linux/drivers/md
Nikos Tsironis 721b1d98fb dm snapshot: Fix excessive memory usage and workqueue stalls
kcopyd has no upper limit to the number of jobs one can allocate and
issue. Under certain workloads this can lead to excessive memory usage
and workqueue stalls. For example, when creating multiple dm-snapshot
targets with a 4K chunk size and then writing to the origin through the
page cache. Syncing the page cache causes a large number of BIOs to be
issued to the dm-snapshot origin target, which itself issues an even
larger (because of the BIO splitting taking place) number of kcopyd
jobs.

Running the following test, from the device mapper test suite [1],

  dmtest run --suite snapshot -n many_snapshots_of_same_volume_N

, with 8 active snapshots, results in the kcopyd job slab cache growing
to 10G. Depending on the available system RAM this can lead to the OOM
killer killing user processes:

[463.492878] kthreadd invoked oom-killer: gfp_mask=0x6040c0(GFP_KERNEL|__GFP_COMP),
              nodemask=(null), order=1, oom_score_adj=0
[463.492894] kthreadd cpuset=/ mems_allowed=0
[463.492948] CPU: 7 PID: 2 Comm: kthreadd Not tainted 4.19.0-rc7 #3
[463.492950] Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS 1.10.2-1 04/01/2014
[463.492952] Call Trace:
[463.492964]  dump_stack+0x7d/0xbb
[463.492973]  dump_header+0x6b/0x2fc
[463.492987]  ? lockdep_hardirqs_on+0xee/0x190
[463.493012]  oom_kill_process+0x302/0x370
[463.493021]  out_of_memory+0x113/0x560
[463.493030]  __alloc_pages_slowpath+0xf40/0x1020
[463.493055]  __alloc_pages_nodemask+0x348/0x3c0
[463.493067]  cache_grow_begin+0x81/0x8b0
[463.493072]  ? cache_grow_begin+0x874/0x8b0
[463.493078]  fallback_alloc+0x1e4/0x280
[463.493092]  kmem_cache_alloc_node+0xd6/0x370
[463.493098]  ? copy_process.part.31+0x1c5/0x20d0
[463.493105]  copy_process.part.31+0x1c5/0x20d0
[463.493115]  ? __lock_acquire+0x3cc/0x1550
[463.493121]  ? __switch_to_asm+0x34/0x70
[463.493129]  ? kthread_create_worker_on_cpu+0x70/0x70
[463.493135]  ? finish_task_switch+0x90/0x280
[463.493165]  _do_fork+0xe0/0x6d0
[463.493191]  ? kthreadd+0x19f/0x220
[463.493233]  kernel_thread+0x25/0x30
[463.493235]  kthreadd+0x1bf/0x220
[463.493242]  ? kthread_create_on_cpu+0x90/0x90
[463.493248]  ret_from_fork+0x3a/0x50
[463.493279] Mem-Info:
[463.493285] active_anon:20631 inactive_anon:4831 isolated_anon:0
[463.493285]  active_file:80216 inactive_file:80107 isolated_file:435
[463.493285]  unevictable:0 dirty:51266 writeback:109372 unstable:0
[463.493285]  slab_reclaimable:31191 slab_unreclaimable:3483521
[463.493285]  mapped:526 shmem:4903 pagetables:1759 bounce:0
[463.493285]  free:33623 free_pcp:2392 free_cma:0
...
[463.493489] Unreclaimable slab info:
[463.493513] Name                      Used          Total
[463.493522] bio-6                   1028KB       1028KB
[463.493525] bio-5                   1028KB       1028KB
[463.493528] dm_snap_pending_exception     236783KB     243789KB
[463.493531] dm_exception              41KB         42KB
[463.493534] bio-4                   1216KB       1216KB
[463.493537] bio-3                 439396KB     439396KB
[463.493539] kcopyd_job           6973427KB    6973427KB
...
[463.494340] Out of memory: Kill process 1298 (ruby2.3) score 1 or sacrifice child
[463.494673] Killed process 1298 (ruby2.3) total-vm:435740kB, anon-rss:20180kB, file-rss:4kB, shmem-rss:0kB
[463.506437] oom_reaper: reaped process 1298 (ruby2.3), now anon-rss:0kB, file-rss:0kB, shmem-rss:0kB

Moreover, issuing a large number of kcopyd jobs results in kcopyd
hogging the CPU, while processing them. As a result, processing of work
items, queued for execution on the same CPU as the currently running
kcopyd thread, is stalled for long periods of time, hurting performance.
Running the aforementioned test we get, in dmesg, messages like the
following:

