Commit 4acb83417c ("sbitmap: fix batched wait_cnt accounting")
is a big improvement: without it, I had to revert to before commit
040b83fcec ("sbitmap: fix possible io hung due to lost wakeup")
to avoid the high system time and freezes which that had introduced.
Now okay on the NVME laptop, but 4acb83417c is a disaster for heavy
swapping (kernel builds in low memory) on another: soon locking up in
sbitmap_queue_wake_up() (into which __sbq_wake_up() is inlined), cycling
around with waitqueue_active() but wait_cnt 0 . Here is a backtrace,
showing the common pattern of outer sbitmap_queue_wake_up() interrupted
before setting wait_cnt 0 back to wake_batch (in some cases other CPUs
are idle, in other cases they're spinning for a lock in dd_bio_merge()):
sbitmap_queue_wake_up < sbitmap_queue_clear < blk_mq_put_tag <
__blk_mq_free_request < blk_mq_free_request < __blk_mq_end_request <
scsi_end_request < scsi_io_completion < scsi_finish_command <
scsi_complete < blk_complete_reqs < blk_done_softirq < __do_softirq <
__irq_exit_rcu < irq_exit_rcu < common_interrupt < asm_common_interrupt <
_raw_spin_unlock_irqrestore < __wake_up_common_lock < __wake_up <
sbitmap_queue_wake_up < sbitmap_queue_clear < blk_mq_put_tag <
__blk_mq_free_request < blk_mq_free_request < dd_bio_merge <
blk_mq_sched_bio_merge < blk_mq_attempt_bio_merge < blk_mq_submit_bio <
__submit_bio < submit_bio_noacct_nocheck < submit_bio_noacct <
submit_bio < __swap_writepage < swap_writepage < pageout <
shrink_folio_list < evict_folios < lru_gen_shrink_lruvec <
shrink_lruvec < shrink_node < do_try_to_free_pages < try_to_free_pages <
__alloc_pages_slowpath < __alloc_pages < folio_alloc < vma_alloc_folio <
do_anonymous_page < __handle_mm_fault < handle_mm_fault <
do_user_addr_fault < exc_page_fault < asm_exc_page_fault
See how the process-context sbitmap_queue_wake_up() has been interrupted,
after bringing wait_cnt down to 0 (and in this example, after doing its
wakeups), before advancing wake_index and refilling wake_cnt: an
interrupt-context sbitmap_queue_wake_up() of the same sbq gets stuck.
I have almost no grasp of all the possible sbitmap races, and their
consequences: but __sbq_wake_up() can do nothing useful while wait_cnt 0,
so it is better if sbq_wake_ptr() skips on to the next ws in that case:
which fixes the lockup and shows no adverse consequence for me.
The check for wait_cnt being 0 is obviously racy, and ultimately can lead
to lost wakeups: for example, when there is only a single waitqueue with
waiters. However, lost wakeups are unlikely to matter in these cases,
and a proper fix requires redesign (and benchmarking) of the batched
wakeup code: so let's plug the hole with this bandaid for now.
Signed-off-by: Hugh Dickins <hughd@google.com>
Reviewed-by: Jan Kara <jack@suse.cz>
Reviewed-by: Keith Busch <kbusch@kernel.org>
Link: https://lore.kernel.org/r/9c2038a7-cdc5-5ee-854c-fbc6168bf16@google.com
Signed-off-by: Jens Axboe <axboe@kernel.dk>
Use atomic_long_try_cmpxchg instead of
atomic_long_cmpxchg (*ptr, old, new) == old in __sbitmap_queue_get_batch.
x86 CMPXCHG instruction returns success in ZF flag, so this change
saves a compare after cmpxchg (and related move instruction in front
of cmpxchg).
Also, atomic_long_cmpxchg implicitly assigns old *ptr value to "old"
when cmpxchg fails, enabling further code simplifications, e.g.
an extra memory read can be avoided in the loop.
No functional change intended.
Cc: Jens Axboe <axboe@kernel.dk>
Signed-off-by: Uros Bizjak <ubizjak@gmail.com>
Link: https://lore.kernel.org/r/20220908151200.9993-1-ubizjak@gmail.com
Signed-off-by: Jens Axboe <axboe@kernel.dk>
When __sbq_wake_up() decrements wait_cnt to 0 but races with someone
else waking the waiter on the waitqueue (so the waitqueue becomes
empty), it exits without reseting wait_cnt to wake_batch number. Once
wait_cnt is 0, nobody will ever reset the wait_cnt or wake the new
waiters resulting in possible deadlocks or busyloops. Fix the problem by
making sure we reset wait_cnt even if we didn't wake up anybody in the
end.
