sched:
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0 c=20972 g=20973 pq=1 pgp=20973 qp=0 dt=545/1/0 df=50 of=0 ri=0 ql=163 qs=NRW. kt=0/W/0 ktl=ebc3 b=10 ci=153737 co=0 ca=0
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1 c=20972 g=20973 pq=1 pgp=20973 qp=0 dt=967/1/0 df=58 of=0 ri=0 ql=634 qs=NRW. kt=0/W/1 ktl=58c b=10 ci=191037 co=0 ca=0
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2 c=20972 g=20973 pq=1 pgp=20973 qp=0 dt=1081/1/0 df=175 of=0 ri=0 ql=74 qs=N.W. kt=0/W/2 ktl=da94 b=10 ci=75991 co=0 ca=0
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3 c=20942 g=20943 pq=1 pgp=20942 qp=1 dt=1846/0/0 df=404 of=0 ri=0 ql=0 qs=.... kt=0/W/3 ktl=d1cd b=10 ci=72261 co=0 ca=0
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4 c=20972 g=20973 pq=1 pgp=20973 qp=0 dt=369/1/0 df=83 of=0 ri=0 ql=48 qs=N.W. kt=0/W/4 ktl=e0e7 b=10 ci=128365 co=0 ca=0
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5 c=20972 g=20973 pq=1 pgp=20973 qp=0 dt=381/1/0 df=64 of=0 ri=0 ql=169 qs=NRW. kt=0/W/5 ktl=fb2f b=10 ci=164360 co=0 ca=0
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6 c=20972 g=20973 pq=1 pgp=20973 qp=0 dt=1037/1/0 df=183 of=0 ri=0 ql=62 qs=N.W. kt=0/W/6 ktl=d2ad b=10 ci=65663 co=0 ca=0
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7 c=20897 g=20897 pq=1 pgp=20896 qp=0 dt=1572/0/0 df=382 of=0 ri=0 ql=0 qs=.... kt=0/W/7 ktl=cf15 b=10 ci=75006 co=0 ca=0
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rcu_bh:
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0 c=1480 g=1480 pq=1 pgp=1480 qp=0 dt=545/1/0 df=6 of=0 ri=1 ql=0 qs=.... kt=0/W/0 ktl=ebc3 b=10 ci=0 co=0 ca=0
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1 c=1480 g=1480 pq=1 pgp=1480 qp=0 dt=967/1/0 df=3 of=0 ri=1 ql=0 qs=.... kt=0/W/1 ktl=58c b=10 ci=151 co=0 ca=0
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2 c=1480 g=1480 pq=1 pgp=1480 qp=0 dt=1081/1/0 df=6 of=0 ri=1 ql=0 qs=.... kt=0/W/2 ktl=da94 b=10 ci=0 co=0 ca=0
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3 c=1480 g=1480 pq=1 pgp=1480 qp=0 dt=1846/0/0 df=8 of=0 ri=1 ql=0 qs=.... kt=0/W/3 ktl=d1cd b=10 ci=0 co=0 ca=0
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4 c=1480 g=1480 pq=1 pgp=1480 qp=0 dt=369/1/0 df=6 of=0 ri=1 ql=0 qs=.... kt=0/W/4 ktl=e0e7 b=10 ci=0 co=0 ca=0
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5 c=1480 g=1480 pq=1 pgp=1480 qp=0 dt=381/1/0 df=4 of=0 ri=1 ql=0 qs=.... kt=0/W/5 ktl=fb2f b=10 ci=0 co=0 ca=0
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6 c=1480 g=1480 pq=1 pgp=1480 qp=0 dt=1037/1/0 df=6 of=0 ri=1 ql=0 qs=.... kt=0/W/6 ktl=d2ad b=10 ci=0 co=0 ca=0
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7 c=1474 g=1474 pq=1 pgp=1473 qp=0 dt=1572/0/0 df=8 of=0 ri=1 ql=0 qs=.... kt=0/W/7 ktl=cf15 b=10 ci=0 co=0 ca=0
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The first section lists the rcu_data structures for rcu_sched, the second
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for rcu_bh. Note that CONFIG_TREE_PREEMPT_RCU kernels will have an
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additional section for rcu_preempt. Each section has one line per CPU,
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or eight for this 8-CPU system. The fields are as follows:
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o The number at the beginning of each line is the CPU number.