[67501.194592] BUG: workqueue lockup - pool cpus=4 node=0 flags=0x0 nice=0 stuck for 27s!
[67501.195586] Showing busy workqueues and worker pools:
[67501.195591] workqueue events: flags=0x0
[67501.195597]   pwq 8: cpus=4 node=0 flags=0x0 nice=0 active=1/256
[67501.195611]     pending: cache_reap
[67501.195641] workqueue mm_percpu_wq: flags=0x8
[67501.195645]   pwq 8: cpus=4 node=0 flags=0x0 nice=0 active=1/256
[67501.195656]     pending: vmstat_update
[67501.195682] workqueue kblockd: flags=0x18
[67501.195687]   pwq 5: cpus=2 node=0 flags=0x0 nice=-20 active=1/256
[67501.195698]     pending: blk_timeout_work
[67501.195753] workqueue kcopyd: flags=0x8
[67501.195757]   pwq 8: cpus=4 node=0 flags=0x0 nice=0 active=1/256
[67501.195768]     pending: do_work [dm_mod]
[67501.195802] workqueue kcopyd: flags=0x8
[67501.195806]   pwq 8: cpus=4 node=0 flags=0x0 nice=0 active=1/256
[67501.195817]     pending: do_work [dm_mod]
[67501.195834] workqueue kcopyd: flags=0x8
[67501.195838]   pwq 8: cpus=4 node=0 flags=0x0 nice=0 active=1/256
[67501.195848]     pending: do_work [dm_mod]
[67501.195881] workqueue kcopyd: flags=0x8
[67501.195885]   pwq 8: cpus=4 node=0 flags=0x0 nice=0 active=1/256
[67501.195896]     pending: do_work [dm_mod]
[67501.195920] workqueue kcopyd: flags=0x8
[67501.195924]   pwq 8: cpus=4 node=0 flags=0x0 nice=0 active=2/256
[67501.195935]     in-flight: 67:do_work [dm_mod]
[67501.195945]     pending: do_work [dm_mod]
[67501.195961] pool 8: cpus=4 node=0 flags=0x0 nice=0 hung=27s workers=3 idle: 129 23765

The root cause for these issues is the way dm-snapshot uses kcopyd. In
particular, the lack of an explicit or implicit limit to the maximum
number of in-flight COW jobs. The merging path is not affected because
it implicitly limits the in-flight kcopyd jobs to one.

Fix these issues by using a semaphore to limit the maximum number of
in-flight kcopyd jobs. We grab the semaphore before allocating a new
kcopyd job in start_copy() and start_full_bio() and release it after the
job finishes in copy_callback().

The initial semaphore value is configurable through a module parameter,
to allow fine tuning the maximum number of in-flight COW jobs. Setting
this parameter to zero initializes the semaphore to INT_MAX.

A default value of 2048 maximum in-flight kcopyd jobs was chosen. This
value was decided experimentally as a trade-off between memory
consumption, stalling the kernel's workqueues and maintaining a high
enough throughput.

Re-running the aforementioned test:

  * Workqueue stalls are eliminated
  * kcopyd's job slab cache uses a maximum of 130MB
  * The time taken by the test to write to the snapshot-origin target is
    reduced from 05m20.48s to 03m26.38s