Fixes: 040b83fcec ("sbitmap: fix possible io hung due to lost wakeup")
Reported-by: Keith Busch <kbusch@kernel.org>
Signed-off-by: Jan Kara <jack@suse.cz>
Link: https://lore.kernel.org/r/20220908130937.2795-1-jack@suse.cz
Signed-off-by: Jens Axboe <axboe@kernel.dk>
This reverts commit 16ede66973.
This is causing issues with CPU stalls on my test box, revert it for
now until we understand what is going on. It looks like infinite
looping off sbitmap_queue_wake_up(), but hard to tell with a lot of
CPUs hitting this issue and the console scrolling infinitely.
Link: https://lore.kernel.org/linux-block/e742813b-ce5c-0d58-205b-1626f639b1bd@kernel.dk/
Signed-off-by: Jens Axboe <axboe@kernel.dk>
There are two problems can lead to lost wakeup:
1) invalid wakeup on the wrong waitqueue:
For example, 2 * wake_batch tags are put, while only wake_batch threads
are woken:
__sbq_wake_up
atomic_cmpxchg -> reset wait_cnt
__sbq_wake_up -> decrease wait_cnt
...
__sbq_wake_up -> wait_cnt is decreased to 0 again
atomic_cmpxchg
sbq_index_atomic_inc -> increase wake_index
wake_up_nr -> wake up and waitqueue might be empty
sbq_index_atomic_inc -> increase again, one waitqueue is skipped
wake_up_nr -> invalid wake up because old wakequeue might be empty
To fix the problem, increasing 'wake_index' before resetting 'wait_cnt'.
2) 'wait_cnt' can be decreased while waitqueue is empty
As pointed out by Jan Kara, following race is possible:
CPU1 CPU2
__sbq_wake_up __sbq_wake_up
sbq_wake_ptr() sbq_wake_ptr() -> the same
wait_cnt = atomic_dec_return()
/* decreased to 0 */
sbq_index_atomic_inc()
/* move to next waitqueue */
atomic_set()
/* reset wait_cnt */
wake_up_nr()
/* wake up on the old waitqueue */
wait_cnt = atomic_dec_return()
/*
* decrease wait_cnt in the old
* waitqueue, while it can be
* empty.
*/
Fix the problem by waking up before updating 'wake_index' and
'wait_cnt'.
With this patch, noted that 'wait_cnt' is still decreased in the old
empty waitqueue, however, the wakeup is redirected to a active waitqueue,
and the extra decrement on the old empty waitqueue is not handled.
Fixes: 88459642cb ("blk-mq: abstract tag allocation out into sbitmap library")
Signed-off-by: Yu Kuai <yukuai3@huawei.com>
Reviewed-by: Jan Kara <jack@suse.cz>
Link: https://lore.kernel.org/r/20220803121504.212071-1-yukuai1@huaweicloud.com
Signed-off-by: Jens Axboe <axboe@kernel.dk>
1. Getting next index before continue branch.
2. Checking free bits when setting the target bits. Otherwise,
it may reuse the busying bits.
Signed-off-by: wuchi <wuchi.zero@gmail.com>
Reviewed-by: Martin Wilck <mwilck@suse.com>
Link: https://lore.kernel.org/r/20220605145835.26916-1-wuchi.zero@gmail.com
Fixes: 9672b0d437 ("sbitmap: add __sbitmap_queue_get_batch()")
Signed-off-by: Jens Axboe <axboe@kernel.dk>
sbitmap has been used in scsi for replacing atomic operations on
sdev->device_busy, so IOPS on some fast scsi storage can be improved.
However, sdev->device_busy can be changed in fast path, so we have to
allocate the sb->map statically. sdev->device_busy has been capped to 1024,
but some drivers may configure the default depth as < 8, then
cause each sbitmap word to hold only one bit. Finally 1024 * 128(
sizeof(sbitmap_word)) bytes is needed for sb->map, given it is order 5
allocation, sometimes it may fail.
Avoid the issue by using kvzalloc_node() for allocating sb->map.
Cc: Ewan D. Milne <emilne@redhat.com>
Signed-off-by: Ming Lei <ming.lei@redhat.com>
Reviewed-by: Chaitanya Kulkarni <kch@nvidia.com>
Link: https://lore.kernel.org/r/20220316012708.354668-1-ming.lei@redhat.com
Signed-off-by: Jens Axboe <axboe@kernel.dk>
Since __sbitmap_queue_get_shallow() was introduced in commit c05e667337
("sbitmap: add sbitmap_get_shallow() operation"), it has not been used.
