Re: [PATCH v7] mm: Add memory allocation watchdog kernel thread.

Re: [PATCH v7] mm: Add memory allocation watchdog kernel thread.

[Tetsuo Handa]

Andrew, do you have any questions on this patch?
I really need this patch for finding bugs which MM people overlook.

Tetsuo Handa wrote:
> This patch adds a watchdog which periodically reports number of memory
> allocating tasks, dying tasks and OOM victim tasks when some task is
> spending too long time inside __alloc_pages_slowpath(). This patch also
> serves as a hook for obtaining additional information using SystemTap
> (e.g. examine other variables using printk(), capture a crash dump by
> calling panic()) by triggering a callback only when a stall is detected.
> Ability to take administrator-controlled actions based on some threshold
> is a big advantage gained by introducing a state tracking.
> 
> Commit 63f53dea0c9866e9 ("mm: warn about allocations which stall for
> too long") was a great step for reducing possibility of silent hang up
> problem caused by memory allocation stalls [1]. However, there are
> reports of long stalls (e.g. [2] is over 30 minutes!) and lockups (e.g.
> [3] is an "unable to invoke the OOM killer due to !__GFP_FS allocation"
> lockup problem) where this patch is more useful than that commit, for
> this patch can report possibly related tasks even if allocating tasks
> are unexpectedly blocked for so long. Regarding premature OOM killer
> invocation, tracepoints which can accumulate samples in short interval
> would be useful. But regarding too late to report allocation stalls,
> this patch which can capture all tasks (for reporting overall situation)
> in longer interval and act as a trigger (for accumulating short interval
> samples) would be useful.

Re: [PATCH v7] mm: Add memory allocation watchdog kernel thread.

[Andrew Morton]

On Thu, 9 Mar 2017 19:46:14 +0900 Tetsuo Handa <penguin-kernel@I-love.SAKURA.ne.jp> wrote:

> Tetsuo Handa wrote:
> > This patch adds a watchdog which periodically reports number of memory
> > allocating tasks, dying tasks and OOM victim tasks when some task is
> > spending too long time inside __alloc_pages_slowpath(). This patch also
> > serves as a hook for obtaining additional information using SystemTap
> > (e.g. examine other variables using printk(), capture a crash dump by
> > calling panic()) by triggering a callback only when a stall is detected.
> > Ability to take administrator-controlled actions based on some threshold
> > is a big advantage gained by introducing a state tracking.
> > 
> > Commit 63f53dea0c9866e9 ("mm: warn about allocations which stall for
> > too long") was a great step for reducing possibility of silent hang up
> > problem caused by memory allocation stalls [1]. However, there are
> > reports of long stalls (e.g. [2] is over 30 minutes!) and lockups (e.g.
> > [3] is an "unable to invoke the OOM killer due to !__GFP_FS allocation"
> > lockup problem) where this patch is more useful than that commit, for
> > this patch can report possibly related tasks even if allocating tasks
> > are unexpectedly blocked for so long. Regarding premature OOM killer
> > invocation, tracepoints which can accumulate samples in short interval
> > would be useful. But regarding too late to report allocation stalls,
> > this patch which can capture all tasks (for reporting overall situation)
> > in longer interval and act as a trigger (for accumulating short interval
> > samples) would be useful.
)
> Andrew, do you have any questions on this patch?
> I really need this patch for finding bugs which MM people overlook.

(top-posting repaired - please don't do that)

Undecided.  I can see the need but it is indeed quite a large lump of
code.  Perhaps some additional examples of how this new code was used
to understand and improve real-world kernel problems would be persuasive.

Re: [PATCH v7] mm: Add memory allocation watchdog kernel thread.

[Michal Hocko]

On Thu 09-03-17 14:37:51, Andrew Morton wrote:
> On Thu, 9 Mar 2017 19:46:14 +0900 Tetsuo Handa <penguin-kernel@I-love.SAKURA.ne.jp> wrote:
> 
> > Tetsuo Handa wrote:
> > > This patch adds a watchdog which periodically reports number of memory
> > > allocating tasks, dying tasks and OOM victim tasks when some task is
> > > spending too long time inside __alloc_pages_slowpath(). This patch also
> > > serves as a hook for obtaining additional information using SystemTap
> > > (e.g. examine other variables using printk(), capture a crash dump by
> > > calling panic()) by triggering a callback only when a stall is detected.
> > > Ability to take administrator-controlled actions based on some threshold
> > > is a big advantage gained by introducing a state tracking.
> > > 
> > > Commit 63f53dea0c9866e9 ("mm: warn about allocations which stall for
> > > too long") was a great step for reducing possibility of silent hang up
> > > problem caused by memory allocation stalls [1]. However, there are
> > > reports of long stalls (e.g. [2] is over 30 minutes!) and lockups (e.g.
> > > [3] is an "unable to invoke the OOM killer due to !__GFP_FS allocation"
> > > lockup problem) where this patch is more useful than that commit, for
> > > this patch can report possibly related tasks even if allocating tasks
> > > are unexpectedly blocked for so long. Regarding premature OOM killer
> > > invocation, tracepoints which can accumulate samples in short interval
> > > would be useful. But regarding too late to report allocation stalls,
> > > this patch which can capture all tasks (for reporting overall situation)
> > > in longer interval and act as a trigger (for accumulating short interval
> > > samples) would be useful.
> )
> > Andrew, do you have any questions on this patch?
> > I really need this patch for finding bugs which MM people overlook.
> 
> (top-posting repaired - please don't do that)
> 
> Undecided.  I can see the need but it is indeed quite a large lump of
> code.  Perhaps some additional examples of how this new code was used
> to understand and improve real-world kernel problems would be persuasive.

Well, it is true that the watchdog can provide much more information
than we can gather with other debugging options we currently have when
allocations run away.  So I am not questioning this is useful when doing
memory stress testing and trying to trigger corner cases but I am not
really sure how much this will be useful in production systems, though.
Tracking is not for free both in runtime and longterm in maintenance.
-- 
Michal Hocko
SUSE Labs

Re: [PATCH v7] mm: Add memory allocation watchdog kernel thread.

[Tetsuo Handa]

Andrew Morton wrote:
> On Thu, 9 Mar 2017 19:46:14 +0900 Tetsuo Handa <penguin-kernel@I-love.SAKURA.ne.jp> wrote:
> > Tetsuo Handa wrote:
> > > This patch adds a watchdog which periodically reports number of memory
> > > allocating tasks, dying tasks and OOM victim tasks when some task is
> > > spending too long time inside __alloc_pages_slowpath(). This patch also
> > > serves as a hook for obtaining additional information using SystemTap
> > > (e.g. examine other variables using printk(), capture a crash dump by
> > > calling panic()) by triggering a callback only when a stall is detected.
> > > Ability to take administrator-controlled actions based on some threshold
> > > is a big advantage gained by introducing a state tracking.
> > > 
> > > Commit 63f53dea0c9866e9 ("mm: warn about allocations which stall for
> > > too long") was a great step for reducing possibility of silent hang up
> > > problem caused by memory allocation stalls [1]. However, there are
> > > reports of long stalls (e.g. [2] is over 30 minutes!) and lockups (e.g.
> > > [3] is an "unable to invoke the OOM killer due to !__GFP_FS allocation"
> > > lockup problem) where this patch is more useful than that commit, for
> > > this patch can report possibly related tasks even if allocating tasks
> > > are unexpectedly blocked for so long. Regarding premature OOM killer
> > > invocation, tracepoints which can accumulate samples in short interval
> > > would be useful. But regarding too late to report allocation stalls,
> > > this patch which can capture all tasks (for reporting overall situation)
> > > in longer interval and act as a trigger (for accumulating short interval
> > > samples) would be useful.
> )
> > Andrew, do you have any questions on this patch?
> > I really need this patch for finding bugs which MM people overlook.
> 
> Undecided.  I can see the need but it is indeed quite a large lump of
> code.  Perhaps some additional examples of how this new code was used
> to understand and improve real-world kernel problems would be persuasive.

This patch is aimed for help bisecting whether unexpected hung cases are related to
memory allocation. By merging this patch (and enabling this watchdog in enterprise
systems via kernels supported by distributors), we can identify patterns/cases of
problems (if related to memory allocation) and improve quality of Linux kernels
by fixing problems related to memory allocation.

