The exact, general goal of the function bfq_split_bfqq() is not that
apparent. Add a comment to make it clear.
Tested-by: Oleksandr Natalenko <oleksandr@natalenko.name>
Signed-off-by: Paolo Valente <paolo.valente@linaro.org>
Signed-off-by: Jens Axboe <axboe@kernel.dk>
The flag on_st in the bfq_entity data structure is true if the entity
is on a service tree or is in service. Yet the name of the field,
confusingly, does not mention the second, very important case. Extend
the name to mention the second case too.
Tested-by: Oleksandr Natalenko <oleksandr@natalenko.name>
Signed-off-by: Paolo Valente <paolo.valente@linaro.org>
Signed-off-by: Jens Axboe <axboe@kernel.dk>
BFQ maintains an ordered list, implemented with an RB tree, of
head-request positions of non-empty bfq_queues. This position tree,
inherited from CFQ, is used to find bfq_queues that contain I/O close
to each other. BFQ merges these bfq_queues into a single shared queue,
if this boosts throughput on the device at hand.
There is however a special-purpose bfq_queue that does not participate
in queue merging, the oom bfq_queue. Yet, also this bfq_queue could be
wrongly added to the position tree. So bfqq_find_close() could return
the oom bfq_queue, which is a source of further troubles in an
out-of-memory situation. This commit prevents the oom bfq_queue from
being inserted into the position tree.
Tested-by: Patrick Dung <patdung100@gmail.com>
Tested-by: Oleksandr Natalenko <oleksandr@natalenko.name>
Signed-off-by: Paolo Valente <paolo.valente@linaro.org>
Signed-off-by: Jens Axboe <axboe@kernel.dk>
Commit 478de3380c ("block, bfq: deschedule empty bfq_queues not
referred by any process") fixed commit 3726112ec7 ("block, bfq:
re-schedule empty queues if they deserve I/O plugging") by
descheduling an empty bfq_queue when it remains with not process
reference. Yet, this still left a case uncovered: an empty bfq_queue
with not process reference that remains in service. This happens for
an in-service sync bfq_queue that is deemed to deserve I/O-dispatch
plugging when it remains empty. Yet no new requests will arrive for
such a bfq_queue if no process sends requests to it any longer. Even
worse, the bfq_queue may happen to be prematurely freed while still in
service (because there may remain no reference to it any longer).
This commit solves this problem by preventing I/O dispatch from being
plugged for the in-service bfq_queue, if the latter has no process
reference (the bfq_queue is then prevented from remaining in service).
Fixes: 3726112ec7 ("block, bfq: re-schedule empty queues if they deserve I/O plugging")
Tested-by: Oleksandr Natalenko <oleksandr@natalenko.name>
Reported-by: Patrick Dung <patdung100@gmail.com>
Tested-by: Patrick Dung <patdung100@gmail.com>
Signed-off-by: Paolo Valente <paolo.valente@linaro.org>
Signed-off-by: Jens Axboe <axboe@kernel.dk>
This macro is never used after introduced from commit aee69d78de
("block, bfq: introduce the BFQ-v0 I/O scheduler as an extra scheduler")
Better to remove it.
Signed-off-by: Alex Shi <alex.shi@linux.alibaba.com>
Cc: Paolo Valente <paolo.valente@linaro.org>
Cc: Jens Axboe <axboe@kernel.dk>
Cc: linux-block@vger.kernel.org
Cc: linux-kernel@vger.kernel.org
Signed-off-by: Jens Axboe <axboe@kernel.dk>
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Merge tag 'for-5.5/block-20191121' of git://git.kernel.dk/linux-block
Pull core block updates from Jens Axboe:
"Due to more granular branches, this one is small and will be followed
with other core branches that add specific features. I meant to just
have a core and drivers branch, but external dependencies we ended up
adding a few more that are also core.
The changes are:
- Fixes and improvements for the zoned device support (Ajay, Damien)
- sed-opal table writing and datastore UID (Revanth)
- blk-cgroup (and bfq) blk-cgroup stat fixes (Tejun)
- Improvements to the block stats tracking (Pavel)
- Fix for overruning sysfs buffer for large number of CPUs (Ming)
- Optimization for small IO (Ming, Christoph)
- Fix typo in RWH lifetime hint (Eugene)
- Dead code removal and documentation (Bart)
- Reduction in memory usage for queue and tag set (Bart)
- Kerneldoc header documentation (André)
- Device/partition revalidation fixes (Jan)
- Stats tracking for flush requests (Konstantin)
- Various other little fixes here and there (et al)"
* tag 'for-5.5/block-20191121' of git://git.kernel.dk/linux-block: (48 commits)
Revert "block: split bio if the only bvec's length is > SZ_4K"
block: add iostat counters for flush requests
block,bfq: Skip tracing hooks if possible
block: sed-opal: Introduce SUM_SET_LIST parameter and append it using 'add_token_u64'
blk-cgroup: cgroup_rstat_updated() shouldn't be called on cgroup1
block: Don't disable interrupts in trigger_softirq()
sbitmap: Delete sbitmap_any_bit_clear()
blk-mq: Delete blk_mq_has_free_tags() and blk_mq_can_queue()
block: split bio if the only bvec's length is > SZ_4K
block: still try to split bio if the bvec crosses pages
blk-cgroup: separate out blkg_rwstat under CONFIG_BLK_CGROUP_RWSTAT
blk-cgroup: reimplement basic IO stats using cgroup rstat
blk-cgroup: remove now unused blkg_print_stat_{bytes|ios}_recursive()
blk-throtl: stop using blkg->stat_bytes and ->stat_ios
bfq-iosched: stop using blkg->stat_bytes and ->stat_ios
bfq-iosched: relocate bfqg_*rwstat*() helpers
block: add zone open, close and finish ioctl support
block: add zone open, close and finish operations
block: Simplify REQ_OP_ZONE_RESET_ALL handling
block: Remove REQ_OP_ZONE_RESET plugging
...
Since commit 3726112ec7 ("block, bfq: re-schedule empty queues if
they deserve I/O plugging"), to prevent the service guarantees of a
bfq_queue from being violated, the bfq_queue may be left busy, i.e.,
scheduled for service, even if empty (see comments in
__bfq_bfqq_expire() for details). But, if no process will send
requests to the bfq_queue any longer, then there is no point in
keeping the bfq_queue scheduled for service.
In addition, keeping the bfq_queue scheduled for service, but with no
process reference any longer, may cause the bfq_queue to be freed when
descheduled from service. But this is assumed to never happen, and
causes a UAF if it happens. This, in turn, caused crashes [1, 2].
This commit fixes this issue by descheduling an empty bfq_queue when
it remains with not process reference.