Delete __sbitmap_queue_get_shallow() and rename public
__sbitmap_queue_get_shallow() -> sbitmap_queue_get_shallow() as it is odd
to have public __foo but no foo at all.
Signed-off-by: John Garry <john.garry@huawei.com>
Link: https://lore.kernel.org/r/1644322024-105340-1-git-send-email-john.garry@huawei.com
Signed-off-by: Jens Axboe <axboe@kernel.dk>
Only the last sbitmap_word can have different depth, and all the others
must have same depth of 1U << sb->shift, so not necessary to store it in
sbitmap_word, and it can be retrieved easily and efficiently by adding
one internal helper of __map_depth(sb, index).
Remove 'depth' field from sbitmap_word, then the annotation of
____cacheline_aligned_in_smp for 'word' isn't needed any more.
Not see performance effect when running high parallel IOPS test on
null_blk.
This way saves us one cacheline(usually 64 words) per each sbitmap_word.
Cc: Martin Wilck <martin.wilck@suse.com>
Signed-off-by: Ming Lei <ming.lei@redhat.com>
Reviewed-by: Martin Wilck <mwilck@suse.com>
Reviewed-by: John Garry <john.garry@huawei.com>
Link: https://lore.kernel.org/r/20220110072945.347535-1-ming.lei@redhat.com
Signed-off-by: Jens Axboe <axboe@kernel.dk>
Commit 180dccb0db ("blk-mq: fix tag_get wait task can't be
awakened") will recalculate wake_batch when incrementing or decrementing
active_queues to avoid wake_batch > hctx_max_depth. At the same time, in
order to not affect performance as much as possible, the minimum wakeup
batch is set to 4. But when the QD is small (such as QD=1), if inc or dec
active_queues increases wakeup batch, that can lead to a hang:
Fix this problem with the following strategies:
QD : >= 32 | < 32
---------------------------------
wakeup batch: 8~4 | 3~1
Fixes: 180dccb0db ("blk-mq: fix tag_get wait task can't be awakened")
Link: https://lore.kernel.org/linux-block/78cafe94-a787-e006-8851-69906f0c2128@huawei.com/T/#t
Reported-by: Alex Xu (Hello71) <alex_y_xu@yahoo.ca>
Signed-off-by: Laibin Qiu <qiulaibin@huawei.com>
Tested-by: Alex Xu (Hello71) <alex_y_xu@yahoo.ca>
Link: https://lore.kernel.org/r/20220127100047.1763746-1-qiulaibin@huawei.com
Signed-off-by: Jens Axboe <axboe@kernel.dk>
In case of shared tags, there might be more than one hctx which
allocates from the same tags, and each hctx is limited to allocate at
most:
hctx_max_depth = max((bt->sb.depth + users - 1) / users, 4U);
tag idle detection is lazy, and may be delayed for 30sec, so there
could be just one real active hctx(queue) but all others are actually
idle and still accounted as active because of the lazy idle detection.
Then if wake_batch is > hctx_max_depth, driver tag allocation may wait
forever on this real active hctx.
Fix this by recalculating wake_batch when inc or dec active_queues.