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CPUs numbers followed by an exclamation mark are offline,
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but have been online at least once since boot. There will be
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no output for CPUs that have never been online, which can be
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a good thing in the surprisingly common case where NR_CPUS is
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substantially larger than the number of actual CPUs.
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o "c" is the count of grace periods that this CPU believes have
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completed. Offlined CPUs and CPUs in dynticks idle mode may
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lag quite a ways behind, for example, CPU 6 under "rcu_sched"
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above, which has been offline through not quite 40,000 RCU grace
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periods. It is not unusual to see CPUs lagging by thousands of
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grace periods.
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o "g" is the count of grace periods that this CPU believes have
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started. Again, offlined CPUs and CPUs in dynticks idle mode
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may lag behind. If the "c" and "g" values are equal, this CPU
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has already reported a quiescent state for the last RCU grace
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period that it is aware of, otherwise, the CPU believes that it
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owes RCU a quiescent state.
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o "pq" indicates that this CPU has passed through a quiescent state
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for the current grace period. It is possible for "pq" to be
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"1" and "c" different than "g", which indicates that although
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the CPU has passed through a quiescent state, either (1) this
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CPU has not yet reported that fact, (2) some other CPU has not
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yet reported for this grace period, or (3) both.
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o "pgp" indicates which grace period the last-observed quiescent
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state for this CPU corresponds to. This is important for handling
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the race between CPU 0 reporting an extended dynticks-idle
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quiescent state for CPU 1 and CPU 1 suddenly waking up and
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reporting its own quiescent state. If CPU 1 was the last CPU
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for the current grace period, then the CPU that loses this race
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will attempt to incorrectly mark CPU 1 as having checked in for
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the next grace period!
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o "qp" indicates that RCU still expects a quiescent state from
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this CPU. Offlined CPUs and CPUs in dyntick idle mode might
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well have qp=1, which is OK: RCU is still ignoring them.
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o "dt" is the current value of the dyntick counter that is incremented
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when entering or leaving dynticks idle state, either by the
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scheduler or by irq. This number is even if the CPU is in
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dyntick idle mode and odd otherwise. The number after the first
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"/" is the interrupt nesting depth when in dyntick-idle state,
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or one greater than the interrupt-nesting depth otherwise.
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The number after the second "/" is the NMI nesting depth.
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o "df" is the number of times that some other CPU has forced a
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quiescent state on behalf of this CPU due to this CPU being in
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dynticks-idle state.
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o "of" is the number of times that some other CPU has forced a
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quiescent state on behalf of this CPU due to this CPU being
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offline. In a perfect world, this might never happen, but it
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turns out that offlining and onlining a CPU can take several grace
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periods, and so there is likely to be an extended period of time
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when RCU believes that the CPU is online when it really is not.
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Please note that erring in the other direction (RCU believing a
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CPU is offline when it is really alive and kicking) is a fatal
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error, so it makes sense to err conservatively.
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o "ri" is the number of times that RCU has seen fit to send a
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reschedule IPI to this CPU in order to get it to report a
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quiescent state.
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o "ql" is the number of RCU callbacks currently residing on
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this CPU. This is the total number of callbacks, regardless
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of what state they are in (new, waiting for grace period to
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start, waiting for grace period to end, ready to invoke).
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o "qs" gives an indication of the state of the callback queue
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with four characters:
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"N" Indicates that there are callbacks queued that are not
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ready to be handled by the next grace period, and thus
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will be handled by the grace period following the next
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one.
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"R" Indicates that there are callbacks queued that are
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ready to be handled by the next grace period.