I was working at a support center for 3 years and had many cases where Linux
systems encountered unexpected silent hangup/reboot problem. In most cases, all
clue available was limited to sar (in sysstat package) showing that free memory
was low just before unexpected hung/reboot.

As a nature of such problems, it is very hard for administrators to collect
information for analysis; let alone identify the cause of problems. As a result,
such problems are left unrecognized/unsolved at the support center, and are
seldom reported to distributors/developers in order to ask for fixes.

Michal Hocko wrote:
> Well, it is true that the watchdog can provide much more information
> than we can gather with other debugging options we currently have when
> allocations run away.  So I am not questioning this is useful when doing
> memory stress testing and trying to trigger corner cases but I am not
> really sure how much this will be useful in production systems, though.
> Tracking is not for free both in runtime and longterm in maintenance.

I don't think that my stress testing is hitting bugs which do not occur in
production systems. But I can't prove it without using watchdog mechanism.
Today I heard about a case where an enterprise system hung last Sunday due to
XFS deadlock by memory allocation. This was rather a lucky case where
administrator of that system was able to capture vmcore. In most cases,
administrators can't capture even SysRq-t; let alone vmcore. Therefore,
automatic watchdog is highly appreciated.



Or, did you mean "some additional examples of how this new code was used
to understand and improve real-world kernel problems" as how this patch
helped in [2] [3] ?

Regarding [3] (now continued as
http://lkml.kernel.org/r/201703031948.CHJ81278.VOHSFFFOOLJQMt@I-love.SAKURA.ne.jp ),
I used this patch for confirming the following things.

  (1) kswapd cannot make progress waiting for lock.

----------
[  518.900038] MemAlloc: kswapd0(69) flags=0xa40840 switches=23883 uninterruptible
[  518.902095] kswapd0         D10776    69      2 0x00000000
[  518.903784] Call Trace:
[  518.904849]  __schedule+0x336/0xe00
[  518.906118]  schedule+0x3d/0x90
[  518.907314]  io_schedule+0x16/0x40
[  518.908622]  __xfs_iflock+0x129/0x140 [xfs]
[  518.910027]  ? autoremove_wake_function+0x60/0x60
[  518.911559]  xfs_reclaim_inode+0x162/0x440 [xfs]
[  518.913068]  xfs_reclaim_inodes_ag+0x2cf/0x4f0 [xfs]
[  518.914611]  ? xfs_reclaim_inodes_ag+0xf2/0x4f0 [xfs]
[  518.916148]  ? trace_hardirqs_on+0xd/0x10
[  518.917465]  ? try_to_wake_up+0x59/0x7a0
[  518.918758]  ? wake_up_process+0x15/0x20
[  518.920067]  xfs_reclaim_inodes_nr+0x33/0x40 [xfs]
[  518.921560]  xfs_fs_free_cached_objects+0x19/0x20 [xfs]
[  518.923114]  super_cache_scan+0x181/0x190
[  518.924435]  shrink_slab+0x29f/0x6d0
[  518.925683]  shrink_node+0x2fa/0x310
[  518.926909]  kswapd+0x362/0x9b0
[  518.928061]  kthread+0x10f/0x150
[  518.929218]  ? mem_cgroup_shrink_node+0x3b0/0x3b0
[  518.930953]  ? kthread_create_on_node+0x70/0x70
[  518.932380]  ret_from_fork+0x31/0x40

[ 1095.632984] MemAlloc: kswapd0(69) flags=0xa40840 switches=23883 uninterruptible
[ 1095.632985] kswapd0         D10776    69      2 0x00000000
[ 1095.632988] Call Trace:
[ 1095.632991]  __schedule+0x336/0xe00
[ 1095.632994]  schedule+0x3d/0x90
[ 1095.632996]  io_schedule+0x16/0x40
[ 1095.633017]  __xfs_iflock+0x129/0x140 [xfs]
[ 1095.633021]  ? autoremove_wake_function+0x60/0x60
[ 1095.633051]  xfs_reclaim_inode+0x162/0x440 [xfs]
[ 1095.633072]  xfs_reclaim_inodes_ag+0x2cf/0x4f0 [xfs]
[ 1095.633106]  ? xfs_reclaim_inodes_ag+0xf2/0x4f0 [xfs]
[ 1095.633114]  ? trace_hardirqs_on+0xd/0x10
[ 1095.633116]  ? try_to_wake_up+0x59/0x7a0
[ 1095.633120]  ? wake_up_process+0x15/0x20
[ 1095.633156]  xfs_reclaim_inodes_nr+0x33/0x40 [xfs]
[ 1095.633178]  xfs_fs_free_cached_objects+0x19/0x20 [xfs]
[ 1095.633180]  super_cache_scan+0x181/0x190
[ 1095.633183]  shrink_slab+0x29f/0x6d0
[ 1095.633189]  shrink_node+0x2fa/0x310
[ 1095.633193]  kswapd+0x362/0x9b0
[ 1095.633200]  kthread+0x10f/0x150
[ 1095.633201]  ? mem_cgroup_shrink_node+0x3b0/0x3b0
[ 1095.633202]  ? kthread_create_on_node+0x70/0x70
[ 1095.633205]  ret_from_fork+0x31/0x40
----------

  (2) All WQ_MEM_RECLAIM threads shown by show_workqueue_state()
      cannot make progress waiting for memory allocation.