Fixes: 0d2602ca30 ("blk-mq: improve support for shared tags maps")
Suggested-by: Ming Lei <ming.lei@redhat.com>
Suggested-by: John Garry <john.garry@huawei.com>
Signed-off-by: Laibin Qiu <qiulaibin@huawei.com>
Reviewed-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Link: https://lore.kernel.org/r/20220113025536.1479653-1-qiulaibin@huawei.com
Signed-off-by: Jens Axboe <axboe@kernel.dk>
KCSAN complaints about the sbitmap hint update:
==================================================================
BUG: KCSAN: data-race in sbitmap_queue_clear / sbitmap_queue_clear
write to 0xffffe8ffffd145b8 of 4 bytes by interrupt on cpu 1:
sbitmap_queue_clear+0xca/0xf0 lib/sbitmap.c:606
blk_mq_put_tag+0x82/0x90
__blk_mq_free_request+0x114/0x180 block/blk-mq.c:507
blk_mq_free_request+0x2c8/0x340 block/blk-mq.c:541
__blk_mq_end_request+0x214/0x230 block/blk-mq.c:565
blk_mq_end_request+0x37/0x50 block/blk-mq.c:574
lo_complete_rq+0xca/0x170 drivers/block/loop.c:541
blk_complete_reqs block/blk-mq.c:584 [inline]
blk_done_softirq+0x69/0x90 block/blk-mq.c:589
__do_softirq+0x12c/0x26e kernel/softirq.c:558
run_ksoftirqd+0x13/0x20 kernel/softirq.c:920
smpboot_thread_fn+0x22f/0x330 kernel/smpboot.c:164
kthread+0x262/0x280 kernel/kthread.c:319
ret_from_fork+0x1f/0x30
write to 0xffffe8ffffd145b8 of 4 bytes by interrupt on cpu 0:
sbitmap_queue_clear+0xca/0xf0 lib/sbitmap.c:606
blk_mq_put_tag+0x82/0x90
__blk_mq_free_request+0x114/0x180 block/blk-mq.c:507
blk_mq_free_request+0x2c8/0x340 block/blk-mq.c:541
__blk_mq_end_request+0x214/0x230 block/blk-mq.c:565
blk_mq_end_request+0x37/0x50 block/blk-mq.c:574
lo_complete_rq+0xca/0x170 drivers/block/loop.c:541
blk_complete_reqs block/blk-mq.c:584 [inline]
blk_done_softirq+0x69/0x90 block/blk-mq.c:589
__do_softirq+0x12c/0x26e kernel/softirq.c:558
run_ksoftirqd+0x13/0x20 kernel/softirq.c:920
smpboot_thread_fn+0x22f/0x330 kernel/smpboot.c:164
kthread+0x262/0x280 kernel/kthread.c:319
ret_from_fork+0x1f/0x30
value changed: 0x00000035 -> 0x00000044
Reported by Kernel Concurrency Sanitizer on:
CPU: 0 PID: 10 Comm: ksoftirqd/0 Not tainted 5.15.0-rc6-syzkaller #0
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 01/01/2011
==================================================================
which is a data race, but not an important one. This is just updating the
percpu alloc hint, and the reader of that hint doesn't ever require it to
be valid.
Just annotate it with data_race() to silence this one.
Reported-by: syzbot+4f8bfd804b4a1f95b8f6@syzkaller.appspotmail.com
Acked-by: Marco Elver <elver@google.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>
sbitmap currently only supports clearing tags one-by-one, add a helper
that allows the caller to pass in an array of tags to clear.
Signed-off-by: Jens Axboe <axboe@kernel.dk>
The block layer tag allocation batching still calls into sbitmap to get
each tag, but we can improve on that. Add __sbitmap_queue_get_batch(),
which returns a mask of tags all at once, along with an offset for
those tags.
An example return would be 0xff, where bits 0..7 are set, with
tag_offset == 128. The valid tags in this case would be 128..135.
A batch is specific to an individual sbitmap_map, hence it cannot be
larger than that. The requested number of tags is automatically reduced
to the max that can be satisfied with a single map.
On failure, 0 is returned. Caller should fall back to single tag
allocation at that point/
Signed-off-by: Jens Axboe <axboe@kernel.dk>
Move code for calculating default shift into a public helper which can be
used by SCSI.
Link: https://lore.kernel.org/r/20210122023317.687987-7-ming.lei@redhat.com
Cc: Omar Sandoval <osandov@fb.com>
Cc: Kashyap Desai <kashyap.desai@broadcom.com>
Cc: Sumanesh Samanta <sumanesh.samanta@broadcom.com>
Cc: Ewan D. Milne <emilne@redhat.com>
Tested-by: Sumanesh Samanta <sumanesh.samanta@broadcom.com>
Reviewed-by: Hannes Reinecke <hare@suse.de>
Signed-off-by: Ming Lei <ming.lei@redhat.com>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>
SCSI's .device_busy will be converted to sbitmap and sbitmap_weight is
needed. Export the helper.
The only existing user of sbitmap_weight() uses it to find out how many
bits are set and not cleared. Align sbitmap_weight() meaning with this
usage model.
Link: https://lore.kernel.org/r/20210122023317.687987-6-ming.lei@redhat.com
Cc: Omar Sandoval <osandov@fb.com>
Cc: Kashyap Desai <kashyap.desai@broadcom.com>
Cc: Sumanesh Samanta <sumanesh.samanta@broadcom.com>
Cc: Ewan D. Milne <emilne@redhat.com>
Tested-by: Sumanesh Samanta <sumanesh.samanta@broadcom.com>
Reviewed-by: Hannes Reinecke <hare@suse.de>
Signed-off-by: Ming Lei <ming.lei@redhat.com>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>
Allocation hint should have belonged to sbitmap. Also, when sbitmap's depth
is high and there is no need to use mulitple wakeup queues, user can
benefit from percpu allocation hint too.