[1] https://bugzilla.redhat.com/show_bug.cgi?id=1767539
[2] https://bugzilla.kernel.org/show_bug.cgi?id=205447
Fixes: 3726112ec7 ("block, bfq: re-schedule empty queues if they deserve I/O plugging")
Reported-by: Chris Evich <cevich@redhat.com>
Reported-by: Patrick Dung <patdung100@gmail.com>
Reported-by: Thorsten Schubert <tschubert@bafh.org>
Tested-by: Thorsten Schubert <tschubert@bafh.org>
Tested-by: Oleksandr Natalenko <oleksandr@natalenko.name>
Signed-off-by: Paolo Valente <paolo.valente@linaro.org>
Signed-off-by: Jens Axboe <axboe@kernel.dk>
When used on cgroup1, bfq uses the blkg->stat_bytes and ->stat_ios
from blk-cgroup core to populate six stat knobs. blk-cgroup core is
moving away from blkg_rwstat to improve scalability and won't be able
to support this usage.
It isn't like the sharing gains all that much. Let's break it out to
dedicated rwstat counters which are updated when on cgroup1. This
makes use of bfqg_*rwstat*() helpers outside of
CONFIG_BFQ_CGROUP_DEBUG. Move them out.
v2: Compile fix when !CONFIG_BFQ_CGROUP_DEBUG.
Signed-off-by: Tejun Heo <tj@kernel.org>
Cc: Paolo Valente <paolo.valente@linaro.org>
Signed-off-by: Jens Axboe <axboe@kernel.dk>
If equal to 0, the injection limit for a bfq_queue is pushed to 1
after a first sample of the total service time of the I/O requests of
the queue is computed (to allow injection to start). Yet, because of a
mistake in the branch that performs this action, the push may happen
also in some other case. This commit fixes this issue.
Tested-by: Oleksandr Natalenko <oleksandr@natalenko.name>
Signed-off-by: Paolo Valente <paolo.valente@linaro.org>
Signed-off-by: Jens Axboe <axboe@kernel.dk>
The update period of the injection limit has been tentatively set to
100 ms, to reduce fluctuations. This value however proved to cause,
occasionally, the limit to be decremented for some bfq_queue only
after the queue underwent excessive injection for a lot of time. This
commit reduces the period to 10 ms.
Tested-by: Oleksandr Natalenko <oleksandr@natalenko.name>
Signed-off-by: Paolo Valente <paolo.valente@linaro.org>
Signed-off-by: Jens Axboe <axboe@kernel.dk>
Upon an increment attempt of the injection limit, the latter is
constrained not to become higher than twice the maximum number
max_rq_in_driver of I/O requests that have happened to be in service
in the drive. This high bound allows the injection limit to grow
beyond max_rq_in_driver, which may then cause max_rq_in_driver itself
to grow.
However, since the limit is incremented by only one unit at a time,
there is no need for such a high bound, and just max_rq_in_driver+1 is
enough.
Tested-by: Oleksandr Natalenko <oleksandr@natalenko.name>
Signed-off-by: Paolo Valente <paolo.valente@linaro.org>
Signed-off-by: Jens Axboe <axboe@kernel.dk>
BFQ updates the injection limit of each bfq_queue as a function of how
much the limit inflates the service times experienced by the I/O
requests of the queue. So only service times affected by injection
must be taken into account. Unfortunately, in the current
implementation of this update scheme, the service time of an I/O
request rq not affected by injection may happen to be considered in
the following case: there is no I/O request in service when rq
arrives.
This commit fixes this issue by making sure that only service times
affected by injection are considered for updating the injection
limit. In particular, the service time of an I/O request rq is now
considered only if at least one of the following two conditions holds:
- the destination bfq_queue for rq underwent injection before rq
arrival, and there is still I/O in service in the drive on rq arrival
(the service of such unfinished I/O may delay the service of rq);
- injection occurs between the arrival and the completion time of rq.
Tested-by: Oleksandr Natalenko <oleksandr@natalenko.name>
Signed-off-by: Paolo Valente <paolo.valente@linaro.org>
Signed-off-by: Jens Axboe <axboe@kernel.dk>
As reported in [1], the call bfq_init_rq(rq) may return NULL in case
of OOM (in particular, if rq->elv.icq is NULL because memory
allocation failed in failed in ioc_create_icq()).
This commit handles this circumstance.
[1] https://lkml.org/lkml/2019/7/22/824
Cc: Hsin-Yi Wang <hsinyi@google.com>
Cc: Nicolas Boichat <drinkcat@chromium.org>
Cc: Doug Anderson <dianders@chromium.org>
Reported-by: Guenter Roeck <linux@roeck-us.net>
Reported-by: Hsin-Yi Wang <hsinyi@google.com>
Reviewed-by: Guenter Roeck <linux@roeck-us.net>
Signed-off-by: Paolo Valente <paolo.valente@linaro.org>
Signed-off-by: Jens Axboe <axboe@kernel.dk>
Since commit 13a857a4c4 ("block, bfq: detect wakers and
unconditionally inject their I/O"), every bfq_queue has a pointer to a
waker bfq_queue and a list of the bfq_queues it may wake. In this
respect, when a bfq_queue, say Q, remains with no I/O source attached
to it, Q cannot be woken by any other bfq_queue, and cannot wake any
other bfq_queue. Then Q must be removed from the woken list of its
possible waker bfq_queue, and all bfq_queues in the woken list of Q
must stop having a waker bfq_queue.
Q remains with no I/O source in two cases: when the last process
associated with Q exits or when such a process gets associated with a
different bfq_queue. Unfortunately, commit 13a857a4c4 ("block, bfq:
detect wakers and unconditionally inject their I/O") performed the
above updates only in the first case.
This commit fixes this bug by moving these updates to when Q gets
freed. This is a simple and safe way to handle all cases, as both the
above events, process exit and re-association, lead to Q being freed
soon, and because dangling references would come out only after Q gets
freed (if no update were performed).
Fixes: 13a857a4c4 ("block, bfq: detect wakers and unconditionally inject their I/O")
Reported-by: Douglas Anderson <dianders@chromium.org>
Tested-by: Douglas Anderson <dianders@chromium.org>
Signed-off-by: Paolo Valente <paolo.valente@linaro.org>
Signed-off-by: Jens Axboe <axboe@kernel.dk>
Since commit 13a857a4c4 ("block, bfq: detect wakers and
unconditionally inject their I/O"), BFQ stores, in a per-device
pointer last_completed_rq_bfqq, the last bfq_queue that had an I/O
request completed. If some bfq_queue receives new I/O right after the
last request of last_completed_rq_bfqq has been completed, then
last_completed_rq_bfqq may be a waker bfq_queue.
But if the bfq_queue last_completed_rq_bfqq points to is freed, then
last_completed_rq_bfqq becomes a dangling reference. This commit
resets last_completed_rq_bfqq if the pointed bfq_queue is freed.