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"W" Indicates that there are callbacks queued that are
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waiting on the current grace period.
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"D" Indicates that there are callbacks queued that have
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already been handled by a prior grace period, and are
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thus waiting to be invoked. Note that callbacks in
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the process of being invoked are not counted here.
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Callbacks in the process of being invoked are those
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that have been removed from the rcu_data structures
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queues by rcu_do_batch(), but which have not yet been
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invoked.
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If there are no callbacks in a given one of the above states,
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the corresponding character is replaced by ".".
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o "kt" is the per-CPU kernel-thread state. The digit preceding
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the first slash is zero if there is no work pending and 1
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otherwise. The character between the first pair of slashes is
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as follows:
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"S" The kernel thread is stopped, in other words, all
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CPUs corresponding to this rcu_node structure are
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offline.
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"R" The kernel thread is running.
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"W" The kernel thread is waiting because there is no work
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for it to do.
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"O" The kernel thread is waiting because it has been
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forced off of its designated CPU or because its
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->cpus_allowed mask permits it to run on other than
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its designated CPU.
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"Y" The kernel thread is yielding to avoid hogging CPU.
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"?" Unknown value, indicates a bug.
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The number after the final slash is the CPU that the kthread
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is actually running on.
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This field is displayed only for CONFIG_RCU_BOOST kernels.
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o "ktl" is the low-order 16 bits (in hexadecimal) of the count of
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the number of times that this CPU's per-CPU kthread has gone
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through its loop servicing invoke_rcu_cpu_kthread() requests.
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This field is displayed only for CONFIG_RCU_BOOST kernels.
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o "b" is the batch limit for this CPU. If more than this number
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of RCU callbacks is ready to invoke, then the remainder will
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be deferred.
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o "ci" is the number of RCU callbacks that have been invoked for
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this CPU. Note that ci+ql is the number of callbacks that have
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been registered in absence of CPU-hotplug activity.
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o "co" is the number of RCU callbacks that have been orphaned due to
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this CPU going offline. These orphaned callbacks have been moved
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to an arbitrarily chosen online CPU.
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o "ca" is the number of RCU callbacks that have been adopted due to
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other CPUs going offline. Note that ci+co-ca+ql is the number of
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RCU callbacks registered on this CPU.
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There is also an rcu/rcudata.csv file with the same information in
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comma-separated-variable spreadsheet format.
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The output of "cat rcu/rcugp" looks as follows:
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rcu_sched: completed=33062 gpnum=33063
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rcu_bh: completed=464 gpnum=464
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Again, this output is for both "rcu_sched" and "rcu_bh". Note that
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kernels built with CONFIG_TREE_PREEMPT_RCU will have an additional
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"rcu_preempt" line. The fields are taken from the rcu_state structure,
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and are as follows:
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o "completed" is the number of grace periods that have completed.
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It is comparable to the "c" field from rcu/rcudata in that a
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CPU whose "c" field matches the value of "completed" is aware
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that the corresponding RCU grace period has completed.
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o "gpnum" is the number of grace periods that have started. It is
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comparable to the "g" field from rcu/rcudata in that a CPU
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whose "g" field matches the value of "gpnum" is aware that the
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corresponding RCU grace period has started.
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If these two fields are equal (as they are for "rcu_bh" above),
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then there is no grace period in progress, in other words, RCU
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is idle. On the other hand, if the two fields differ (as they
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do for "rcu_sched" above), then an RCU grace period is in progress.