Move allocation hint into sbitmap, then SCSI device queue can benefit from
allocation hint when converting to plain sbitmap.
Convert vhost/scsi.c to use sbitmap allocation with percpu alloc hint. This
is more efficient than the previous approach.
Link: https://lore.kernel.org/r/20210122023317.687987-5-ming.lei@redhat.com
Cc: Omar Sandoval <osandov@fb.com>
Cc: Kashyap Desai <kashyap.desai@broadcom.com>
Cc: Sumanesh Samanta <sumanesh.samanta@broadcom.com>
Cc: Ewan D. Milne <emilne@redhat.com>
Cc: Mike Christie <michael.christie@oracle.com>
Cc: virtualization@lists.linux-foundation.org
Tested-by: Sumanesh Samanta <sumanesh.samanta@broadcom.com>
Reviewed-by: Hannes Reinecke <hare@suse.de>
Signed-off-by: Ming Lei <ming.lei@redhat.com>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>
Add helpers for updating allocation hint so that we can avoid duplicate
code.
Prepare for moving allocation hint into sbitmap.
Link: https://lore.kernel.org/r/20210122023317.687987-4-ming.lei@redhat.com
Cc: Omar Sandoval <osandov@fb.com>
Cc: Kashyap Desai <kashyap.desai@broadcom.com>
Cc: Sumanesh Samanta <sumanesh.samanta@broadcom.com>
Cc: Ewan D. Milne <emilne@redhat.com>
Cc: Hannes Reinecke <hare@suse.de>
Tested-by: Sumanesh Samanta <sumanesh.samanta@broadcom.com>
Reviewed-by: Hannes Reinecke <hare@suse.de>
Signed-off-by: Ming Lei <ming.lei@redhat.com>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>
Currently the allocation round_robin info is maintained by sbitmap_queue.
However, bit allocation really belongs to sbitmap. Move it there.
Link: https://lore.kernel.org/r/20210122023317.687987-3-ming.lei@redhat.com
Cc: Omar Sandoval <osandov@fb.com>
Cc: Kashyap Desai <kashyap.desai@broadcom.com>
Cc: Sumanesh Samanta <sumanesh.samanta@broadcom.com>
Cc: Ewan D. Milne <emilne@redhat.com>
Cc: Hannes Reinecke <hare@suse.de>
Cc: virtualization@lists.linux-foundation.org
Tested-by: Sumanesh Samanta <sumanesh.samanta@broadcom.com>
Reviewed-by: Hannes Reinecke <hare@suse.de>
Signed-off-by: Ming Lei <ming.lei@redhat.com>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>
__sbitmap_get_word() doesn't warp if it's starting from the beginning
(i.e. initial hint is 0). Instead of stashing the original hint just set
@wrap accordingly.
Signed-off-by: Pavel Begunkov <asml.silence@gmail.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>
sbitmap_deferred_clear() does CAS loop to propagate cleared bits,
replace it with equivalent atomic bitwise and. That's slightly faster
and makes wait-free instead of lock-free as before.
The atomic can be relaxed (i.e. barrier-less) because following
sbitmap_get*() deal with synchronisation, see comments in
sbitmap_queue_clear().
It's ok to cast to atomic_long_t, that's what bitops/lock.h does.
Signed-off-by: Pavel Begunkov <asml.silence@gmail.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>
map->swap_lock protects map->cleared from concurrent modification,
however sbitmap_deferred_clear() is already atomically drains it, so
it's guaranteed to not loose bits on concurrent
sbitmap_deferred_clear().
A one threaded tag heavy test on top of nullbk showed ~1.5% t-put
increase, and 3% -> 1% cycle reduction of sbitmap_get() according to perf.
Signed-off-by: Pavel Begunkov <asml.silence@gmail.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>
Because of spinlocks and atomics sbitmap_deferred_clear() have to reload
&sb->map[index] on each access even though the map address won't change.
Pass in sbitmap_word instead of {sb, index}, so it's cached in a
variable. It also improves code generation of
sbitmap_find_bit_in_index().
Signed-off-by: Pavel Begunkov <asml.silence@gmail.com>
Reviewed-by: John Garry <john.garry@huawei.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>
sbitmap works by maintaining separate bitmaps of set and cleared bits.