Fixes: 13a857a4c4 ("block, bfq: detect wakers and unconditionally inject their I/O")
Reported-by: Douglas Anderson <dianders@chromium.org>
Tested-by: Douglas Anderson <dianders@chromium.org>
Signed-off-by: Paolo Valente <paolo.valente@linaro.org>
Signed-off-by: Jens Axboe <axboe@kernel.dk>
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Merge tag 'for-linus-20190726' of git://git.kernel.dk/linux-block
Pull block fixes from Jens Axboe:
- Several io_uring fixes/improvements:
- Blocking fix for O_DIRECT (me)
- Latter page slowness for registered buffers (me)
- Fix poll hang under certain conditions (me)
- Defer sequence check fix for wrapped rings (Zhengyuan)
- Mismatch in async inc/dec accounting (Zhengyuan)
- Memory ordering issue that could cause stall (Zhengyuan)
- Track sequential defer in bytes, not pages (Zhengyuan)
- NVMe pull request from Christoph
- Set of hang fixes for wbt (Josef)
- Redundant error message kill for libahci (Ding)
- Remove unused blk_mq_sched_started_request() and related ops (Marcos)
- drbd dynamic alloc shash descriptor to reduce stack use (Arnd)
- blkcg ->pd_stat() non-debug print (Tejun)
- bcache memory leak fix (Wei)
- Comment fix (Akinobu)
- BFQ perf regression fix (Paolo)
* tag 'for-linus-20190726' of git://git.kernel.dk/linux-block: (24 commits)
io_uring: ensure ->list is initialized for poll commands
Revert "nvme-pci: don't create a read hctx mapping without read queues"
nvme: fix multipath crash when ANA is deactivated
nvme: fix memory leak caused by incorrect subsystem free
nvme: ignore subnqn for ADATA SX6000LNP
drbd: dynamically allocate shash descriptor
block: blk-mq: Remove blk_mq_sched_started_request and started_request
bcache: fix possible memory leak in bch_cached_dev_run()
io_uring: track io length in async_list based on bytes
io_uring: don't use iov_iter_advance() for fixed buffers
block: properly handle IOCB_NOWAIT for async O_DIRECT IO
blk-mq: allow REQ_NOWAIT to return an error inline
io_uring: add a memory barrier before atomic_read
rq-qos: use a mb for got_token
rq-qos: set ourself TASK_UNINTERRUPTIBLE after we schedule
rq-qos: don't reset has_sleepers on spurious wakeups
rq-qos: fix missed wake-ups in rq_qos_throttle
wait: add wq_has_single_sleeper helper
block, bfq: check also in-flight I/O in dispatch plugging
block: fix sysfs module parameters directory path in comment
...
Consider a sync bfq_queue Q that remains empty while in service, and
suppose that, when this happens, there is a fair amount of already
in-flight I/O not belonging to Q. In such a situation, I/O dispatching
may need to be plugged (until new I/O arrives for Q), for the
following reason.
The drive may decide to serve in-flight non-Q's I/O requests before
Q's ones, thereby delaying the arrival of new I/O requests for Q
(recall that Q is sync). If I/O-dispatching is not plugged, then,
while Q remains empty, a basically uncontrolled amount of I/O from
other queues may be dispatched too, possibly causing the service of
Q's I/O to be delayed even longer in the drive. This problem gets more
and more serious as the speed and the queue depth of the drive grow,
because, as these two quantities grow, the probability to find no
queue busy but many requests in flight grows too.
If Q has the same weight and priority as the other queues, then the
above delay is unlikely to cause any issue, because all queues tend to
undergo the same treatment. So, since not plugging I/O dispatching is
convenient for throughput, it is better not to plug. Things change in
case Q has a higher weight or priority than some other queue, because
Q's service guarantees may simply be violated. For this reason,
commit 1de0c4cd9e ("block, bfq: reduce idling only in symmetric
scenarios") does plug I/O in such an asymmetric scenario. Plugging
minimizes the delay induced by already in-flight I/O, and enables Q to
recover the bandwidth it may lose because of this delay.
Yet the above commit does not cover the case of weight-raised queues,
for efficiency concerns. For weight-raised queues, I/O-dispatch
plugging is activated simply if not all bfq_queues are
weight-raised. But this check does not handle the case of in-flight
requests, because a bfq_queue may become non busy *before* all its
in-flight requests are completed.
This commit performs I/O-dispatch plugging for weight-raised queues if
there are some in-flight requests.
As a practical example of the resulting recover of control, under
write load on a Samsung SSD 970 PRO, gnome-terminal starts in 1.5
seconds after this fix, against 15 seconds before the fix (as a
reference, gnome-terminal takes about 35 seconds to start with any of
the other I/O schedulers).
Fixes: 1de0c4cd9e ("block, bfq: reduce idling only in symmetric scenarios")
Signed-off-by: Paolo Valente <paolo.valente@linaro.org>
Signed-off-by: Jens Axboe <axboe@kernel.dk>
Rename the block documentation files to ReST, add an
index for them and adjust in order to produce a nice html
output via the Sphinx build system.
At its new index.rst, let's add a :orphan: while this is not linked to
the main index.rst file, in order to avoid build warnings.
Signed-off-by: Mauro Carvalho Chehab <mchehab+samsung@kernel.org>
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Merge tag 'for-5.3/block-20190708' of git://git.kernel.dk/linux-block
Pull block updates from Jens Axboe:
"This is the main block updates for 5.3. Nothing earth shattering or
major in here, just fixes, additions, and improvements all over the
map. This contains:
- Series of documentation fixes (Bart)
- Optimization of the blk-mq ctx get/put (Bart)
- null_blk removal race condition fix (Bob)
- req/bio_op() cleanups (Chaitanya)
- Series cleaning up the segment accounting, and request/bio mapping
(Christoph)
- Series cleaning up the page getting/putting for bios (Christoph)
- block cgroup cleanups and moving it to where it is used (Christoph)
- block cgroup fixes (Tejun)
- Series of fixes and improvements to bcache, most notably a write
deadlock fix (Coly)
- blk-iolatency STS_AGAIN and accounting fixes (Dennis)
- Series of improvements and fixes to BFQ (Douglas, Paolo)
- debugfs_create() return value check removal for drbd (Greg)
- Use struct_size(), where appropriate (Gustavo)
- Two lighnvm fixes (Heiner, Geert)
- MD fixes, including a read balance and corruption fix (Guoqing,
Marcos, Xiao, Yufen)
- block opal shadow mbr additions (Jonas, Revanth)
- sbitmap compare-and-exhange improvemnts (Pavel)
- Fix for potential bio->bi_size overflow (Ming)
- NVMe pull requests:
- improved PCIe suspent support (Keith Busch)
- error injection support for the admin queue (Akinobu Mita)
- Fibre Channel discovery improvements (James Smart)
- tracing improvements including nvmetc tracing support (Minwoo Im)
- misc fixes and cleanups (Anton Eidelman, Minwoo Im, Chaitanya
Kulkarni)"
- Various little fixes and improvements to drivers and core"
* tag 'for-5.3/block-20190708' of git://git.kernel.dk/linux-block: (153 commits)
blk-iolatency: fix STS_AGAIN handling
block: nr_phys_segments needs to be zero for REQ_OP_WRITE_ZEROES
blk-mq: simplify blk_mq_make_request()
blk-mq: remove blk_mq_put_ctx()
sbitmap: Replace cmpxchg with xchg
block: fix .bi_size overflow
block: sed-opal: check size of shadow mbr
block: sed-opal: ioctl for writing to shadow mbr
block: sed-opal: add ioctl for done-mark of shadow mbr
block: never take page references for ITER_BVEC
direct-io: use bio_release_pages in dio_bio_complete
block_dev: use bio_release_pages in bio_unmap_user
block_dev: use bio_release_pages in blkdev_bio_end_io
iomap: use bio_release_pages in iomap_dio_bio_end_io
block: use bio_release_pages in bio_map_user_iov
block: use bio_release_pages in bio_unmap_user
block: optionally mark pages dirty in bio_release_pages
block: move the BIO_NO_PAGE_REF check into bio_release_pages
block: skd_main.c: Remove call to memset after dma_alloc_coherent
block: mtip32xx: Remove call to memset after dma_alloc_coherent
...