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The output of "cat rcu/rcuhier" looks as follows, with very long lines:
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c=6902 g=6903 s=2 jfq=3 j=72c7 nfqs=13142/nfqsng=0(13142) fqlh=6
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1/1 ..>. 0:127 ^0
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3/3 ..>. 0:35 ^0 0/0 ..>. 36:71 ^1 0/0 ..>. 72:107 ^2 0/0 ..>. 108:127 ^3
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3/3f ..>. 0:5 ^0 2/3 ..>. 6:11 ^1 0/0 ..>. 12:17 ^2 0/0 ..>. 18:23 ^3 0/0 ..>. 24:29 ^4 0/0 ..>. 30:35 ^5 0/0 ..>. 36:41 ^0 0/0 ..>. 42:47 ^1 0/0 ..>. 48:53 ^2 0/0 ..>. 54:59 ^3 0/0 ..>. 60:65 ^4 0/0 ..>. 66:71 ^5 0/0 ..>. 72:77 ^0 0/0 ..>. 78:83 ^1 0/0 ..>. 84:89 ^2 0/0 ..>. 90:95 ^3 0/0 ..>. 96:101 ^4 0/0 ..>. 102:107 ^5 0/0 ..>. 108:113 ^0 0/0 ..>. 114:119 ^1 0/0 ..>. 120:125 ^2 0/0 ..>. 126:127 ^3
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rcu_bh:
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c=-226 g=-226 s=1 jfq=-5701 j=72c7 nfqs=88/nfqsng=0(88) fqlh=0
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0/1 ..>. 0:127 ^0
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0/3 ..>. 0:35 ^0 0/0 ..>. 36:71 ^1 0/0 ..>. 72:107 ^2 0/0 ..>. 108:127 ^3
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0/3f ..>. 0:5 ^0 0/3 ..>. 6:11 ^1 0/0 ..>. 12:17 ^2 0/0 ..>. 18:23 ^3 0/0 ..>. 24:29 ^4 0/0 ..>. 30:35 ^5 0/0 ..>. 36:41 ^0 0/0 ..>. 42:47 ^1 0/0 ..>. 48:53 ^2 0/0 ..>. 54:59 ^3 0/0 ..>. 60:65 ^4 0/0 ..>. 66:71 ^5 0/0 ..>. 72:77 ^0 0/0 ..>. 78:83 ^1 0/0 ..>. 84:89 ^2 0/0 ..>. 90:95 ^3 0/0 ..>. 96:101 ^4 0/0 ..>. 102:107 ^5 0/0 ..>. 108:113 ^0 0/0 ..>. 114:119 ^1 0/0 ..>. 120:125 ^2 0/0 ..>. 126:127 ^3
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This is once again split into "rcu_sched" and "rcu_bh" portions,
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and CONFIG_TREE_PREEMPT_RCU kernels will again have an additional
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"rcu_preempt" section. The fields are as follows:
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o "c" is exactly the same as "completed" under rcu/rcugp.
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o "g" is exactly the same as "gpnum" under rcu/rcugp.
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o "s" is the "signaled" state that drives force_quiescent_state()'s
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state machine.
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o "jfq" is the number of jiffies remaining for this grace period
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before force_quiescent_state() is invoked to help push things
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along. Note that CPUs in dyntick-idle mode throughout the grace
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period will not report on their own, but rather must be check by
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some other CPU via force_quiescent_state().
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o "j" is the low-order four hex digits of the jiffies counter.
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Yes, Paul did run into a number of problems that turned out to
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be due to the jiffies counter no longer counting. Why do you ask?
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o "nfqs" is the number of calls to force_quiescent_state() since
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boot.
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o "nfqsng" is the number of useless calls to force_quiescent_state(),
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where there wasn't actually a grace period active. This can
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happen due to races. The number in parentheses is the difference
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between "nfqs" and "nfqsng", or the number of times that
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force_quiescent_state() actually did some real work.
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o "fqlh" is the number of calls to force_quiescent_state() that
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exited immediately (without even being counted in nfqs above)
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due to contention on ->fqslock.
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o Each element of the form "1/1 0:127 ^0" represents one struct
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rcu_node. Each line represents one level of the hierarchy, from
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root to leaves. It is best to think of the rcu_data structures
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as forming yet another level after the leaves. Note that there
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might be either one, two, or three levels of rcu_node structures,
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depending on the relationship between CONFIG_RCU_FANOUT and
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CONFIG_NR_CPUS.