The set bits are cleared in a batch, to save the burden of continuously
locking the "word" map to unset.
sbitmap_bitmap_show() only shows the set bits (in "word"), which is not
too much use, so mask out the cleared bits.
Fixes: ea86ea2cdc ("sbitmap: ammortize cost of clearing bits")
Signed-off-by: John Garry <john.garry@huawei.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>
Under heavy loads where the kyber I/O scheduler hits the token limits for
its scheduling domains, kyber can become stuck. When active requests
complete, kyber may not be woken up leaving the I/O requests in kyber
stuck.
This stuck state is due to a race condition with kyber and the sbitmap
functions it uses to run a callback when enough requests have completed.
The running of a sbt_wait callback can race with the attempt to insert the
sbt_wait. Since sbitmap_del_wait_queue removes the sbt_wait from the list
first then sets the sbq field to NULL, kyber can see the item as not on a
list but the call to sbitmap_add_wait_queue will see sbq as non-NULL. This
results in the sbt_wait being inserted onto the wait list but ws_active
doesn't get incremented. So the sbitmap queue does not know there is a
waiter on a wait list.
Since sbitmap doesn't think there is a waiter, kyber may never be
informed that there are domain tokens available and the I/O never advances.
With the sbt_wait on a wait list, kyber believes it has an active waiter
so cannot insert a new waiter when reaching the domain's full state.
This race can be fixed by only adding the sbt_wait to the queue if the
sbq field is NULL. If sbq is not NULL, there is already an action active
which will trigger the re-running of kyber. Let it run and add the
sbt_wait to the wait list if still needing to wait.
Reviewed-by: Omar Sandoval <osandov@fb.com>
Signed-off-by: David Jeffery <djeffery@redhat.com>
Reported-by: John Pittman <jpittman@redhat.com>
Tested-by: John Pittman <jpittman@redhat.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>
Since the only caller of this function has been deleted, delete this one
also.
Signed-off-by: John Garry <john.garry@huawei.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>
cmpxchg() with an immediate value could be replaced with less expensive
xchg(). The same true if new value don't _depend_ on the old one.
In the second block, atomic_cmpxchg() return value isn't checked, so
after atomic_cmpxchg() -> atomic_xchg() conversion it could be replaced
with atomic_set(). Comparison with atomic_read() in the second chunk was
left as an optimisation (if that was the initial intention).
Reviewed-by: Omar Sandoval <osandov@fb.com>
Signed-off-by: Pavel Begunkov <asml.silence@gmail.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>
Based on 1 normalized pattern(s):
this program is free software you can redistribute it and or modify
it under the terms of the gnu general public license v2 as published
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GPL-2.0-only
has been chosen to replace the boilerplate/reference in 2 file(s).
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Reviewed-by: Alexios Zavras <alexios.zavras@intel.com>
Reviewed-by: Armijn Hemel <armijn@tjaldur.nl>
Reviewed-by: Kate Stewart <kstewart@linuxfoundation.org>
Reviewed-by: Allison Randal <allison@lohutok.net>
Cc: linux-spdx@vger.kernel.org
Link: https://lkml.kernel.org/r/20190530000435.923873561@linutronix.de
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
This barrier only applies to the read-modify-write operations; in
particular, it does not apply to the atomic_set() primitive.
Replace the barrier with an smp_mb().
Fixes: 6c0ca7ae29 ("sbitmap: fix wakeup hang after sbq resize")
Cc: stable@vger.kernel.org
Reported-by: "Paul E. McKenney" <paulmck@linux.ibm.com>
Reported-by: Peter Zijlstra <peterz@infradead.org>
Signed-off-by: Andrea Parri <andrea.parri@amarulasolutions.com>
Reviewed-by: Ming Lei <ming.lei@redhat.com>
Cc: Jens Axboe <axboe@kernel.dk>
Cc: Omar Sandoval <osandov@fb.com>
Cc: Ming Lei <ming.lei@redhat.com>
Cc: linux-block@vger.kernel.org
Cc: "Paul E. McKenney" <paulmck@linux.ibm.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Signed-off-by: Jens Axboe <axboe@kernel.dk>
Because we may call blk_mq_get_driver_tag() directly from
blk_mq_dispatch_rq_list() without holding any lock, then HARDIRQ may
come and the above DEADLOCK is triggered.
Commit ab53dcfb3e7b ("sbitmap: Protect swap_lock from hardirq") tries to
fix this issue by using 'spin_lock_bh', which isn't enough because we
complete request from hardirq context direclty in case of multiqueue.