In reboot tests on several devices we were seeing a "use after free"
when slub_debug or KASAN was enabled. The kernel complained about:
Unable to handle kernel paging request at virtual address 6b6b6c2b
...which is a classic sign of use after free under slub_debug. The
stack crawl in kgdb looked like:
0 test_bit (addr=<optimized out>, nr=<optimized out>)
1 bfq_bfqq_busy (bfqq=<optimized out>)
2 bfq_select_queue (bfqd=<optimized out>)
3 __bfq_dispatch_request (hctx=<optimized out>)
4 bfq_dispatch_request (hctx=<optimized out>)
5 0xc056ef00 in blk_mq_do_dispatch_sched (hctx=0xed249440)
6 0xc056f728 in blk_mq_sched_dispatch_requests (hctx=0xed249440)
7 0xc0568d24 in __blk_mq_run_hw_queue (hctx=0xed249440)
8 0xc0568d94 in blk_mq_run_work_fn (work=<optimized out>)
9 0xc024c5c4 in process_one_work (worker=0xec6d4640, work=0xed249480)
10 0xc024cff4 in worker_thread (__worker=0xec6d4640)
Digging in kgdb, it could be found that, though bfqq looked fine,
bfqq->bic had been freed.
Through further digging, I postulated that perhaps it is illegal to
access a "bic" (AKA an "icq") after bfq_exit_icq() had been called
because the "bic" can be freed at some point in time after this call
is made. I confirmed that there certainly were cases where the exact
crashing code path would access the "bic" after bfq_exit_icq() had
been called. Sspecifically I set the "bfqq->bic" to (void *)0x7 and
saw that the bic was 0x7 at the time of the crash.
To understand a bit more about why this crash was fairly uncommon (I
saw it only once in a few hundred reboots), you can see that much of
the time bfq_exit_icq_fbqq() fully frees the bfqq and thus it can't
access the ->bic anymore. The only case it doesn't is if
bfq_put_queue() sees a reference still held.
However, even in the case when bfqq isn't freed, the crash is still
rare. Why? I tracked what happened to the "bic" after the exit
routine. It doesn't get freed right away. Rather,
put_io_context_active() eventually called put_io_context() which
queued up freeing on a workqueue. The freeing then actually happened
later than that through call_rcu(). Despite all these delays, some
extra debugging showed that all the hoops could be jumped through in
time and the memory could be freed causing the original crash. Phew!
To make a long story short, assuming it truly is illegal to access an
icq after the "exit_icq" callback is finished, this patch is needed.
Cc: stable@vger.kernel.org
Reviewed-by: Paolo Valente <paolo.valente@unimore.it>
Signed-off-by: Douglas Anderson <dianders@chromium.org>
Signed-off-by: Jens Axboe <axboe@kernel.dk>
Some debug code suggested by Paolo was tripping when I did reboot
stress tests. Specifically in bfq_bfqq_resume_state()
"bic->saved_wr_start_at_switch_to_srt" was later than the current
value of "jiffies". A bit of debugging showed that
"bic->saved_wr_start_at_switch_to_srt" was actually 0 and a bit more
debugging showed that was because we had run through the "unlikely"
case in the bfq_bfqq_save_state() function.
Let's init "saved_wr_start_at_switch_to_srt" in the unlikely case to
something sane.
NOTE: this fixes no known real-world errors.
Reviewed-by: Paolo Valente <paolo.valente@linaro.org>
Reviewed-by: Guenter Roeck <groeck@chromium.org>
Signed-off-by: Douglas Anderson <dianders@chromium.org>
Signed-off-by: Jens Axboe <axboe@kernel.dk>
By mistake, there is a '&' instead of a '==' in the definition of the
macro BFQQ_TOTALLY_SEEKY. This commit replaces the wrong operator with
the correct one.
Fixes: 7074f076ff ("block, bfq: do not tag totally seeky queues as soft rt")
Signed-off-by: Paolo Valente <paolo.valente@linaro.org>
Signed-off-by: Jens Axboe <axboe@kernel.dk>
Consider, on one side, a bfq_queue Q that remains empty while in
service, and, on the other side, the pending I/O of bfq_queues that,
according to their timestamps, have to be served after Q. If an
uncontrolled amount of I/O from the latter bfq_queues were dispatched
while Q is waiting for its new I/O to arrive, then Q's bandwidth
guarantees would be violated. To prevent this, I/O dispatch is plugged
until Q receives new I/O (except for a properly controlled amount of
injected I/O). Unfortunately, preemption breaks I/O-dispatch plugging,
for the following reason.
Preemption is performed in two steps. First, Q is expired and
re-scheduled. Second, the new bfq_queue to serve is chosen. The first
step is needed by the second, as the second can be performed only
after Q's timestamps have been properly updated (done in the
expiration step), and Q has been re-queued for service. This
dependency is a consequence of the way how BFQ's scheduling algorithm
is currently implemented.
But Q is not re-scheduled at all in the first step, because Q is
empty. As a consequence, an uncontrolled amount of I/O may be
dispatched until Q becomes non empty again. This breaks Q's service
guarantees.
This commit addresses this issue by re-scheduling Q even if it is
empty. This in turn breaks the assumption that all scheduled queues
are non empty. Then a few extra checks are now needed.
Signed-off-by: Paolo Valente <paolo.valente@linaro.org>
Signed-off-by: Jens Axboe <axboe@kernel.dk>
BFQ enqueues the I/O coming from each process into a separate
bfq_queue, and serves bfq_queues one at a time. Each bfq_queue may be
served for at most timeout_sync milliseconds (default: 125 ms). This
service scheme is prone to the following inaccuracy.