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o The numbers separated by the "/" are the qsmask followed
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by the qsmaskinit. The qsmask will have one bit
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set for each entity in the next lower level that
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has not yet checked in for the current grace period.
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The qsmaskinit will have one bit for each entity that is
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currently expected to check in during each grace period.
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The value of qsmaskinit is assigned to that of qsmask
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at the beginning of each grace period.
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For example, for "rcu_sched", the qsmask of the first
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entry of the lowest level is 0x14, meaning that we
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are still waiting for CPUs 2 and 4 to check in for the
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current grace period.
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o The characters separated by the ">" indicate the state
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of the blocked-tasks lists. A "G" preceding the ">"
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indicates that at least one task blocked in an RCU
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read-side critical section blocks the current grace
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period, while a "E" preceding the ">" indicates that
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at least one task blocked in an RCU read-side critical
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section blocks the current expedited grace period.
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A "T" character following the ">" indicates that at
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least one task is blocked within an RCU read-side
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critical section, regardless of whether any current
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grace period (expedited or normal) is inconvenienced.
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A "." character appears if the corresponding condition
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does not hold, so that "..>." indicates that no tasks
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are blocked. In contrast, "GE>T" indicates maximal
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inconvenience from blocked tasks.
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o The numbers separated by the ":" are the range of CPUs
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served by this struct rcu_node. This can be helpful
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in working out how the hierarchy is wired together.
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For example, the first entry at the lowest level shows
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"0:5", indicating that it covers CPUs 0 through 5.
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o The number after the "^" indicates the bit in the
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next higher level rcu_node structure that this
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rcu_node structure corresponds to.
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For example, the first entry at the lowest level shows
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"^0", indicating that it corresponds to bit zero in
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the first entry at the middle level.
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The output of "cat rcu/rcu_pending" looks as follows:
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rcu_sched:
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0 np=255892 qsp=53936 rpq=85 cbr=0 cng=14417 gpc=10033 gps=24320 nf=6445 nn=146741
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1 np=261224 qsp=54638 rpq=33 cbr=0 cng=25723 gpc=16310 gps=2849 nf=5912 nn=155792
|
|
2 np=237496 qsp=49664 rpq=23 cbr=0 cng=2762 gpc=45478 gps=1762 nf=1201 nn=136629
|
|
3 np=236249 qsp=48766 rpq=98 cbr=0 cng=286 gpc=48049 gps=1218 nf=207 nn=137723
|
|
4 np=221310 qsp=46850 rpq=7 cbr=0 cng=26 gpc=43161 gps=4634 nf=3529 nn=123110
|
|
5 np=237332 qsp=48449 rpq=9 cbr=0 cng=54 gpc=47920 gps=3252 nf=201 nn=137456
|
|
6 np=219995 qsp=46718 rpq=12 cbr=0 cng=50 gpc=42098 gps=6093 nf=4202 nn=120834
|
|
7 np=249893 qsp=49390 rpq=42 cbr=0 cng=72 gpc=38400 gps=17102 nf=41 nn=144888
|
|
rcu_bh:
|
|
0 np=146741 qsp=1419 rpq=6 cbr=0 cng=6 gpc=0 gps=0 nf=2 nn=145314
|
|
1 np=155792 qsp=12597 rpq=3 cbr=0 cng=0 gpc=4 gps=8 nf=3 nn=143180
|
|
2 np=136629 qsp=18680 rpq=1 cbr=0 cng=0 gpc=7 gps=6 nf=0 nn=117936
|
|
3 np=137723 qsp=2843 rpq=0 cbr=0 cng=0 gpc=10 gps=7 nf=0 nn=134863
|
|
4 np=123110 qsp=12433 rpq=0 cbr=0 cng=0 gpc=4 gps=2 nf=0 nn=110671
|
|
5 np=137456 qsp=4210 rpq=1 cbr=0 cng=0 gpc=6 gps=5 nf=0 nn=133235
|
|
6 np=120834 qsp=9902 rpq=2 cbr=0 cng=0 gpc=6 gps=3 nf=2 nn=110921
|
|
7 np=144888 qsp=26336 rpq=0 cbr=0 cng=0 gpc=8 gps=2 nf=0 nn=118542
|
|
|
|
As always, this is once again split into "rcu_sched" and "rcu_bh"
|
|
portions, with CONFIG_TREE_PREEMPT_RCU kernels having an additional
|
|
"rcu_preempt" section. The fields are as follows:
|
|
|
|
o "np" is the number of times that __rcu_pending() has been invoked
|
|
for the corresponding flavor of RCU.