----------
[ 1095.633625] MemAlloc: xfs-data/sda1(451) flags=0x4228060 switches=45509 seq=1 gfp=0x1604240(GFP_NOFS|__GFP_NOWARN|__GFP_COMP|__GFP_NOTRACK) order=0 delay=652073
[ 1095.633626] xfs-data/sda1   R  running task    12696   451      2 0x00000000
[ 1095.633663] Workqueue: xfs-data/sda1 xfs_end_io [xfs]
[ 1095.633665] Call Trace:
[ 1095.633668]  __schedule+0x336/0xe00
[ 1095.633671]  schedule+0x3d/0x90
[ 1095.633672]  schedule_timeout+0x20d/0x510
[ 1095.633675]  ? lock_timer_base+0xa0/0xa0
[ 1095.633678]  schedule_timeout_uninterruptible+0x2a/0x30
[ 1095.633680]  __alloc_pages_slowpath+0x2b5/0xd95
[ 1095.633687]  __alloc_pages_nodemask+0x3e4/0x460
[ 1095.633699]  alloc_pages_current+0x97/0x1b0
[ 1095.633702]  new_slab+0x4cb/0x6b0
[ 1095.633706]  ___slab_alloc+0x3a3/0x620
[ 1095.633728]  ? kmem_alloc+0x96/0x120 [xfs]
[ 1095.633730]  ? ___slab_alloc+0x5c6/0x620
[ 1095.633732]  ? cpuacct_charge+0x38/0x1e0
[ 1095.633767]  ? kmem_alloc+0x96/0x120 [xfs]
[ 1095.633770]  __slab_alloc+0x46/0x7d
[ 1095.633773]  __kmalloc+0x301/0x3b0
[ 1095.633802]  kmem_alloc+0x96/0x120 [xfs]
[ 1095.633804]  ? kfree+0x1fa/0x330
[ 1095.633842]  xfs_log_commit_cil+0x489/0x710 [xfs]
[ 1095.633864]  __xfs_trans_commit+0x83/0x260 [xfs]
[ 1095.633883]  xfs_trans_commit+0x10/0x20 [xfs]
[ 1095.633901]  __xfs_setfilesize+0xdb/0x240 [xfs]
[ 1095.633936]  xfs_setfilesize_ioend+0x89/0xb0 [xfs]
[ 1095.633954]  ? xfs_setfilesize_ioend+0x5/0xb0 [xfs]
[ 1095.633971]  xfs_end_io+0x81/0x110 [xfs]
[ 1095.633973]  process_one_work+0x22b/0x760
[ 1095.633975]  ? process_one_work+0x194/0x760
[ 1095.633997]  rescuer_thread+0x1f2/0x3d0
[ 1095.634002]  kthread+0x10f/0x150
[ 1095.634003]  ? worker_thread+0x4b0/0x4b0
[ 1095.634004]  ? kthread_create_on_node+0x70/0x70
[ 1095.634007]  ret_from_fork+0x31/0x40
[ 1095.634013] MemAlloc: xfs-eofblocks/s(456) flags=0x4228860 switches=15435 seq=1 gfp=0x1400240(GFP_NOFS|__GFP_NOWARN) order=0 delay=293074
[ 1095.634014] xfs-eofblocks/s R  running task    12032   456      2 0x00000000
[ 1095.634037] Workqueue: xfs-eofblocks/sda1 xfs_eofblocks_worker [xfs]
[ 1095.634038] Call Trace:
[ 1095.634040]  ? _raw_spin_lock+0x3d/0x80
[ 1095.634042]  ? vmpressure+0xd0/0x120
[ 1095.634044]  ? vmpressure+0xd0/0x120
[ 1095.634047]  ? vmpressure_prio+0x21/0x30
[ 1095.634049]  ? do_try_to_free_pages+0x70/0x300
[ 1095.634052]  ? try_to_free_pages+0x131/0x3f0
[ 1095.634058]  ? __alloc_pages_slowpath+0x3ec/0xd95
[ 1095.634065]  ? __alloc_pages_nodemask+0x3e4/0x460
[ 1095.634069]  ? alloc_pages_current+0x97/0x1b0
[ 1095.634111]  ? xfs_buf_allocate_memory+0x160/0x2a3 [xfs]
[ 1095.634133]  ? xfs_buf_get_map+0x2be/0x480 [xfs]
[ 1095.634169]  ? xfs_buf_read_map+0x2c/0x400 [xfs]
[ 1095.634204]  ? xfs_trans_read_buf_map+0x186/0x830 [xfs]
[ 1095.634222]  ? xfs_btree_read_buf_block.constprop.34+0x78/0xc0 [xfs]
[ 1095.634239]  ? xfs_btree_lookup_get_block+0x8a/0x180 [xfs]
[ 1095.634257]  ? xfs_btree_lookup+0xd0/0x3f0 [xfs]
[ 1095.634296]  ? kmem_zone_alloc+0x96/0x120 [xfs]
[ 1095.634299]  ? _raw_spin_unlock+0x27/0x40
[ 1095.634315]  ? xfs_bmbt_lookup_eq+0x1f/0x30 [xfs]
[ 1095.634348]  ? xfs_bmap_del_extent+0x1b2/0x1610 [xfs]
[ 1095.634380]  ? kmem_zone_alloc+0x96/0x120 [xfs]
[ 1095.634400]  ? __xfs_bunmapi+0x4db/0xda0 [xfs]
[ 1095.634421]  ? xfs_bunmapi+0x2b/0x40 [xfs]
[ 1095.634459]  ? xfs_itruncate_extents+0x1df/0x780 [xfs]
[ 1095.634502]  ? xfs_rename+0xc70/0x1080 [xfs]
[ 1095.634525]  ? xfs_free_eofblocks+0x1c4/0x230 [xfs]
[ 1095.634546]  ? xfs_inode_free_eofblocks+0x18d/0x280 [xfs]
[ 1095.634565]  ? xfs_inode_ag_walk.isra.13+0x2b5/0x620 [xfs]
[ 1095.634582]  ? xfs_inode_ag_walk.isra.13+0x91/0x620 [xfs]
[ 1095.634618]  ? xfs_inode_clear_eofblocks_tag+0x1a0/0x1a0 [xfs]
[ 1095.634630]  ? radix_tree_next_chunk+0x10b/0x2d0
[ 1095.634635]  ? radix_tree_gang_lookup_tag+0xd7/0x150
[ 1095.634672]  ? xfs_perag_get_tag+0x11d/0x370 [xfs]
[ 1095.634690]  ? xfs_perag_get_tag+0x5/0x370 [xfs]
[ 1095.634709]  ? xfs_inode_ag_iterator_tag+0x71/0xa0 [xfs]
[ 1095.634726]  ? xfs_inode_clear_eofblocks_tag+0x1a0/0x1a0 [xfs]
[ 1095.634744]  ? __xfs_icache_free_eofblocks+0x3b/0x40 [xfs]
[ 1095.634759]  ? xfs_eofblocks_worker+0x27/0x40 [xfs]
[ 1095.634762]  ? process_one_work+0x22b/0x760
[ 1095.634763]  ? process_one_work+0x194/0x760
[ 1095.634784]  ? rescuer_thread+0x1f2/0x3d0
[ 1095.634788]  ? kthread+0x10f/0x150
[ 1095.634789]  ? worker_thread+0x4b0/0x4b0
[ 1095.634790]  ? kthread_create_on_node+0x70/0x70
[ 1095.634793]  ? ret_from_fork+0x31/0x40
----------

  (3) order-0 GFP_NOIO allocation request cannot make progress
      waiting for memory allocation.

----------
[ 1095.631687] MemAlloc: kworker/2:0(27) flags=0x4208860 switches=38727 seq=21 gfp=0x1400000(GFP_NOIO) order=0 delay=652160
[ 1095.631688] kworker/2:0     R  running task    12680    27      2 0x00000000
[ 1095.631739] Workqueue: events_freezable_power_ disk_events_workfn
[ 1095.631740] Call Trace:
[ 1095.631743]  __schedule+0x336/0xe00
[ 1095.631746]  preempt_schedule_common+0x1f/0x31
[ 1095.631747]  _cond_resched+0x1c/0x30
[ 1095.631749]  shrink_slab+0x339/0x6d0
[ 1095.631754]  shrink_node+0x2fa/0x310
[ 1095.631758]  do_try_to_free_pages+0xe1/0x300
[ 1095.631761]  try_to_free_pages+0x131/0x3f0
[ 1095.631765]  __alloc_pages_slowpath+0x3ec/0xd95
[ 1095.631771]  __alloc_pages_nodemask+0x3e4/0x460
[ 1095.631775]  alloc_pages_current+0x97/0x1b0
[ 1095.631779]  bio_copy_kern+0xc9/0x180
[ 1095.631830]  blk_rq_map_kern+0x70/0x140
[ 1095.631835]  __scsi_execute.isra.22+0x13a/0x1e0
[ 1095.631838]  scsi_execute_req_flags+0x94/0x100
[ 1095.631844]  sr_check_events+0xbf/0x2b0 [sr_mod]
[ 1095.631852]  cdrom_check_events+0x18/0x30 [cdrom]
[ 1095.631854]  sr_block_check_events+0x2a/0x30 [sr_mod]
[ 1095.631856]  disk_check_events+0x60/0x170
[ 1095.631859]  disk_events_workfn+0x1c/0x20
[ 1095.631862]  process_one_work+0x22b/0x760
[ 1095.631863]  ? process_one_work+0x194/0x760
[ 1095.631867]  worker_thread+0x137/0x4b0
[ 1095.631887]  kthread+0x10f/0x150
[ 1095.631889]  ? process_one_work+0x760/0x760
[ 1095.631890]  ? kthread_create_on_node+0x70/0x70
[ 1095.631893]  ret_from_fork+0x31/0x40
----------

  (4) Number of stalling threads does not decrease over time while
      number of out_of_memory() calls increases over time.