[1] https://github.com/jthornber/device-mapper-test-suite

Signed-off-by: Nikos Tsironis <ntsironis@arrikto.com>
Signed-off-by: Ilias Tsitsimpis <iliastsi@arrikto.com>
Signed-off-by: Mike Snitzer <snitzer@redhat.com>
2018-12-18 09:02:26 -05:00
..
bcache bcache: print number of keys in trace_bcache_journal_write 2018-12-13 08:15:54 -07:00
persistent-data dm: Avoid namespace collision with bitmap API 2018-08-01 15:49:38 -07:00
dm-bio-prison-v1.c dm: adjust structure members to improve alignment 2018-06-08 11:53:14 -04:00
dm-bio-prison-v1.h block: switch bios to blk_status_t 2017-06-09 09:27:32 -06:00
dm-bio-prison-v2.c dm: adjust structure members to improve alignment 2018-06-08 11:53:14 -04:00
dm-bio-prison-v2.h
dm-bio-record.h block: replace bi_bdev with a gendisk pointer and partitions index 2017-08-23 12:49:55 -06:00
dm-bufio.c dm bufio: update comment in dm-bufio.c 2018-12-18 09:02:26 -05:00
dm-builtin.c License cleanup: add SPDX GPL-2.0 license identifier to files with no license 2017-11-02 11:10:55 +01:00
dm-cache-background-tracker.c dm cache background tracker: fix sparse warning 2018-04-30 15:40:40 -04:00
dm-cache-background-tracker.h
dm-cache-block-types.h
dm-cache-metadata.c dm cache metadata: ignore hints array being too small during resize 2018-10-04 15:20:51 -04:00
dm-cache-metadata.h
dm-cache-policy-internal.h
dm-cache-policy-smq.c dm: remove unnecessary unlikely() around WARN_ON_ONCE() 2018-10-16 14:34:59 -04:00
dm-cache-policy.c
dm-cache-policy.h
dm-cache-target.c dm cache: destroy migration_cache if cache target registration failed 2018-10-09 13:53:03 -04:00
dm-core.h dm: remove the pending IO accounting 2018-12-10 08:30:38 -07:00
dm-crypt.c dm crypt: make workqueue names device-specific 2018-10-18 12:17:17 -04:00
dm-delay.c dm delay: add flush as a third class of IO 2018-07-27 15:24:19 -04:00
dm-era-target.c dm: allow targets to return output from messages they are sent 2018-04-03 15:04:10 -04:00
dm-exception-store.c
dm-exception-store.h
dm-flakey.c block: add a report_zones method 2018-10-25 11:17:40 -06:00
dm-integrity.c Merge branch 'linus' of git://git.kernel.org/pub/scm/linux/kernel/git/herbert/crypto-2.6 2018-10-25 16:43:35 -07:00
dm-io.c dm: Use kzalloc for all structs with embedded biosets/mempools 2018-06-05 08:47:43 -06:00
dm-ioctl.c dm ioctl: harden copy_params()'s copy_from_user() from malicious users 2018-10-18 11:54:07 -04:00
dm-kcopyd.c dm kcopyd: avoid softlockup in run_complete_job 2018-08-08 09:16:24 -04:00
dm-linear.c block: add a report_zones method 2018-10-25 11:17:40 -06:00
dm-log-userspace-base.c dm: convert to bioset_init()/mempool_init() 2018-05-30 15:33:32 -06:00
dm-log-userspace-transfer.c
dm-log-userspace-transfer.h
dm-log-writes.c dax: Introduce a ->copy_to_iter dax operation 2018-05-22 23:18:31 -07:00
dm-log.c
dm-mpath.c dm mpath: only flush workqueue when needed 2018-12-18 09:02:25 -05:00
dm-mpath.h
dm-path-selector.c
dm-path-selector.h
dm-queue-length.c dm mpath selector: more evenly distribute ties 2018-01-29 13:44:58 -05:00
dm-raid1.c dm kcopyd: return void from dm_kcopyd_copy() 2018-07-31 17:33:21 -04:00
dm-raid.c dm raid: avoid bitmap with raid4/5/6 journal device 2018-10-18 15:13:48 -04:00
dm-region-hash.c - Error path bug fix for overflow tests (Dan) 2018-06-12 18:28:00 -07:00
dm-round-robin.c
dm-rq.c dm rq: remove unused arguments from rq_completed() 2018-12-18 09:02:25 -05:00
dm-rq.h dm: remove legacy request-based IO path 2018-10-11 11:36:09 -04:00
dm-service-time.c dm mpath selector: more evenly distribute ties 2018-01-29 13:44:58 -05:00
dm-snap-persistent.c dm bufio: move dm-bufio.h to include/linux/ 2018-04-03 15:04:23 -04:00
dm-snap-transient.c
dm-snap.c dm snapshot: Fix excessive memory usage and workqueue stalls 2018-12-18 09:02:26 -05:00
dm-stats.c treewide: kmalloc() -> kmalloc_array() 2018-06-12 16:19:22 -07:00
dm-stats.h License cleanup: add SPDX GPL-2.0 license identifier to files with no license 2017-11-02 11:10:55 +01:00
dm-stripe.c dax: Introduce a ->copy_to_iter dax operation 2018-05-22 23:18:31 -07:00