|
|
|
|
o "qsp" is the number of times that the RCU was waiting for a
|
|
quiescent state from this CPU.
|
|
|
|
o "rpq" is the number of times that the CPU had passed through
|
|
a quiescent state, but not yet reported it to RCU.
|
|
|
|
o "cbr" is the number of times that this CPU had RCU callbacks
|
|
that had passed through a grace period, and were thus ready
|
|
to be invoked.
|
|
|
|
o "cng" is the number of times that this CPU needed another
|
|
grace period while RCU was idle.
|
|
|
|
o "gpc" is the number of times that an old grace period had
|
|
completed, but this CPU was not yet aware of it.
|
|
|
|
o "gps" is the number of times that a new grace period had started,
|
|
but this CPU was not yet aware of it.
|
|
|
|
o "nf" is the number of times that this CPU suspected that the
|
|
current grace period had run for too long, and thus needed to
|
|
be forced.
|
|
|
|
Please note that "forcing" consists of sending resched IPIs
|
|
to holdout CPUs. If that CPU really still is in an old RCU
|
|
read-side critical section, then we really do have to wait for it.
|
|
The assumption behing "forcing" is that the CPU is not still in
|
|
an old RCU read-side critical section, but has not yet responded
|
|
for some other reason.
|
|
|
|
o "nn" is the number of times that this CPU needed nothing. Alert
|
|
readers will note that the rcu "nn" number for a given CPU very
|
|
closely matches the rcu_bh "np" number for that same CPU. This
|
|
is due to short-circuit evaluation in rcu_pending().
|
|
|
|
|
|
The output of "cat rcu/rcutorture" looks as follows:
|
|
|
|
rcutorture test sequence: 0 (test in progress)
|
|
rcutorture update version number: 615
|
|
|
|
The first line shows the number of rcutorture tests that have completed
|
|
since boot. If a test is currently running, the "(test in progress)"
|
|
string will appear as shown above. The second line shows the number of
|
|
update cycles that the current test has started, or zero if there is
|
|
no test in progress.
|
|
|
|
|
|
The output of "cat rcu/rcuboost" looks as follows:
|
|
|
|
0:5 tasks=.... kt=W ntb=0 neb=0 nnb=0 j=2f95 bt=300f
|
|
balk: nt=0 egt=989 bt=0 nb=0 ny=0 nos=16
|
|
6:7 tasks=.... kt=W ntb=0 neb=0 nnb=0 j=2f95 bt=300f
|
|
balk: nt=0 egt=225 bt=0 nb=0 ny=0 nos=6
|
|
|
|
This information is output only for rcu_preempt. Each two-line entry
|
|
corresponds to a leaf rcu_node strcuture. The fields are as follows:
|
|
|
|
o "n:m" is the CPU-number range for the corresponding two-line
|
|
entry. In the sample output above, the first entry covers
|
|
CPUs zero through five and the second entry covers CPUs 6
|
|
and 7.
|
|
|
|
o "tasks=TNEB" gives the state of the various segments of the
|
|
rnp->blocked_tasks list:
|
|
|
|
"T" This indicates that there are some tasks that blocked
|
|
while running on one of the corresponding CPUs while
|
|
in an RCU read-side critical section.
|
|
|
|
"N" This indicates that some of the blocked tasks are preventing
|
|
the current normal (non-expedited) grace period from
|
|
completing.
|
|
|
|
"E" This indicates that some of the blocked tasks are preventing
|
|
the current expedited grace period from completing.