While a bfq_queue Q1 is in service, some empty bfq_queue Q2 may
receive I/O, and, according to BFQ's scheduling policy, may become the
right bfq_queue to serve, in place of the currently in-service
bfq_queue. In this respect, postponing the service of Q2 to after the
service of Q1 finishes may delay the completion of Q2's I/O, compared
with an ideal service in which all non-empty bfq_queues are served in
parallel, and every non-empty bfq_queue is served at a rate
proportional to the bfq_queue's weight. This additional delay is equal
at most to the time Q1 may unjustly remain in service before switching
to Q2.
If Q1 and Q2 have the same weight, then this time is most likely
negligible compared with the completion time to be guaranteed to Q2's
I/O. In addition, first, one of the reasons why BFQ may want to serve
Q1 for a while is that this boosts throughput and, second, serving Q1
longer reduces BFQ's overhead. As a conclusion, it is usually better
not to preempt Q1 if both Q1 and Q2 have the same weight.
In contrast, as Q2's weight or priority becomes higher and higher
compared with that of Q1, the above delay becomes larger and larger,
compared with the I/O completion times that have to be guaranteed to
Q2 according to Q2's weight. So reducing this delay may be more
important than avoiding the costs of preempting Q1.
Accordingly, this commit preempts Q1 if Q2 has a higher weight or a
higher priority than Q1. Preemption causes Q1 to be re-scheduled, and
triggers a new choice of the next bfq_queue to serve. If Q2 really is
the next bfq_queue to serve, then Q2 will be set in service
immediately.
This change reduces the component of the I/O latency caused by the
above delay by about 80%. For example, on an (old) PLEXTOR PX-256M5
SSD, the maximum latency reported by fio drops from 15.1 to 3.2 ms for
a process doing sporadic random reads while another process is doing
continuous sequential reads.
Signed-off-by: Nicola Bottura <bottura.nicola95@gmail.com>
Signed-off-by: Paolo Valente <paolo.valente@linaro.org>
Signed-off-by: Jens Axboe <axboe@kernel.dk>
A bfq_queue Q may happen to be synchronized with another
bfq_queue Q2, i.e., the I/O of Q2 may need to be completed for Q to
receive new I/O. We call Q2 "waker queue".
If I/O plugging is being performed for Q, and Q is not receiving any
more I/O because of the above synchronization, then, thanks to BFQ's
injection mechanism, the waker queue is likely to get served before
the I/O-plugging timeout fires.
Unfortunately, this fact may not be sufficient to guarantee a high
throughput during the I/O plugging, because the inject limit for Q may
be too low to guarantee a lot of injected I/O. In addition, the
duration of the plugging, i.e., the time before Q finally receives new
I/O, may not be minimized, because the waker queue may happen to be
served only after other queues.
To address these issues, this commit introduces the explicit detection
of the waker queue, and the unconditional injection of a pending I/O
request of the waker queue on each invocation of
bfq_dispatch_request().
One may be concerned that this systematic injection of I/O from the
waker queue delays the service of Q's I/O. Fortunately, it doesn't. On
the contrary, next Q's I/O is brought forward dramatically, for it is
not blocked for milliseconds.
Reported-by: Srivatsa S. Bhat (VMware) <srivatsa@csail.mit.edu>
Tested-by: Srivatsa S. Bhat (VMware) <srivatsa@csail.mit.edu>
Signed-off-by: Paolo Valente <paolo.valente@linaro.org>
Signed-off-by: Jens Axboe <axboe@kernel.dk>
Until the base value for request service times gets finally computed
for a bfq_queue, the inject limit for that queue does depend on the
think-time state (short|long) of the queue. A timely update of the
think time then guarantees a quicker activation or deactivation of the
injection. Fortunately, the think time of a bfq_queue is updated in
the same code path as the inject limit; but after the inject limit.
This commits moves the update of the think time before the update of
the inject limit. For coherence, it moves the update of the seek time
too.
Reported-by: Srivatsa S. Bhat (VMware) <srivatsa@csail.mit.edu>
Tested-by: Srivatsa S. Bhat (VMware) <srivatsa@csail.mit.edu>
Signed-off-by: Paolo Valente <paolo.valente@linaro.org>
Signed-off-by: Jens Axboe <axboe@kernel.dk>
I/O injection gets reduced if it increases the request service times
of the victim queue beyond a certain threshold. The threshold, in its
turn, is computed as a function of the base service time enjoyed by
the queue when it undergoes no injection.
As a consequence, for injection to work properly, the above base value
has to be accurate. In this respect, such a value may vary over
time. For example, it varies if the size or the spatial locality of
the I/O requests in the queue change. It is then important to update
this value whenever possible. This commit performs this update.
Reported-by: Srivatsa S. Bhat (VMware) <srivatsa@csail.mit.edu>
Tested-by: Srivatsa S. Bhat (VMware) <srivatsa@csail.mit.edu>
Signed-off-by: Paolo Valente <paolo.valente@linaro.org>
Signed-off-by: Jens Axboe <axboe@kernel.dk>
One of the cases where the parameters for injection may be updated is
when there are no more in-flight I/O requests. The number of in-flight
requests is stored in the field bfqd->rq_in_driver of the descriptor
bfqd of the device. So, the controlled condition is
bfqd->rq_in_driver == 0.
Unfortunately, this is wrong because, the instruction that checks this
condition is in the code path that handles the completion of a
request, and, in particular, the instruction is executed before
bfqd->rq_in_driver is decremented in such a code path.
This commit fixes this issue by just replacing 0 with 1 in the
comparison.
Reported-by: Srivatsa S. Bhat (VMware) <srivatsa@csail.mit.edu>
Tested-by: Srivatsa S. Bhat (VMware) <srivatsa@csail.mit.edu>
Signed-off-by: Paolo Valente <paolo.valente@linaro.org>
Signed-off-by: Jens Axboe <axboe@kernel.dk>
Until the base value of the request service times gets finally
computed for a bfq_queue, the inject limit does depend on the
think-time state (short|long). The limit must be 0 or 1 if the think
time is deemed, respectively, as short or long. However, such a check
and possible limit update is performed only periodically, once per
second. So, to make the injection mechanism much more reactive, this
commit performs the update also every time the think-time state
changes.
In addition, in the following special case, this commit lets the
inject limit of a bfq_queue bfqq remain equal to 1 even if bfqq's
think time is short: bfqq's I/O is synchronized with that of some
other queue, i.e., bfqq may receive new I/O only after the I/O of the
other queue is completed. Keeping the inject limit to 1 allows the
blocking I/O to be served while bfqq is in service. And this is very
convenient both for bfqq and for the total throughput, as explained
in detail in the comments in bfq_update_has_short_ttime().
Reported-by: Srivatsa S. Bhat (VMware) <srivatsa@csail.mit.edu>
Tested-by: Srivatsa S. Bhat (VMware) <srivatsa@csail.mit.edu>
Signed-off-by: Paolo Valente <paolo.valente@linaro.org>
Signed-off-by: Jens Axboe <axboe@kernel.dk>
This option is entirely bfq specific, give it an appropinquate name.