Cc: Clark Williams <williams@redhat.com>
Fixes: ab53dcfb3e7b ("sbitmap: Protect swap_lock from hardirq")
Cc: Jens Axboe <axboe@kernel.dk>
Cc: Ming Lei <ming.lei@redhat.com>
Cc: Guenter Roeck <linux@roeck-us.net>
Cc: Steven Rostedt (VMware) <rostedt@goodmis.org>
Signed-off-by: Ming Lei <ming.lei@redhat.com>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
The swap_lock used by sbitmap has a chain with locks taken from softirq,
but the swap_lock is not protected from being preempted by softirqs.
A chain exists of:
sbq->ws[i].wait -> dispatch_wait_lock -> swap_lock
Where the sbq->ws[i].wait lock can be taken from softirq context, which
means all locks below it in the chain must also be protected from
softirqs.
Reported-by: Clark Williams <williams@redhat.com>
Fixes: 58ab5e32e6 ("sbitmap: silence bogus lockdep IRQ warning")
Fixes: ea86ea2cdc ("sbitmap: amortize cost of clearing bits")
Cc: Jens Axboe <axboe@kernel.dk>
Cc: Ming Lei <ming.lei@redhat.com>
Cc: Guenter Roeck <linux@roeck-us.net>
Signed-off-by: Steven Rostedt (VMware) <rostedt@goodmis.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
After commit 5d2ee7122c, users of sbitmap that need wait queue
handling must use the provided helpers. But we only added
prepare_to_wait()/finish_wait() style helpers, add the equivalent
add_wait_queue/list_del wrappers as we..
This is needed to ensure kyber plays by the sbitmap waitqueue
rules.
Tested-by: Ming Lei <ming.lei@redhat.com>
Reviewed-by: Omar Sandoval <osandov@fb.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>
We're missing a deferred clear off the shallow get, which can cause
a hang. Additionally, when we resize the sbitmap, we should also
flush deferred clears for good measure.
Ensure we have full coverage on batch clears, even for paths where
we would not be doing deferred clear. This makes it less error
prone for future additions.
Reported-by: Bart Van Assche <bvanassche@acm.org>
Tested-by: Ming Lei <ming.lei@redhat.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>
Even if we have no waiters on any of the sbitmap_queue wait states, we
still have to loop every entry to check. We do this for every IO, so
the cost adds up.
Shift a bit of the cost to the slow path, when we actually have waiters.
Wrap prepare_to_wait_exclusive() and finish_wait(), so we can maintain
an internal count of how many are currently active. Then we can simply
check this count in sbq_wake_ptr() and not have to loop if we don't
have any sleepers.
Convert the two users of sbitmap with waiting, blk-mq-tag and iSCSI.
Reviewed-by: Omar Sandoval <osandov@fb.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>
sbitmap maintains a set of words that we use to set and clear bits, with
each bit representing a tag for blk-mq. Even though we spread the bits
out and maintain a hint cache, one particular bit allocated will end up
being cleared in the exact same spot.
This introduces batched clearing of bits. Instead of clearing a given
bit, the same bit is set in a cleared/free mask instead. If we fail
allocating a bit from a given word, then we check the free mask, and
batch move those cleared bits at that time. This trades 64 atomic bitops
for 2 cmpxchg().
In a threaded poll test case, half the overhead of getting and clearing
tags is removed with this change. On another poll test case with a
single thread, performance is unchanged.
Reviewed-by: Omar Sandoval <osandov@fb.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>
If we aren't forced to do round robin tag allocation, just use the
allocation hint to find the index for the tag word, don't use it for the
offset inside the word. This avoids a potential extra round trip in the
bit looping, and since we're fetching this cacheline, we may as well
check the whole word from the start.
Reviewed-by: Omar Sandoval <osandov@fb.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>
When the allocation process is scheduled back and the mapped hw queue is
changed, fake one extra wake up on previous queue for compensating wake
up miss, so other allocations on the previous queue won't be starved.
This patch fixes one request allocation hang issue, which can be
triggered easily in case of very low nr_request.
The race is as follows:
1) 2 hw queues, nr_requests are 2, and wake_batch is one
2) there are 3 waiters on hw queue 0
3) two in-flight requests in hw queue 0 are completed, and only two
waiters of 3 are waken up because of wake_batch, but both the two
waiters can be scheduled to another CPU and cause to switch to hw
queue 1
4) then the 3rd waiter will wait for ever, since no in-flight request
is in hw queue 0 any more.