|
|
|
|
"B" This indicates that some of the blocked tasks are in
|
|
need of RCU priority boosting.
|
|
|
|
Each character is replaced with "." if the corresponding
|
|
condition does not hold.
|
|
|
|
o "kt" is the state of the RCU priority-boosting kernel
|
|
thread associated with the corresponding rcu_node structure.
|
|
The state can be one of the following:
|
|
|
|
"S" The kernel thread is stopped, in other words, all
|
|
CPUs corresponding to this rcu_node structure are
|
|
offline.
|
|
|
|
"R" The kernel thread is running.
|
|
|
|
"W" The kernel thread is waiting because there is no work
|
|
for it to do.
|
|
|
|
"Y" The kernel thread is yielding to avoid hogging CPU.
|
|
|
|
"?" Unknown value, indicates a bug.
|
|
|
|
o "ntb" is the number of tasks boosted.
|
|
|
|
o "neb" is the number of tasks boosted in order to complete an
|
|
expedited grace period.
|
|
|
|
o "nnb" is the number of tasks boosted in order to complete a
|
|
normal (non-expedited) grace period. When boosting a task
|
|
that was blocking both an expedited and a normal grace period,
|
|
it is counted against the expedited total above.
|
|
|
|
o "j" is the low-order 16 bits of the jiffies counter in
|
|
hexadecimal.
|
|
|
|
o "bt" is the low-order 16 bits of the value that the jiffies
|
|
counter will have when we next start boosting, assuming that
|
|
the current grace period does not end beforehand. This is
|
|
also in hexadecimal.
|
|
|
|
o "balk: nt" counts the number of times we didn't boost (in
|
|
other words, we balked) even though it was time to boost because
|
|
there were no blocked tasks to boost. This situation occurs
|
|
when there is one blocked task on one rcu_node structure and
|
|
none on some other rcu_node structure.
|
|
|
|
o "egt" counts the number of times we balked because although
|
|
there were blocked tasks, none of them were blocking the
|
|
current grace period, whether expedited or otherwise.
|
|
|
|
o "bt" counts the number of times we balked because boosting
|
|
had already been initiated for the current grace period.
|
|
|
|
o "nb" counts the number of times we balked because there
|
|
was at least one task blocking the current non-expedited grace
|
|
period that never had blocked. If it is already running, it
|
|
just won't help to boost its priority!
|
|
|
|
o "ny" counts the number of times we balked because it was
|
|
not yet time to start boosting.
|
|
|
|
o "nos" counts the number of times we balked for other
|
|
reasons, e.g., the grace period ended first.
|
|
|
|
|
|
CONFIG_TINY_RCU and CONFIG_TINY_PREEMPT_RCU debugfs Files and Formats
|
|
|
|
These implementations of RCU provides a single debugfs file under the
|
|
top-level directory RCU, namely rcu/rcudata, which displays fields in
|
|
rcu_bh_ctrlblk, rcu_sched_ctrlblk and, for CONFIG_TINY_PREEMPT_RCU,
|
|
rcu_preempt_ctrlblk.
|
|
|
|
The output of "cat rcu/rcudata" is as follows:
|
|
|
|
rcu_preempt: qlen=24 gp=1097669 g197/p197/c197 tasks=...
|
|
ttb=. btg=no ntb=184 neb=0 nnb=183 j=01f7 bt=0274
|
|
normal balk: nt=1097669 gt=0 bt=371 b=0 ny=25073378 nos=0
|
|
exp balk: bt=0 nos=0
|
|
rcu_sched: qlen: 0
|
|
rcu_bh: qlen: 0
|
|
|
|
This is split into rcu_preempt, rcu_sched, and rcu_bh sections, with the
|
|
rcu_preempt section appearing only in CONFIG_TINY_PREEMPT_RCU builds.