Also make it depend on CONFIG_BFQ_GROUP_IOSCHED in Kconfig, as all
the functionality already does so anyway.
Acked-by: Tejun Heo <tj@kernel.org>
Acked-by: Paolo Valente <paolo.valente@linaro.org>
Signed-off-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Jens Axboe <axboe@kernel.dk>
We only need the number of segments in the blk-mq submission path.
Remove the field from struct bio, and return it from a variant of
blk_queue_split instead of that it can passed as an argument to
those functions that need the value.
This also means we stop recounting segments except for cloning
and partial segments.
To keep the number of arguments in this how path down remove
pointless struct request_queue arguments from any of the functions
that had it and grew a nr_segs argument.
Signed-off-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Jens Axboe <axboe@kernel.dk>
All these files have some form of the usual GPLv2 or later boilerplate.
Switch them to use SPDX tags instead.
Reviewed-by: Chaitanya Kulkarni <chaitanya.kulkarni@wdc.com>
Signed-off-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Jens Axboe <axboe@kernel.dk>
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Merge tag 'v5.1-rc6' into for-5.2/block
Pull in v5.1-rc6 to resolve two conflicts. One is in BFQ, in just a
comment, and is trivial. The other one is a conflict due to a later fix
in the bio multi-page work, and needs a bit more care.
* tag 'v5.1-rc6': (770 commits)
Linux 5.1-rc6
block: make sure that bvec length can't be overflow
block: kill all_q_node in request_queue
x86/cpu/intel: Lower the "ENERGY_PERF_BIAS: Set to normal" message's log priority
coredump: fix race condition between mmget_not_zero()/get_task_mm() and core dumping
mm/kmemleak.c: fix unused-function warning
init: initialize jump labels before command line option parsing
kernel/watchdog_hld.c: hard lockup message should end with a newline
kcov: improve CONFIG_ARCH_HAS_KCOV help text
mm: fix inactive list balancing between NUMA nodes and cgroups
mm/hotplug: treat CMA pages as unmovable
proc: fixup proc-pid-vm test
proc: fix map_files test on F29
mm/vmstat.c: fix /proc/vmstat format for CONFIG_DEBUG_TLBFLUSH=y CONFIG_SMP=n
mm/memory_hotplug: do not unlock after failing to take the device_hotplug_lock
mm: swapoff: shmem_unuse() stop eviction without igrab()
mm: swapoff: take notice of completion sooner
mm: swapoff: remove too limiting SWAP_UNUSE_MAX_TRIES
mm: swapoff: shmem_find_swap_entries() filter out other types
slab: store tagged freelist for off-slab slabmgmt
...
Signed-off-by: Jens Axboe <axboe@kernel.dk>
A previous commit moved the shallow depth and BFQ depth map calculations
to be done at init time, moving it outside of the hotter IO path. This
potentially causes hangs if the users changes the depth of the scheduler
map, by writing to the 'nr_requests' sysfs file for that device.
Add a blk-mq-sched hook that allows blk-mq to inform the scheduler if
the depth changes, so that the scheduler can update its internal state.
Tested-by: Kai Krakow <kai@kaishome.de>
Reported-by: Paolo Valente <paolo.valente@linaro.org>
Fixes: f0635b8a41 ("bfq: calculate shallow depths at init time")
Signed-off-by: Jens Axboe <axboe@kernel.dk>
The function bfq_bfqq_expire() invokes the function
__bfq_bfqq_expire(), and the latter may free the in-service bfq-queue.
If this happens, then no other instruction of bfq_bfqq_expire() must
be executed, or a use-after-free will occur.
Basing on the assumption that __bfq_bfqq_expire() invokes
bfq_put_queue() on the in-service bfq-queue exactly once, the queue is
assumed to be freed if its refcounter is equal to one right before
invoking __bfq_bfqq_expire().
But, since commit 9dee8b3b05 ("block, bfq: fix queue removal from
weights tree") this assumption is false. __bfq_bfqq_expire() may also
invoke bfq_weights_tree_remove() and, since commit 9dee8b3b05
("block, bfq: fix queue removal from weights tree"), also
the latter function may invoke bfq_put_queue(). So __bfq_bfqq_expire()
may invoke bfq_put_queue() twice, and this is the actual case where
the in-service queue may happen to be freed.
To address this issue, this commit moves the check on the refcounter
of the queue right around the last bfq_put_queue() that may be invoked
on the queue.
Fixes: 9dee8b3b05 ("block, bfq: fix queue removal from weights tree")
Reported-by: Dmitrii Tcvetkov <demfloro@demfloro.ru>
Reported-by: Douglas Anderson <dianders@chromium.org>
Tested-by: Dmitrii Tcvetkov <demfloro@demfloro.ru>
Tested-by: Douglas Anderson <dianders@chromium.org>
Signed-off-by: Paolo Valente <paolo.valente@linaro.org>
Signed-off-by: Jens Axboe <axboe@kernel.dk>
Some of the comments in the bfq files had typos. This patch fixes them.
Signed-off-by: Angelo Ruocco <angeloruocco90@gmail.com>
Signed-off-by: Paolo Valente <paolo.valente@linaro.org>
Signed-off-by: Jens Axboe <axboe@kernel.dk>
bfq saves the state of a queue each time a merge occurs, to be
able to resume such a state when the queue is associated again
with its original process, on a split.
Unfortunately bfq does not save & restore also the weight of the
queue. If the weight is not correctly resumed when the queue is
recycled, then the weight of the recycled queue could differ
from the weight of the original queue.
This commit adds the missing save & resume of the weight.
Tested-by: Holger Hoffstätte <holger@applied-asynchrony.com>
Tested-by: Oleksandr Natalenko <oleksandr@natalenko.name>
Signed-off-by: Francesco Pollicino <fra.fra.800@gmail.com>
Signed-off-by: Paolo Valente <paolo.valente@linaro.org>
Signed-off-by: Jens Axboe <axboe@kernel.dk>
The function "bfq_log_bfqq" prints the pid of the process
associated with the queue passed as input.
Unfortunately, if the queue is shared, then more than one process
is associated with the queue. The pid that gets printed in this
case is the pid of one of the associated processes.
Which process gets printed depends on the exact sequence of merge
events the queue underwent. So printing such a pid is rather
useless and above all is often rather confusing because it
reports a random pid between those of the associated processes.
This commit addresses this issue by printing SHARED instead of a pid
if the queue is shared.