----------
[  518.090012] MemAlloc-Info: stalling=184 dying=1 exiting=0 victim=1 oom_count=8441307
[  553.070829] MemAlloc-Info: stalling=184 dying=1 exiting=0 victim=1 oom_count=10318507
[  616.394649] MemAlloc-Info: stalling=186 dying=1 exiting=0 victim=1 oom_count=13908219
[  642.266252] MemAlloc-Info: stalling=186 dying=1 exiting=0 victim=1 oom_count=15180673
[  702.412189] MemAlloc-Info: stalling=187 dying=1 exiting=0 victim=1 oom_count=18732529
[  736.787879] MemAlloc-Info: stalling=187 dying=1 exiting=0 victim=1 oom_count=20565244
[  800.715759] MemAlloc-Info: stalling=188 dying=1 exiting=0 victim=1 oom_count=24411576
[  837.571405] MemAlloc-Info: stalling=188 dying=1 exiting=0 victim=1 oom_count=26463562
[  899.021495] MemAlloc-Info: stalling=189 dying=1 exiting=0 victim=1 oom_count=30144879
[  936.282709] MemAlloc-Info: stalling=189 dying=1 exiting=0 victim=1 oom_count=32129234
[  997.328119] MemAlloc-Info: stalling=190 dying=1 exiting=0 victim=1 oom_count=35657983
[ 1033.977265] MemAlloc-Info: stalling=190 dying=1 exiting=0 victim=1 oom_count=37659912
[ 1095.630961] MemAlloc-Info: stalling=190 dying=1 exiting=0 victim=1 oom_count=40639677
[ 1095.821248] MemAlloc-Info: stalling=190 dying=1 exiting=0 victim=1 oom_count=40646791
----------

Further interpretation is up to XFS people (Brian and Dave). So far, boosting priority
(lowering watermark) of WQ_MEM_RECLAIM threads based on current_is_workqueue_rescuer()
seems to be considered as a fix because writeback path already has a special case
handling for the rescuer. But I'm not sure whether it helps getting writeback unstuck
when writeback path is blocked on a lock rather than memory allocation. (Threads doing
writeback and/or mempool allocations might be candidates for always reporting as with
kswapd. That would be in followup patches.)

----------
[ 1044.701393] kworker/u256:0  D10024  8444      2 0x00000080
[ 1044.703017] Workqueue: writeback wb_workfn (flush-8:0)
[ 1044.704550] Call Trace:
[ 1044.705550]  __schedule+0x336/0xe00
[ 1044.706761]  schedule+0x3d/0x90
[ 1044.707891]  io_schedule+0x16/0x40
[ 1044.709073]  __lock_page+0x126/0x180
[ 1044.710272]  ? page_cache_tree_insert+0x120/0x120
[ 1044.711728]  write_cache_pages+0x39a/0x6b0
[ 1044.713077]  ? xfs_map_blocks+0x5a0/0x5a0 [xfs]
[ 1044.714499]  ? xfs_vm_writepages+0x48/0xa0 [xfs]
[ 1044.715944]  xfs_vm_writepages+0x6b/0xa0 [xfs]
[ 1044.717362]  do_writepages+0x21/0x40
[ 1044.718577]  __writeback_single_inode+0x72/0x9d0
[ 1044.719998]  ? _raw_spin_unlock+0x27/0x40
[ 1044.721308]  writeback_sb_inodes+0x322/0x750
[ 1044.722662]  __writeback_inodes_wb+0x8c/0xc0
[ 1044.724009]  wb_writeback+0x3be/0x6e0
[ 1044.725238]  wb_workfn+0x146/0x7d0
[ 1044.726413]  ? process_one_work+0x194/0x760
[ 1044.727736]  process_one_work+0x22b/0x760
[ 1044.729017]  ? process_one_work+0x194/0x760
[ 1044.730343]  worker_thread+0x137/0x4b0
[ 1044.731577]  kthread+0x10f/0x150
[ 1044.732702]  ? process_one_work+0x760/0x760
[ 1044.734021]  ? kthread_create_on_node+0x70/0x70
[ 1044.735415]  ? do_syscall_64+0x195/0x200
[ 1044.736684]  ret_from_fork+0x31/0x40
----------

Regarding "infinite too_many_isolated() loop" case
( http://lkml.kernel.org/r/201702212335.DJB30777.JOFMHSFtVLQOOF@I-love.SAKURA.ne.jp ),
I used this patch for confirming the following things.

  (1) kswapd cannot make progress waiting for lock.

----------
[ 1209.790966] MemAlloc: kswapd0(67) flags=0xa60840 switches=51139 uninterruptible
[ 1209.799726] kswapd0         D10936    67      2 0x00000000
[ 1209.807326] Call Trace:
[ 1209.812581]  __schedule+0x336/0xe00
[ 1209.818599]  schedule+0x3d/0x90
[ 1209.823907]  schedule_timeout+0x26a/0x510
[ 1209.827218]  ? trace_hardirqs_on+0xd/0x10
[ 1209.830535]  __down_common+0xfb/0x131
[ 1209.833801]  ? _xfs_buf_find+0x2cb/0xc10 [xfs]
[ 1209.837372]  __down+0x1d/0x1f
[ 1209.840331]  down+0x41/0x50
[ 1209.843243]  xfs_buf_lock+0x64/0x370 [xfs]
[ 1209.846597]  _xfs_buf_find+0x2cb/0xc10 [xfs]
[ 1209.850031]  ? _xfs_buf_find+0xa4/0xc10 [xfs]
[ 1209.853514]  xfs_buf_get_map+0x2a/0x480 [xfs]
[ 1209.855831]  xfs_buf_read_map+0x2c/0x400 [xfs]
[ 1209.857388]  ? free_debug_processing+0x27d/0x2af
[ 1209.859037]  xfs_trans_read_buf_map+0x186/0x830 [xfs]
[ 1209.860707]  xfs_read_agf+0xc8/0x2b0 [xfs]
[ 1209.862184]  xfs_alloc_read_agf+0x7a/0x300 [xfs]
[ 1209.863728]  ? xfs_alloc_space_available+0x7b/0x120 [xfs]
[ 1209.865385]  xfs_alloc_fix_freelist+0x3bc/0x490 [xfs]
[ 1209.866974]  ? __radix_tree_lookup+0x84/0xf0
[ 1209.868374]  ? xfs_perag_get+0x1a0/0x310 [xfs]
[ 1209.869798]  ? xfs_perag_get+0x5/0x310 [xfs]
[ 1209.871288]  xfs_alloc_vextent+0x161/0xda0 [xfs]
[ 1209.872757]  xfs_bmap_btalloc+0x46c/0x8b0 [xfs]
[ 1209.874182]  ? save_stack_trace+0x1b/0x20
[ 1209.875542]  xfs_bmap_alloc+0x17/0x30 [xfs]
[ 1209.876847]  xfs_bmapi_write+0x74e/0x11d0 [xfs]
[ 1209.878190]  xfs_iomap_write_allocate+0x199/0x3a0 [xfs]
[ 1209.879632]  xfs_map_blocks+0x2cc/0x5a0 [xfs]
[ 1209.880909]  xfs_do_writepage+0x215/0x920 [xfs]
[ 1209.882255]  ? clear_page_dirty_for_io+0xb4/0x310
[ 1209.883598]  xfs_vm_writepage+0x3b/0x70 [xfs]
[ 1209.884841]  pageout.isra.54+0x1a4/0x460
[ 1209.886210]  shrink_page_list+0xa86/0xcf0
[ 1209.887441]  shrink_inactive_list+0x1c5/0x660
[ 1209.888682]  shrink_node_memcg+0x535/0x7f0
[ 1209.889975]  ? mem_cgroup_iter+0x14d/0x720
[ 1209.891197]  shrink_node+0xe1/0x310
[ 1209.892288]  kswapd+0x362/0x9b0
[ 1209.893308]  kthread+0x10f/0x150
[ 1209.894383]  ? mem_cgroup_shrink_node+0x3b0/0x3b0
[ 1209.895703]  ? kthread_create_on_node+0x70/0x70
[ 1209.896956]  ret_from_fork+0x31/0x40
----------

  (2) Both GFP_IO and GFP_KERNEL allocations are stuck at
      too_many_isolated() loop.