5) this patch fixes it by the fake wakeup when waiter is scheduled to
another hw queue
Cc: <stable@vger.kernel.org>
Reviewed-by: Omar Sandoval <osandov@fb.com>
Signed-off-by: Ming Lei <ming.lei@redhat.com>
Modified commit message to make it clearer, and make it apply on
top of the 4.18 branch.
Signed-off-by: Jens Axboe <axboe@kernel.dk>
If we have multiple callers of sbq_wake_up(), we can end up in a
situation where the wait_cnt will continually go more and more
negative. Consider the case where our wake batch is 1, hence
wait_cnt will start out as 1.
wait_cnt == 1
CPU0 CPU1
atomic_dec_return(), cnt == 0
atomic_dec_return(), cnt == -1
cmpxchg(-1, 0) (succeeds)
[wait_cnt now 0]
cmpxchg(0, 1) (fails)
This ends up with wait_cnt being 0, we'll wakeup immediately
next time. Going through the same loop as above again, and
we'll have wait_cnt -1.
For the case where we have a larger wake batch, the only
difference is that the starting point will be higher. We'll
still end up with continually smaller batch wakeups, which
defeats the purpose of the rolling wakeups.
Always reset the wait_cnt to the batch value. Then it doesn't
matter who wins the race. But ensure that whomever does win
the race is the one that increments the ws index and wakes up
our batch count, loser gets to call __sbq_wake_up() again to
account his wakeups towards the next active wait state index.
Fixes: 6c0ca7ae29 ("sbitmap: fix wakeup hang after sbq resize")
Reviewed-by: Omar Sandoval <osandov@fb.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>
Make sure the user passed the right value to
sbitmap_queue_min_shallow_depth().
Acked-by: Paolo Valente <paolo.valente@linaro.org>
Signed-off-by: Omar Sandoval <osandov@fb.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>
The sbitmap queue wake batch is calculated such that once allocations
start blocking, all of the bits which are already allocated must be
enough to fulfill the batch counters of all of the waitqueues. However,
the shallow allocation depth can break this invariant, since we block
before our full depth is being utilized. Add
sbitmap_queue_min_shallow_depth(), which saves the minimum shallow depth
the sbq will use, and update sbq_calc_wake_batch() to take it into
account.
Acked-by: Paolo Valente <paolo.valente@linaro.org>
Signed-off-by: Omar Sandoval <osandov@fb.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>
sbitmap_queue_get()/sbitmap_queue_clear() are used for
allocating/freeing a resource, so they should provide acquire/release
barrier semantics, respectively. sbitmap_get() currently contains a full
barrier, which is unnecessary, so use test_and_set_bit_lock() instead of
test_and_set_bit() (these are equivalent on x86_64). sbitmap_clear_bit()
does not imply any barriers, which is incorrect, as accesses of the
resource (e.g., request) could potentially get reordered to after the
clear_bit(). Introduce sbitmap_clear_bit_unlock() and use it for
sbitmap_queue_clear() (this only adds a compiler barrier on x86_64). The
other existing user of sbitmap_clear_bit() (the blk-mq software queue
pending map) is serialized through a spinlock and does not need this.
Reported-by: Tejun Heo <tj@kernel.org>
Acked-by: Tejun Heo <tj@kernel.org>
Signed-off-by: Omar Sandoval <osandov@fb.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>
Even with a number of waitqueues, we can get into a situation where we
are heavily contended on the waitqueue lock. I got a report on spc1
where we're spending seconds doing this. Arguably the use case is nasty,
I reproduce it with one device and 1000 threads banging on the device.
But that doesn't mean we shouldn't be handling it better.
What ends up happening is that a thread will fail to get a tag, add
itself to the waitqueue, and subsequently get woken up when a tag is
freed - only to find itself going back to sleep on the waitqueue.
Instead of waking all threads, use an exclusive wait and wake up our
sbitmap batch count instead. This seems to work well for me (massive
improvement for this use case), and it survives basic testing. But I
haven't fully verified it yet.
An additional improvement is running the queue and checking for a new
tag BEFORE needing to add ourselves to the waitqueue.
Signed-off-by: Jens Axboe <axboe@kernel.dk>
This operation supports the use case of limiting the number of bits that
can be allocated for a given operation. Rather than setting aside some
bits at the end of the bitmap, we can set aside bits in each word of the
bitmap. This means we can keep the allocation hints spread out and
support sbitmap_resize() nicely at the cost of lower granularity for the
allowed depth.
Signed-off-by: Omar Sandoval <osandov@fb.com>
Signed-off-by: Jens Axboe <axboe@fb.com>
Hugh Dickins