dm-switch.c treewide: Use array_size() in vmalloc() 2018-06-12 16:19:22 -07:00
dm-sysfs.c dm: remove legacy request-based IO path 2018-10-11 11:36:09 -04:00
dm-table.c block: add queue_is_mq() helper 2018-11-16 08:34:06 -07:00
dm-target.c dm: remove unused macro DM_MOD_NAME_SIZE 2018-04-03 15:04:15 -04:00
dm-thin-metadata.c dm thin metadata: fix __udivdi3 undefined on 32-bit 2018-09-17 11:49:34 -04:00
dm-thin-metadata.h
dm-thin.c dm thin: use refcount_t for thin_c reference counting 2018-10-16 14:27:03 -04:00
dm-uevent.c
dm-uevent.h
dm-unstripe.c dm unstripe: remove unnecessary header includes 2018-04-03 15:04:15 -04:00
dm-verity-fec.c dm: Remove VLA usage from hashes 2018-09-14 14:08:52 +08:00
dm-verity-fec.h dm: convert to bioset_init()/mempool_init() 2018-05-30 15:33:32 -06:00
dm-verity-target.c dm verity: fix crash on bufio buffer that was allocated with vmalloc 2018-09-04 11:25:25 -04:00
dm-verity.h dm verity: add 'check_at_most_once' option to only validate hashes once 2018-04-03 15:04:29 -04:00
dm-writecache.c dm writecache: fix typo in error msg for creating writecache_flush_thread 2018-12-18 09:02:26 -05:00
dm-zero.c dm: don't return errnos from ->map 2017-06-09 09:27:32 -06:00
dm-zoned-metadata.c dm zoned: fix various dmz_get_mblock() issues 2018-10-18 15:17:03 -04:00
dm-zoned-reclaim.c dm kcopyd: return void from dm_kcopyd_copy() 2018-07-31 17:33:21 -04:00
dm-zoned-target.c - Biggest change this cycle is to remove support for the legacy IO path 2018-10-26 12:57:38 -07:00
dm-zoned.h dm zoned: drive-managed zoned block device target 2017-06-19 11:05:20 -04:00
dm.c dm: remove indirect calls from __send_changing_extent_only() 2018-12-18 09:02:26 -05:00
dm.h dm: remove legacy request-based IO path 2018-10-11 11:36:09 -04:00
Kconfig dm: remove legacy request-based IO path 2018-10-11 11:36:09 -04:00
Makefile dm: add writecache target 2018-06-08 11:59:51 -04:00
md-bitmap.c md/bitmap: use mddev_suspend/resume instead of ->quiesce() 2018-10-10 11:03:34 -07:00
md-bitmap.h md: Avoid namespace collision with bitmap API 2018-08-01 15:49:39 -07:00
md-cluster.c md-cluster: remove suspend_info 2018-10-18 09:41:25 -07:00
md-cluster.h md-cluster: introduce resync_info_get interface for sanity check 2018-10-18 09:36:35 -07:00
md-faulty.c md: convert to bioset_init()/mempool_init() 2018-05-30 15:33:32 -06:00
md-linear.c md: convert to bioset_init()/mempool_init() 2018-05-30 15:33:32 -06:00
md-linear.h Merge branch 'for-next' of git://git.kernel.org/pub/scm/linux/kernel/git/shli/md 2017-11-14 16:07:26 -08:00
md-multipath.c treewide: kzalloc() -> kcalloc() 2018-06-12 16:19:22 -07:00
md-multipath.h md: convert to bioset_init()/mempool_init() 2018-05-30 15:33:32 -06:00
md.c block: stop passing 'cpu' to all percpu stats methods 2018-12-10 08:30:37 -07:00
md.h md-cluster/raid10: support add disk under grow mode 2018-10-18 09:34:56 -07:00
raid0.c blkcg: remove bio->bi_css and instead use bio->bi_blkg 2018-12-07 22:26:37 -07:00
raid0.h License cleanup: add SPDX GPL-2.0 license identifier to files with no license 2017-11-02 11:10:55 +01:00
raid1-10.c License cleanup: add SPDX GPL-2.0 license identifier to files with no license 2017-11-02 11:10:55 +01:00
raid1.c MD: fix invalid stored role for a disk - try2 2018-10-14 17:05:07 -07:00
raid1.h md: convert to bioset_init()/mempool_init() 2018-05-30 15:33:32 -06:00
raid5-cache.c md: remove redundant code that is no longer reachable 2018-10-10 10:45:15 -07:00
raid5-log.h md/raid5-cache: disable reshape completely 2018-08-31 17:38:09 -07:00
raid5-ppl.c md: convert to bioset_init()/mempool_init() 2018-05-30 15:33:32 -06:00
raid5.c raid5: block failing device if raid will be failed 2018-09-28 11:13:15 -07:00
raid5.h Merge branch 'for-next' of git://git.kernel.org/pub/scm/linux/kernel/git/shli/md 2018-06-09 12:01:36 -07:00
raid10.c md-cluster: introduce resync_info_get interface for sanity check 2018-10-18 09:36:35 -07:00
raid10.h md: convert to bioset_init()/mempool_init() 2018-05-30 15:33:32 -06:00