----------
[ 1209.898117] MemAlloc: systemd-journal(526) flags=0x400900 switches=33248 seq=121659 gfp=0x14201ca(GFP_HIGHUSER_MOVABLE|__GFP_COLD) order=0 delay=52772 uninterruptible
[ 1209.902154] systemd-journal D11240   526      1 0x00000000
[ 1209.903642] Call Trace:
[ 1209.904574]  __schedule+0x336/0xe00
[ 1209.905734]  schedule+0x3d/0x90
[ 1209.906817]  schedule_timeout+0x20d/0x510
[ 1209.908025]  ? prepare_to_wait+0x2b/0xc0
[ 1209.909268]  ? lock_timer_base+0xa0/0xa0
[ 1209.910460]  io_schedule_timeout+0x1e/0x50
[ 1209.911681]  congestion_wait+0x86/0x260
[ 1209.912853]  ? remove_wait_queue+0x60/0x60
[ 1209.914115]  shrink_inactive_list+0x5b4/0x660
[ 1209.915385]  ? __list_lru_count_one.isra.2+0x22/0x80
[ 1209.916768]  shrink_node_memcg+0x535/0x7f0
[ 1209.918173]  shrink_node+0xe1/0x310
[ 1209.919288]  do_try_to_free_pages+0xe1/0x300
[ 1209.920548]  try_to_free_pages+0x131/0x3f0
[ 1209.921827]  __alloc_pages_slowpath+0x3ec/0xd95
[ 1209.923137]  __alloc_pages_nodemask+0x3e4/0x460
[ 1209.924454]  ? __radix_tree_lookup+0x84/0xf0
[ 1209.925790]  alloc_pages_current+0x97/0x1b0
[ 1209.927021]  ? find_get_entry+0x5/0x300
[ 1209.928189]  __page_cache_alloc+0x15d/0x1a0
[ 1209.929471]  ? pagecache_get_page+0x2c/0x2b0
[ 1209.930716]  filemap_fault+0x4df/0x8b0
[ 1209.931867]  ? filemap_fault+0x373/0x8b0
[ 1209.933111]  ? xfs_ilock+0x22c/0x360 [xfs]
[ 1209.934510]  ? xfs_filemap_fault+0x64/0x1e0 [xfs]
[ 1209.935857]  ? down_read_nested+0x7b/0xc0
[ 1209.937123]  ? xfs_ilock+0x22c/0x360 [xfs]
[ 1209.938373]  xfs_filemap_fault+0x6c/0x1e0 [xfs]
[ 1209.939691]  __do_fault+0x1e/0xa0
[ 1209.940807]  ? _raw_spin_unlock+0x27/0x40
[ 1209.942002]  __handle_mm_fault+0xbb1/0xf40
[ 1209.943228]  ? mutex_unlock+0x12/0x20
[ 1209.944410]  ? devkmsg_read+0x15c/0x330
[ 1209.945912]  handle_mm_fault+0x16b/0x390
[ 1209.947297]  ? handle_mm_fault+0x49/0x390
[ 1209.948868]  __do_page_fault+0x24a/0x530
[ 1209.950351]  do_page_fault+0x30/0x80
[ 1209.951615]  page_fault+0x28/0x30

[ 1210.538496] MemAlloc: kworker/3:0(6345) flags=0x4208860 switches=10134 seq=22 gfp=0x1400000(GFP_NOIO) order=0 delay=45953 uninterruptible
[ 1210.541487] kworker/3:0     D12560  6345      2 0x00000080
[ 1210.542991] Workqueue: events_freezable_power_ disk_events_workfn
[ 1210.544577] Call Trace:
[ 1210.545468]  __schedule+0x336/0xe00
[ 1210.546606]  schedule+0x3d/0x90
[ 1210.547616]  schedule_timeout+0x20d/0x510
[ 1210.548778]  ? prepare_to_wait+0x2b/0xc0
[ 1210.550013]  ? lock_timer_base+0xa0/0xa0
[ 1210.551208]  io_schedule_timeout+0x1e/0x50
[ 1210.552519]  congestion_wait+0x86/0x260
[ 1210.553650]  ? remove_wait_queue+0x60/0x60
[ 1210.554900]  shrink_inactive_list+0x5b4/0x660
[ 1210.556119]  ? __list_lru_count_one.isra.2+0x22/0x80
[ 1210.557447]  shrink_node_memcg+0x535/0x7f0
[ 1210.558714]  shrink_node+0xe1/0x310
[ 1210.559803]  do_try_to_free_pages+0xe1/0x300
[ 1210.561009]  try_to_free_pages+0x131/0x3f0
[ 1210.562250]  __alloc_pages_slowpath+0x3ec/0xd95
[ 1210.563506]  __alloc_pages_nodemask+0x3e4/0x460
[ 1210.564777]  alloc_pages_current+0x97/0x1b0
[ 1210.566017]  bio_copy_kern+0xc9/0x180
[ 1210.567116]  blk_rq_map_kern+0x70/0x140
[ 1210.568356]  __scsi_execute.isra.22+0x13a/0x1e0
[ 1210.569839]  scsi_execute_req_flags+0x94/0x100
[ 1210.571218]  sr_check_events+0xbf/0x2b0 [sr_mod]
[ 1210.572500]  cdrom_check_events+0x18/0x30 [cdrom]
[ 1210.573934]  sr_block_check_events+0x2a/0x30 [sr_mod]
[ 1210.575335]  disk_check_events+0x60/0x170
[ 1210.576509]  disk_events_workfn+0x1c/0x20
[ 1210.577744]  process_one_work+0x22b/0x760
[ 1210.578934]  ? process_one_work+0x194/0x760
[ 1210.580147]  worker_thread+0x137/0x4b0
[ 1210.581336]  kthread+0x10f/0x150
[ 1210.582365]  ? process_one_work+0x760/0x760
[ 1210.583603]  ? kthread_create_on_node+0x70/0x70
[ 1210.584961]  ? do_syscall_64+0x6c/0x200
[ 1210.586343]  ret_from_fork+0x31/0x40
----------

  (3) Number of stalling threads does not decrease over time and
      number of out_of_memory() calls does not increase over time.

----------
[ 1209.781787] MemAlloc-Info: stalling=32 dying=1 exiting=0 victim=1 oom_count=45896
[ 1212.195351] MemAlloc-Info: stalling=32 dying=1 exiting=0 victim=1 oom_count=45896
[ 1242.551629] MemAlloc-Info: stalling=36 dying=1 exiting=0 victim=1 oom_count=45896
[ 1245.149165] MemAlloc-Info: stalling=36 dying=1 exiting=0 victim=1 oom_count=45896
[ 1275.319189] MemAlloc-Info: stalling=40 dying=1 exiting=0 victim=1 oom_count=45896
[ 1278.241813] MemAlloc-Info: stalling=40 dying=1 exiting=0 victim=1 oom_count=45896
----------

Regarding [2], Alexander, Brian and Dave can explain it better than I. But
I think that "check threads which cannot make progress" principle is same.



> (top-posting repaired - please don't do that)

Hmm, I don't see anything wrong with mail headers of replied one that can
cause top-posting. I think something wired occurred outside of my control.

  Subject: Re: [PATCH v7] mm: Add memory allocation watchdog kernel thread.
  From: Tetsuo Handa <penguin-kernel@I-love.SAKURA.ne.jp>
  References: <1488244908-57586-1-git-send-email-penguin-kernel@I-love.SAKURA.ne.jp>
  In-Reply-To: <1488244908-57586-1-git-send-email-penguin-kernel@I-love.SAKURA.ne.jp>
  Message-Id: <201703091946.GDC21885.OQFFOtJHSOFVML@I-love.SAKURA.ne.jp>
  Date: Thu, 9 Mar 2017 19:46:14 +0900

Re: [PATCH v7] mm: Add memory allocation watchdog kernel thread.

[Brian Foster]