Tested-by: Holger Hoffstätte <holger@applied-asynchrony.com>
Tested-by: Oleksandr Natalenko <oleksandr@natalenko.name>
Signed-off-by: Francesco Pollicino <fra.fra.800@gmail.com>
Signed-off-by: Paolo Valente <paolo.valente@linaro.org>
Signed-off-by: Jens Axboe <axboe@kernel.dk>
If many bfq_queues belonging to the same group happen to be created
shortly after each other, then the processes associated with these
queues have typically a common goal. In particular, bursts of queue
creations are usually caused by services or applications that spawn
many parallel threads/processes. Examples are systemd during boot, or
git grep. If there are no other active queues, then, to help these
processes get their job done as soon as possible, the best thing to do
is to reach a high throughput. To this goal, it is usually better to
not grant either weight-raising or device idling to the queues
associated with these processes. And this is exactly what BFQ
currently does.
There is however a drawback: if, in contrast, some other queues are
already active, then the newly created queues must be protected from
the I/O flowing through the already existing queues. In this case, the
best thing to do is the opposite as in the other case: it is much
better to grant weight-raising and device idling to the newly-created
queues, if they deserve it. This commit addresses this issue by doing
so if there are already other active queues.
This change also helps eliminating false positives, which occur when
the newly-created queues do not belong to an actual large burst of
creations, but some background task (e.g., a service) happens to
trigger the creation of new queues in the middle, i.e., very close to
when the victim queues are created. These false positive may cause
total loss of control on process latencies.
Tested-by: Holger Hoffstätte <holger@applied-asynchrony.com>
Tested-by: Oleksandr Natalenko <oleksandr@natalenko.name>
Signed-off-by: Paolo Valente <paolo.valente@linaro.org>
Signed-off-by: Jens Axboe <axboe@kernel.dk>
Sync random I/O is likely to be confused with soft real-time I/O,
because it is characterized by limited throughput and apparently
isochronous arrival pattern. To avoid false positives, this commits
prevents bfq_queues containing only random (seeky) I/O from being
tagged as soft real-time.
Tested-by: Holger Hoffstätte <holger@applied-asynchrony.com>
Tested-by: Oleksandr Natalenko <oleksandr@natalenko.name>
Signed-off-by: Paolo Valente <paolo.valente@linaro.org>
Signed-off-by: Jens Axboe <axboe@kernel.dk>
To boost throughput with a set of processes doing interleaved I/O
(i.e., a set of processes whose individual I/O is random, but whose
merged cumulative I/O is sequential), BFQ merges the queues associated
with these processes, i.e., redirects the I/O of these processes into a
common, shared queue. In the shared queue, I/O requests are ordered by
their position on the medium, thus sequential I/O gets dispatched to
the device when the shared queue is served.
Queue merging costs execution time, because, to detect which queues to
merge, BFQ must maintain a list of the head I/O requests of active
queues, ordered by request positions. Measurements showed that this
costs about 10% of BFQ's total per-request processing time.
Request processing time becomes more and more critical as the speed of
the underlying storage device grows. Yet, fortunately, queue merging
is basically useless on the very devices that are so fast to make
request processing time critical. To reach a high throughput, these
devices must have many requests queued at the same time. But, in this
configuration, the internal scheduling algorithms of these devices do
also the job of queue merging: they reorder requests so as to obtain
as much as possible a sequential I/O pattern. As a consequence, with
processes doing interleaved I/O, the throughput reached by one such
device is likely to be the same, with and without queue merging.
In view of this fact, this commit disables queue merging, and all
related housekeeping, for non-rotational devices with internal
queueing. The total, single-lock-protected, per-request processing
time of BFQ drops to, e.g., 1.9 us on an Intel Core i7-2760QM@2.40GHz
(time measured with simple code instrumentation, and using the
throughput-sync.sh script of the S suite [1], in performance-profiling
mode). To put this result into context, the total,
single-lock-protected, per-request execution time of the lightest I/O
scheduler available in blk-mq, mq-deadline, is 0.7 us (mq-deadline is
~800 LOC, against ~10500 LOC for BFQ).
Disabling merging provides a further, remarkable benefit in terms of
throughput. Merging tends to make many workloads artificially more
uneven, mainly because of shared queues remaining non empty for
incomparably more time than normal queues. So, if, e.g., one of the
queues in a set of merged queues has a higher weight than a normal
queue, then the shared queue may inherit such a high weight and, by
staying almost always active, may force BFQ to perform I/O plugging
most of the time. This evidently makes it harder for BFQ to let the
device reach a high throughput.
As a practical example of this problem, and of the benefits of this
commit, we measured again the throughput in the nasty scenario
considered in previous commit messages: dbench test (in the Phoronix
suite), with 6 clients, on a filesystem with journaling, and with the
journaling daemon enjoying a higher weight than normal processes. With
this commit, the throughput grows from ~150 MB/s to ~200 MB/s on a
PLEXTOR PX-256M5 SSD. This is the same peak throughput reached by any
of the other I/O schedulers. As such, this is also likely to be the
maximum possible throughput reachable with this workload on this
device, because I/O is mostly random, and the other schedulers
basically just pass I/O requests to the drive as fast as possible.
[1] https://github.com/Algodev-github/S
Tested-by: Holger Hoffstätte <holger@applied-asynchrony.com>
Tested-by: Oleksandr Natalenko <oleksandr@natalenko.name>
Tested-by: Francesco Pollicino <fra.fra.800@gmail.com>
Signed-off-by: Alessio Masola <alessio.masola@gmail.com>
Signed-off-by: Paolo Valente <paolo.valente@linaro.org>
Signed-off-by: Jens Axboe <axboe@kernel.dk>
The processes associated with a bfq_queue, say Q, may happen to
generate their cumulative I/O at a lower rate than the rate at which
the device could serve the same I/O. This is rather probable, e.g., if
only one process is associated with Q and the device is an SSD. It
results in Q becoming often empty while in service. If BFQ is not
allowed to switch to another queue when Q becomes empty, then, during
the service of Q, there will be frequent "service holes", i.e., time
intervals during which Q gets empty and the device can only consume
the I/O already queued in its hardware queues. This easily causes
considerable losses of throughput.
To counter this problem, BFQ implements a request injection mechanism,
which tries to fill the above service holes with I/O requests taken
from other bfq_queues. The hard part in this mechanism is finding the
right amount of I/O to inject, so as to both boost throughput and not
break Q's bandwidth and latency guarantees. To this goal, the current
version of this mechanism measures the bandwidth enjoyed by Q while it
is being served, and tries to inject the maximum possible amount of
extra service that does not cause Q's bandwidth to decrease too
much.
This solution has an important shortcoming. For bandwidth measurements
to be stable and reliable, Q must remain in service for a much longer
time than that needed to serve a single I/O request. Unfortunately,
this does not hold with many workloads. This commit addresses this
issue by changing the way the amount of injection allowed is
dynamically computed. It tunes injection as a function of the service
times of single I/O requests of Q, instead of Q's
bandwidth. Single-request service times are evidently meaningful even
if Q gets very few I/O requests completed while it is in service.