|
|
The last three lines of the rcu_preempt section appear only in
|
|
CONFIG_RCU_BOOST kernel builds. The fields are as follows:
|
|
|
|
o "qlen" is the number of RCU callbacks currently waiting either
|
|
for an RCU grace period or waiting to be invoked. This is the
|
|
only field present for rcu_sched and rcu_bh, due to the
|
|
short-circuiting of grace period in those two cases.
|
|
|
|
o "gp" is the number of grace periods that have completed.
|
|
|
|
o "g197/p197/c197" displays the grace-period state, with the
|
|
"g" number being the number of grace periods that have started
|
|
(mod 256), the "p" number being the number of grace periods
|
|
that the CPU has responded to (also mod 256), and the "c"
|
|
number being the number of grace periods that have completed
|
|
(once again mode 256).
|
|
|
|
Why have both "gp" and "g"? Because the data flowing into
|
|
"gp" is only present in a CONFIG_RCU_TRACE kernel.
|
|
|
|
o "tasks" is a set of bits. The first bit is "T" if there are
|
|
currently tasks that have recently blocked within an RCU
|
|
read-side critical section, the second bit is "N" if any of the
|
|
aforementioned tasks are blocking the current RCU grace period,
|
|
and the third bit is "E" if any of the aforementioned tasks are
|
|
blocking the current expedited grace period. Each bit is "."
|
|
if the corresponding condition does not hold.
|
|
|
|
o "ttb" is a single bit. It is "B" if any of the blocked tasks
|
|
need to be priority boosted and "." otherwise.
|
|
|
|
o "btg" indicates whether boosting has been carried out during
|
|
the current grace period, with "exp" indicating that boosting
|
|
is in progress for an expedited grace period, "no" indicating
|
|
that boosting has not yet started for a normal grace period,
|
|
"begun" indicating that boosting has bebug for a normal grace
|
|
period, and "done" indicating that boosting has completed for
|
|
a normal grace period.
|
|
|
|
o "ntb" is the total number of tasks subjected to RCU priority boosting
|
|
periods since boot.
|
|
|
|
o "neb" is the number of expedited grace periods that have had
|
|
to resort to RCU priority boosting since boot.
|
|
|
|
o "nnb" is the number of normal grace periods that have had
|
|
to resort to RCU priority boosting since boot.
|
|
|
|
o "j" is the low-order 16 bits of the jiffies counter in hexadecimal.
|
|
|
|
o "bt" is the low-order 16 bits of the value that the jiffies counter
|
|
will have at the next time that boosting is scheduled to begin.
|
|
|
|
o In the line beginning with "normal balk", the fields are as follows:
|
|
|
|
o "nt" is the number of times that the system balked from
|
|
boosting because there were no blocked tasks to boost.
|
|
Note that the system will balk from boosting even if the
|
|
grace period is overdue when the currently running task
|
|
is looping within an RCU read-side critical section.
|
|
There is no point in boosting in this case, because
|
|
boosting a running task won't make it run any faster.
|
|
|
|
o "gt" is the number of times that the system balked
|
|
from boosting because, although there were blocked tasks,
|
|
none of them were preventing the current grace period
|
|
from completing.
|
|
|
|
o "bt" is the number of times that the system balked
|
|
from boosting because boosting was already in progress.
|
|
|
|
o "b" is the number of times that the system balked from
|
|
boosting because boosting had already completed for
|
|
the grace period in question.
|
|
|
|
o "ny" is the number of times that the system balked from
|
|
boosting because it was not yet time to start boosting
|
|
the grace period in question.
|
|
|
|
o "nos" is the number of times that the system balked from
|
|
boosting for inexplicable ("not otherwise specified")
|
|
reasons. This can actually happen due to races involving
|
|
increments of the jiffies counter.
|
|
|
|
o In the line beginning with "exp balk", the fields are as follows:
|
|
|
|
o "bt" is the number of times that the system balked from
|
|
boosting because there were no blocked tasks to boost.
|
|
|
|
o "nos" is the number of times that the system balked from
|
|
boosting for inexplicable ("not otherwise specified")
|
|
reasons.
|