On Fri, Mar 10, 2017 at 08:19:58PM +0900, Tetsuo Handa wrote:
> Andrew Morton wrote:
> > On Thu, 9 Mar 2017 19:46:14 +0900 Tetsuo Handa <penguin-kernel@I-love.SAKURA.ne.jp> wrote:
> > > Tetsuo Handa wrote:
> > > > This patch adds a watchdog which periodically reports number of memory
> > > > allocating tasks, dying tasks and OOM victim tasks when some task is
> > > > spending too long time inside __alloc_pages_slowpath(). This patch also
> > > > serves as a hook for obtaining additional information using SystemTap
> > > > (e.g. examine other variables using printk(), capture a crash dump by
> > > > calling panic()) by triggering a callback only when a stall is detected.
> > > > Ability to take administrator-controlled actions based on some threshold
> > > > is a big advantage gained by introducing a state tracking.
> > > > 
> > > > Commit 63f53dea0c9866e9 ("mm: warn about allocations which stall for
> > > > too long") was a great step for reducing possibility of silent hang up
> > > > problem caused by memory allocation stalls [1]. However, there are
> > > > reports of long stalls (e.g. [2] is over 30 minutes!) and lockups (e.g.
> > > > [3] is an "unable to invoke the OOM killer due to !__GFP_FS allocation"
> > > > lockup problem) where this patch is more useful than that commit, for
> > > > this patch can report possibly related tasks even if allocating tasks
> > > > are unexpectedly blocked for so long. Regarding premature OOM killer
> > > > invocation, tracepoints which can accumulate samples in short interval
> > > > would be useful. But regarding too late to report allocation stalls,
> > > > this patch which can capture all tasks (for reporting overall situation)
> > > > in longer interval and act as a trigger (for accumulating short interval
> > > > samples) would be useful.
> > )
> > > Andrew, do you have any questions on this patch?
> > > I really need this patch for finding bugs which MM people overlook.
> > 
> > Undecided.  I can see the need but it is indeed quite a large lump of
> > code.  Perhaps some additional examples of how this new code was used
> > to understand and improve real-world kernel problems would be persuasive.
> 
> This patch is aimed for help bisecting whether unexpected hung cases are related to
> memory allocation. By merging this patch (and enabling this watchdog in enterprise
> systems via kernels supported by distributors), we can identify patterns/cases of
> problems (if related to memory allocation) and improve quality of Linux kernels
> by fixing problems related to memory allocation.
> 
> I was working at a support center for 3 years and had many cases where Linux
> systems encountered unexpected silent hangup/reboot problem. In most cases, all
> clue available was limited to sar (in sysstat package) showing that free memory
> was low just before unexpected hung/reboot.
> 
> As a nature of such problems, it is very hard for administrators to collect
> information for analysis; let alone identify the cause of problems. As a result,
> such problems are left unrecognized/unsolved at the support center, and are
> seldom reported to distributors/developers in order to ask for fixes.
> 
> Michal Hocko wrote:
> > Well, it is true that the watchdog can provide much more information
> > than we can gather with other debugging options we currently have when
> > allocations run away.  So I am not questioning this is useful when doing
> > memory stress testing and trying to trigger corner cases but I am not
> > really sure how much this will be useful in production systems, though.
> > Tracking is not for free both in runtime and longterm in maintenance.
> 
> I don't think that my stress testing is hitting bugs which do not occur in
> production systems. But I can't prove it without using watchdog mechanism.
> Today I heard about a case where an enterprise system hung last Sunday due to
> XFS deadlock by memory allocation. This was rather a lucky case where
> administrator of that system was able to capture vmcore. In most cases,
> administrators can't capture even SysRq-t; let alone vmcore. Therefore,
> automatic watchdog is highly appreciated.
> 
> 
> 
> Or, did you mean "some additional examples of how this new code was used
> to understand and improve real-world kernel problems" as how this patch
> helped in [2] [3] ?
> 
> Regarding [3] (now continued as
> http://lkml.kernel.org/r/201703031948.CHJ81278.VOHSFFFOOLJQMt@I-love.SAKURA.ne.jp ),
> I used this patch for confirming the following things.
> 
>   (1) kswapd cannot make progress waiting for lock.
> 
> ----------
> [  518.900038] MemAlloc: kswapd0(69) flags=0xa40840 switches=23883 uninterruptible
> [  518.902095] kswapd0         D10776    69      2 0x00000000
> [  518.903784] Call Trace:
> [  518.904849]  __schedule+0x336/0xe00
> [  518.906118]  schedule+0x3d/0x90
> [  518.907314]  io_schedule+0x16/0x40
> [  518.908622]  __xfs_iflock+0x129/0x140 [xfs]
> [  518.910027]  ? autoremove_wake_function+0x60/0x60
> [  518.911559]  xfs_reclaim_inode+0x162/0x440 [xfs]
> [  518.913068]  xfs_reclaim_inodes_ag+0x2cf/0x4f0 [xfs]
> [  518.914611]  ? xfs_reclaim_inodes_ag+0xf2/0x4f0 [xfs]
> [  518.916148]  ? trace_hardirqs_on+0xd/0x10
> [  518.917465]  ? try_to_wake_up+0x59/0x7a0
> [  518.918758]  ? wake_up_process+0x15/0x20
> [  518.920067]  xfs_reclaim_inodes_nr+0x33/0x40 [xfs]
> [  518.921560]  xfs_fs_free_cached_objects+0x19/0x20 [xfs]
> [  518.923114]  super_cache_scan+0x181/0x190
> [  518.924435]  shrink_slab+0x29f/0x6d0
> [  518.925683]  shrink_node+0x2fa/0x310
> [  518.926909]  kswapd+0x362/0x9b0
> [  518.928061]  kthread+0x10f/0x150
> [  518.929218]  ? mem_cgroup_shrink_node+0x3b0/0x3b0
> [  518.930953]  ? kthread_create_on_node+0x70/0x70
> [  518.932380]  ret_from_fork+0x31/0x40
> 
> [ 1095.632984] MemAlloc: kswapd0(69) flags=0xa40840 switches=23883 uninterruptible