As a testbed for this new solution, we measured the throughput reached
by BFQ for one of the nastiest workloads and configurations for this
scheduler: the workload generated by the dbench test (in the Phoronix
suite), with 6 clients, on a filesystem with journaling, and with the
journaling daemon enjoying a higher weight than normal processes.
With this commit, the throughput grows from ~100 MB/s to ~150 MB/s on
a PLEXTOR PX-256M5.
Tested-by: Holger Hoffstätte <holger@applied-asynchrony.com>
Tested-by: Oleksandr Natalenko <oleksandr@natalenko.name>
Tested-by: Francesco Pollicino <fra.fra.800@gmail.com>
Signed-off-by: Paolo Valente <paolo.valente@linaro.org>
Signed-off-by: Jens Axboe <axboe@kernel.dk>
In most cases, it is detrimental for throughput to plug I/O dispatch
when the in-service bfq_queue becomes temporarily empty (plugging is
performed to wait for the possible arrival, soon, of new I/O from the
in-service queue). There is however a case where plugging is needed
for service guarantees. If a bfq_queue, say Q, has a higher weight
than some other active bfq_queue, and is sync, i.e., contains sync
I/O, then, to guarantee that Q does receive a higher share of the
throughput than other lower-weight queues, it is necessary to plug I/O
dispatch when Q remains temporarily empty while being served.
For this reason, BFQ performs I/O plugging when some active bfq_queue
has a higher weight than some other active bfq_queue. But this is
unnecessarily overkill. In fact, if the in-service bfq_queue actually
has a weight lower than or equal to the other queues, then the queue
*must not* be guaranteed a higher share of the throughput than the
other queues. So, not plugging I/O cannot cause any harm to the
queue. And can boost throughput.
Taking advantage of this fact, this commit does not plug I/O for sync
bfq_queues with a weight lower than or equal to the weights of the
other queues. Here is an example of the resulting throughput boost
with the dbench workload, which is particularly nasty for BFQ. With
the dbench test in the Phoronix suite, BFQ reaches its lowest total
throughput with 6 clients on a filesystem with journaling, in case the
journaling daemon has a higher weight than normal processes. Before
this commit, the total throughput was ~80 MB/sec on a PLEXTOR PX-256M5,
after this commit it is ~100 MB/sec.
Tested-by: Holger Hoffstätte <holger@applied-asynchrony.com>
Tested-by: Oleksandr Natalenko <oleksandr@natalenko.name>
Signed-off-by: Paolo Valente <paolo.valente@linaro.org>
Signed-off-by: Jens Axboe <axboe@kernel.dk>
If a sync bfq_queue has a higher weight than some other queue, and
remains temporarily empty while in service, then, to preserve the
bandwidth share of the queue, it is necessary to plug I/O dispatching
until a new request arrives for the queue. In addition, a timeout
needs to be set, to avoid waiting for ever if the process associated
with the queue has actually finished its I/O.
Even with the above timeout, the device is however not fed with new
I/O for a while, if the process has finished its I/O. If this happens
often, then throughput drops and latencies grow. For this reason, the
timeout is kept rather low: 8 ms is the current default.
Unfortunately, such a low value may cause, on the opposite end, a
violation of bandwidth guarantees for a process that happens to issue
new I/O too late. The higher the system load, the higher the
probability that this happens to some process. This is a problem in
scenarios where service guarantees matter more than throughput. One
important case are weight-raised queues, which need to be granted a
very high fraction of the bandwidth.
To address this issue, this commit lower-bounds the plugging timeout
for weight-raised queues to 20 ms. This simple change provides
relevant benefits. For example, on a PLEXTOR PX-256M5S, with which
gnome-terminal starts in 0.6 seconds if there is no other I/O in
progress, the same applications starts in
- 0.8 seconds, instead of 1.2 seconds, if ten files are being read
sequentially in parallel
- 1 second, instead of 2 seconds, if, in parallel, five files are
being read sequentially, and five more files are being written
sequentially
Tested-by: Holger Hoffstätte <holger@applied-asynchrony.com>
Tested-by: Oleksandr Natalenko <oleksandr@natalenko.name>
Signed-off-by: Paolo Valente <paolo.valente@linaro.org>
Signed-off-by: Jens Axboe <axboe@kernel.dk>
When a new I/O request arrives for a bfq_queue, say Q, bfq checks
whether that request is close to
(a) the head request of some other queue waiting to be served, or
(b) the last request dispatched for the in-service queue (in case Q
itself is not the in-service queue)
If a queue, say Q2, is found for which the above condition holds, then
bfq merges Q and Q2, to hopefully get a more sequential I/O in the
resulting merged queue, and thus a possibly higher throughput.
Case (b) is checked by comparing the new request for Q with the last
request dispatched, assuming that the latter necessarily belonged to the
in-service queue. Unfortunately, this assumption is no longer always
correct, since commit d0edc2473b ("block, bfq: inject other-queue I/O
into seeky idle queues on NCQ flash").
When the assumption does not hold, queues that must not be merged may be
merged, causing unexpected loss of control on per-queue service
guarantees.
This commit solves this problem by adding an extra field, which stores
the actual last request dispatched for the in-service queue, and by
using this new field to correctly check case (b).
Signed-off-by: Paolo Valente <paolo.valente@linaro.org>
Signed-off-by: Jens Axboe <axboe@kernel.dk>
Writes tend to starve reads. bfq counters this problem by overcharging
writes with an inflated service w.r.t. the actual service (number of
sector written) they receive.
Yet his overcharging is useless, and actually causes unfairness in the
opposite direction, when bfq happens to be enforcing strong I/O control.
bfq does this enforcing when the scenario is asymmetric, i.e., when some
bfq_queue or group of bfq_queues is to be granted a different bandwidth
than some other bfq_queue or group of bfq_queues. So, in such a
scenario, this commit disables write overcharging.
Signed-off-by: Paolo Valente <paolo.valente@linaro.org>
Signed-off-by: Jens Axboe <axboe@kernel.dk>
The original commit is commit 1a1238a7dd ("cfq-iosched: improve hw_tag
detection") and has the following commit message:
If active queue hasn't enough requests and idle window opens, cfq will
not dispatch sufficient requests to hardware. In such situation, current
code will zero hw_tag. But this is because cfq doesn't dispatch enough
requests instead of hardware queue doesn't work. Don't zero hw_tag in
such case.
Signed-off-by: Paolo Valente <paolo.valente@linaro.org>
Signed-off-by: Jens Axboe <axboe@kernel.dk>
bfq simple heuristic from cfq for detecting whether the drive performs
command queueing: check whether the average number of in-flight requests
is above a given threshold. Unfortunately this heuristic does fail to
detect queueing (on drives with queueing) if processes doing I/O are few
and issue I/O with a low depth.
To reduce false negatives, this commit lowers the threshold.
Signed-off-by: Paolo Valente <paolo.valente@linaro.org>
Signed-off-by: Jens Axboe <axboe@kernel.dk>