> [ 1095.632985] kswapd0         D10776    69      2 0x00000000
> [ 1095.632988] Call Trace:
> [ 1095.632991]  __schedule+0x336/0xe00
> [ 1095.632994]  schedule+0x3d/0x90
> [ 1095.632996]  io_schedule+0x16/0x40
> [ 1095.633017]  __xfs_iflock+0x129/0x140 [xfs]
> [ 1095.633021]  ? autoremove_wake_function+0x60/0x60
> [ 1095.633051]  xfs_reclaim_inode+0x162/0x440 [xfs]
> [ 1095.633072]  xfs_reclaim_inodes_ag+0x2cf/0x4f0 [xfs]
> [ 1095.633106]  ? xfs_reclaim_inodes_ag+0xf2/0x4f0 [xfs]
> [ 1095.633114]  ? trace_hardirqs_on+0xd/0x10
> [ 1095.633116]  ? try_to_wake_up+0x59/0x7a0
> [ 1095.633120]  ? wake_up_process+0x15/0x20
> [ 1095.633156]  xfs_reclaim_inodes_nr+0x33/0x40 [xfs]
> [ 1095.633178]  xfs_fs_free_cached_objects+0x19/0x20 [xfs]
> [ 1095.633180]  super_cache_scan+0x181/0x190
> [ 1095.633183]  shrink_slab+0x29f/0x6d0
> [ 1095.633189]  shrink_node+0x2fa/0x310
> [ 1095.633193]  kswapd+0x362/0x9b0
> [ 1095.633200]  kthread+0x10f/0x150
> [ 1095.633201]  ? mem_cgroup_shrink_node+0x3b0/0x3b0
> [ 1095.633202]  ? kthread_create_on_node+0x70/0x70
> [ 1095.633205]  ret_from_fork+0x31/0x40
> ----------
> 
>   (2) All WQ_MEM_RECLAIM threads shown by show_workqueue_state()
>       cannot make progress waiting for memory allocation.
> 
> ----------
> [ 1095.633625] MemAlloc: xfs-data/sda1(451) flags=0x4228060 switches=45509 seq=1 gfp=0x1604240(GFP_NOFS|__GFP_NOWARN|__GFP_COMP|__GFP_NOTRACK) order=0 delay=652073
> [ 1095.633626] xfs-data/sda1   R  running task    12696   451      2 0x00000000
> [ 1095.633663] Workqueue: xfs-data/sda1 xfs_end_io [xfs]
> [ 1095.633665] Call Trace:
> [ 1095.633668]  __schedule+0x336/0xe00
> [ 1095.633671]  schedule+0x3d/0x90
> [ 1095.633672]  schedule_timeout+0x20d/0x510
> [ 1095.633675]  ? lock_timer_base+0xa0/0xa0
> [ 1095.633678]  schedule_timeout_uninterruptible+0x2a/0x30
> [ 1095.633680]  __alloc_pages_slowpath+0x2b5/0xd95
> [ 1095.633687]  __alloc_pages_nodemask+0x3e4/0x460
> [ 1095.633699]  alloc_pages_current+0x97/0x1b0
> [ 1095.633702]  new_slab+0x4cb/0x6b0
> [ 1095.633706]  ___slab_alloc+0x3a3/0x620
> [ 1095.633728]  ? kmem_alloc+0x96/0x120 [xfs]
> [ 1095.633730]  ? ___slab_alloc+0x5c6/0x620
> [ 1095.633732]  ? cpuacct_charge+0x38/0x1e0
> [ 1095.633767]  ? kmem_alloc+0x96/0x120 [xfs]
> [ 1095.633770]  __slab_alloc+0x46/0x7d
> [ 1095.633773]  __kmalloc+0x301/0x3b0
> [ 1095.633802]  kmem_alloc+0x96/0x120 [xfs]
> [ 1095.633804]  ? kfree+0x1fa/0x330
> [ 1095.633842]  xfs_log_commit_cil+0x489/0x710 [xfs]
> [ 1095.633864]  __xfs_trans_commit+0x83/0x260 [xfs]
> [ 1095.633883]  xfs_trans_commit+0x10/0x20 [xfs]
> [ 1095.633901]  __xfs_setfilesize+0xdb/0x240 [xfs]
> [ 1095.633936]  xfs_setfilesize_ioend+0x89/0xb0 [xfs]
> [ 1095.633954]  ? xfs_setfilesize_ioend+0x5/0xb0 [xfs]
> [ 1095.633971]  xfs_end_io+0x81/0x110 [xfs]
> [ 1095.633973]  process_one_work+0x22b/0x760
> [ 1095.633975]  ? process_one_work+0x194/0x760
> [ 1095.633997]  rescuer_thread+0x1f2/0x3d0
> [ 1095.634002]  kthread+0x10f/0x150
> [ 1095.634003]  ? worker_thread+0x4b0/0x4b0
> [ 1095.634004]  ? kthread_create_on_node+0x70/0x70
> [ 1095.634007]  ret_from_fork+0x31/0x40
> [ 1095.634013] MemAlloc: xfs-eofblocks/s(456) flags=0x4228860 switches=15435 seq=1 gfp=0x1400240(GFP_NOFS|__GFP_NOWARN) order=0 delay=293074
> [ 1095.634014] xfs-eofblocks/s R  running task    12032   456      2 0x00000000
> [ 1095.634037] Workqueue: xfs-eofblocks/sda1 xfs_eofblocks_worker [xfs]
> [ 1095.634038] Call Trace:
> [ 1095.634040]  ? _raw_spin_lock+0x3d/0x80
> [ 1095.634042]  ? vmpressure+0xd0/0x120
> [ 1095.634044]  ? vmpressure+0xd0/0x120
> [ 1095.634047]  ? vmpressure_prio+0x21/0x30
> [ 1095.634049]  ? do_try_to_free_pages+0x70/0x300
> [ 1095.634052]  ? try_to_free_pages+0x131/0x3f0
> [ 1095.634058]  ? __alloc_pages_slowpath+0x3ec/0xd95
> [ 1095.634065]  ? __alloc_pages_nodemask+0x3e4/0x460
> [ 1095.634069]  ? alloc_pages_current+0x97/0x1b0
> [ 1095.634111]  ? xfs_buf_allocate_memory+0x160/0x2a3 [xfs]
> [ 1095.634133]  ? xfs_buf_get_map+0x2be/0x480 [xfs]
> [ 1095.634169]  ? xfs_buf_read_map+0x2c/0x400 [xfs]
> [ 1095.634204]  ? xfs_trans_read_buf_map+0x186/0x830 [xfs]
> [ 1095.634222]  ? xfs_btree_read_buf_block.constprop.34+0x78/0xc0 [xfs]
> [ 1095.634239]  ? xfs_btree_lookup_get_block+0x8a/0x180 [xfs]
> [ 1095.634257]  ? xfs_btree_lookup+0xd0/0x3f0 [xfs]
> [ 1095.634296]  ? kmem_zone_alloc+0x96/0x120 [xfs]
> [ 1095.634299]  ? _raw_spin_unlock+0x27/0x40
> [ 1095.634315]  ? xfs_bmbt_lookup_eq+0x1f/0x30 [xfs]
> [ 1095.634348]  ? xfs_bmap_del_extent+0x1b2/0x1610 [xfs]
> [ 1095.634380]  ? kmem_zone_alloc+0x96/0x120 [xfs]
> [ 1095.634400]  ? __xfs_bunmapi+0x4db/0xda0 [xfs]
> [ 1095.634421]  ? xfs_bunmapi+0x2b/0x40 [xfs]
> [ 1095.634459]  ? xfs_itruncate_extents+0x1df/0x780 [xfs]
> [ 1095.634502]  ? xfs_rename+0xc70/0x1080 [xfs]
> [ 1095.634525]  ? xfs_free_eofblocks+0x1c4/0x230 [xfs]
> [ 1095.634546]  ? xfs_inode_free_eofblocks+0x18d/0x280 [xfs]
> [ 1095.634565]  ? xfs_inode_ag_walk.isra.13+0x2b5/0x620 [xfs]
> [ 1095.634582]  ? xfs_inode_ag_walk.isra.13+0x91/0x620 [xfs]
> [ 1095.634618]  ? xfs_inode_clear_eofblocks_tag+0x1a0/0x1a0 [xfs]
> [ 1095.634630]  ? radix_tree_next_chunk+0x10b/0x2d0
> [ 1095.634635]  ? radix_tree_gang_lookup_tag+0xd7/0x150
> [ 1095.634672]  ? xfs_perag_get_tag+0x11d/0x370 [xfs]
> [ 1095.634690]  ? xfs_perag_get_tag+0x5/0x370 [xfs]
> [ 1095.634709]  ? xfs_inode_ag_iterator_tag+0x71/0xa0 [xfs]
> [ 1095.634726]  ? xfs_inode_clear_eofblocks_tag+0x1a0/0x1a0 [xfs]
> [ 1095.634744]  ? __xfs_icache_free_eofblocks+0x3b/0x40 [xfs]
> [ 1095.634759]  ? xfs_eofblocks_worker+0x27/0x40 [xfs]
> [ 1095.634762]  ? process_one_work+0x22b/0x760
> [ 1095.634763]  ? process_one_work+0x194/0x760
> [ 1095.634784]  ? rescuer_thread+0x1f2/0x3d0
> [ 1095.634788]  ? kthread+0x10f/0x150
> [ 1095.634789]  ? worker_thread+0x4b0/0x4b0
> [ 1095.634790]  ? kthread_create_on_node+0x70/0x70
> [ 1095.634793]  ? ret_from_fork+0x31/0x40
> ----------
> 

Clearly we have some rescuer threads blocked on memory reclaim. What
about the rescuer threads that are not blocked in reclaim? For example,
the one that could very well be responsible for allowing xfsaild (and
thus kswapd) to proceed is here:

[ 1041.555227] xfs-buf/sda1    S14904   450      2 0x00000000
[ 1041.556813] Call Trace:
[ 1041.557796]  __schedule+0x336/0xe00
[ 1041.558983]  schedule+0x3d/0x90
[ 1041.560085]  rescuer_thread+0x322/0x3d0
[ 1041.561333]  kthread+0x10f/0x150
[ 1041.562464]  ? worker_thread+0x4b0/0x4b0
[ 1041.563732]  ? kthread_create_on_node+0x70/0x70
[ 1041.565123]  ret_from_fork+0x31/0x40

I've pointed this out at least twice now in the original thread. :/

>   (3) order-0 GFP_NOIO allocation request cannot make progress
>       waiting for memory allocation.
> 
> ----------
> [ 1095.631687] MemAlloc: kworker/2:0(27) flags=0x4208860 switches=38727 seq=21 gfp=0x1400000(GFP_NOIO) order=0 delay=652160
> [ 1095.631688] kworker/2:0     R  running task    12680    27      2 0x00000000
> [ 1095.631739] Workqueue: events_freezable_power_ disk_events_workfn
> [ 1095.631740] Call Trace:
> [ 1095.631743]  __schedule+0x336/0xe00
> [ 1095.631746]  preempt_schedule_common+0x1f/0x31
> [ 1095.631747]  _cond_resched+0x1c/0x30
> [ 1095.631749]  shrink_slab+0x339/0x6d0
> [ 1095.631754]  shrink_node+0x2fa/0x310
> [ 1095.631758]  do_try_to_free_pages+0xe1/0x300
> [ 1095.631761]  try_to_free_pages+0x131/0x3f0
> [ 1095.631765]  __alloc_pages_slowpath+0x3ec/0xd95
> [ 1095.631771]  __alloc_pages_nodemask+0x3e4/0x460
> [ 1095.631775]  alloc_pages_current+0x97/0x1b0
> [ 1095.631779]  bio_copy_kern+0xc9/0x180
> [ 1095.631830]  blk_rq_map_kern+0x70/0x140
> [ 1095.631835]  __scsi_execute.isra.22+0x13a/0x1e0
> [ 1095.631838]  scsi_execute_req_flags+0x94/0x100
> [ 1095.631844]  sr_check_events+0xbf/0x2b0 [sr_mod]
> [ 1095.631852]  cdrom_check_events+0x18/0x30 [cdrom]
> [ 1095.631854]  sr_block_check_events+0x2a/0x30 [sr_mod]
> [ 1095.631856]  disk_check_events+0x60/0x170
> [ 1095.631859]  disk_events_workfn+0x1c/0x20
> [ 1095.631862]  process_one_work+0x22b/0x760
> [ 1095.631863]  ? process_one_work+0x194/0x760
> [ 1095.631867]  worker_thread+0x137/0x4b0
> [ 1095.631887]  kthread+0x10f/0x150
> [ 1095.631889]  ? process_one_work+0x760/0x760
> [ 1095.631890]  ? kthread_create_on_node+0x70/0x70
> [ 1095.631893]  ret_from_fork+0x31/0x40
> ----------
> 
>   (4) Number of stalling threads does not decrease over time while
>       number of out_of_memory() calls increases over time.
> 
> ----------
> [  518.090012] MemAlloc-Info: stalling=184 dying=1 exiting=0 victim=1 oom_count=8441307
> [  553.070829] MemAlloc-Info: stalling=184 dying=1 exiting=0 victim=1 oom_count=10318507
> [  616.394649] MemAlloc-Info: stalling=186 dying=1 exiting=0 victim=1 oom_count=13908219
> [  642.266252] MemAlloc-Info: stalling=186 dying=1 exiting=0 victim=1 oom_count=15180673
> [  702.412189] MemAlloc-Info: stalling=187 dying=1 exiting=0 victim=1 oom_count=18732529
> [  736.787879] MemAlloc-Info: stalling=187 dying=1 exiting=0 victim=1 oom_count=20565244
> [  800.715759] MemAlloc-Info: stalling=188 dying=1 exiting=0 victim=1 oom_count=24411576
> [  837.571405] MemAlloc-Info: stalling=188 dying=1 exiting=0 victim=1 oom_count=26463562
> [  899.021495] MemAlloc-Info: stalling=189 dying=1 exiting=0 victim=1 oom_count=30144879
> [  936.282709] MemAlloc-Info: stalling=189 dying=1 exiting=0 victim=1 oom_count=32129234
> [  997.328119] MemAlloc-Info: stalling=190 dying=1 exiting=0 victim=1 oom_count=35657983
> [ 1033.977265] MemAlloc-Info: stalling=190 dying=1 exiting=0 victim=1 oom_count=37659912
> [ 1095.630961] MemAlloc-Info: stalling=190 dying=1 exiting=0 victim=1 oom_count=40639677
> [ 1095.821248] MemAlloc-Info: stalling=190 dying=1 exiting=0 victim=1 oom_count=40646791
> ----------
> 
> Further interpretation is up to XFS people (Brian and Dave). So far, boosting priority
> (lowering watermark) of WQ_MEM_RECLAIM threads based on current_is_workqueue_rescuer()
> seems to be considered as a fix because writeback path already has a special case
> handling for the rescuer. But I'm not sure whether it helps getting writeback unstuck
> when writeback path is blocked on a lock rather than memory allocation. (Threads doing
> writeback and/or mempool allocations might be candidates for always reporting as with
> kswapd. That would be in followup patches.)
> 

Using the rescuer context in those associated workqueues is probably a
good idea, it may even resolve (or workaround) this problem. As noted
above, however, I don't think we've proven that addresses this problem.

How would you expect current_is_workqueue_rescuer() to affect the idle
xfs-buf rescuer thread above? Is there something else going on that
could be blocking metadata buffer I/O completion via xfs-buf? We do have
the following, which I suppose suggests there is a pending work item:

[  273.412600] workqueue xfs-buf/sda1: flags=0xc
[  273.414486]   pwq 4: cpus=2 node=0 flags=0x0 nice=0 active=1/1
[  273.416415]     pending: xfs_buf_ioend_work [xfs]

... but otherwise I don't see anything executing xfs_buf_ioend()..? Am I
missing something?

AFAICT, all the information we have so far shows is that kswapd is
blocked in XFS, and as a result of that, we've ultimately deadlocked on
memory allocation.

Brian

> ----------
> [ 1044.701393] kworker/u256:0  D10024  8444      2 0x00000080
> [ 1044.703017] Workqueue: writeback wb_workfn (flush-8:0)
> [ 1044.704550] Call Trace:
> [ 1044.705550]  __schedule+0x336/0xe00
> [ 1044.706761]  schedule+0x3d/0x90
> [ 1044.707891]  io_schedule+0x16/0x40
> [ 1044.709073]  __lock_page+0x126/0x180
> [ 1044.710272]  ? page_cache_tree_insert+0x120/0x120
> [ 1044.711728]  write_cache_pages+0x39a/0x6b0
> [ 1044.713077]  ? xfs_map_blocks+0x5a0/0x5a0 [xfs]
> [ 1044.714499]  ? xfs_vm_writepages+0x48/0xa0 [xfs]
> [ 1044.715944]  xfs_vm_writepages+0x6b/0xa0 [xfs]
> [ 1044.717362]  do_writepages+0x21/0x40
> [ 1044.718577]  __writeback_single_inode+0x72/0x9d0
> [ 1044.719998]  ? _raw_spin_unlock+0x27/0x40
> [ 1044.721308]  writeback_sb_inodes+0x322/0x750
> [ 1044.722662]  __writeback_inodes_wb+0x8c/0xc0
> [ 1044.724009]  wb_writeback+0x3be/0x6e0
> [ 1044.725238]  wb_workfn+0x146/0x7d0
> [ 1044.726413]  ? process_one_work+0x194/0x760
> [ 1044.727736]  process_one_work+0x22b/0x760
> [ 1044.729017]  ? process_one_work+0x194/0x760
> [ 1044.730343]  worker_thread+0x137/0x4b0
> [ 1044.731577]  kthread+0x10f/0x150
> [ 1044.732702]  ? process_one_work+0x760/0x760
> [ 1044.734021]  ? kthread_create_on_node+0x70/0x70
> [ 1044.735415]  ? do_syscall_64+0x195/0x200
> [ 1044.736684]  ret_from_fork+0x31/0x40
> ----------
> 
> Regarding "infinite too_many_isolated() loop" case
> ( http://lkml.kernel.org/r/201702212335.DJB30777.JOFMHSFtVLQOOF@I-love.SAKURA.ne.jp ),
> I used this patch for confirming the following things.
> 
>   (1) kswapd cannot make progress waiting for lock.
> 
> ----------
> [ 1209.790966] MemAlloc: kswapd0(67) flags=0xa60840 switches=51139 uninterruptible
> [ 1209.799726] kswapd0         D10936    67      2 0x00000000
> [ 1209.807326] Call Trace:
> [ 1209.812581]  __schedule+0x336/0xe00
> [ 1209.818599]  schedule+0x3d/0x90
> [ 1209.823907]  schedule_timeout+0x26a/0x510
> [ 1209.827218]  ? trace_hardirqs_on+0xd/0x10
> [ 1209.830535]  __down_common+0xfb/0x131
> [ 1209.833801]  ? _xfs_buf_find+0x2cb/0xc10 [xfs]
> [ 1209.837372]  __down+0x1d/0x1f
> [ 1209.840331]  down+0x41/0x50
> [ 1209.843243]  xfs_buf_lock+0x64/0x370 [xfs]
> [ 1209.846597]  _xfs_buf_find+0x2cb/0xc10 [xfs]
> [ 1209.850031]  ? _xfs_buf_find+0xa4/0xc10 [xfs]
> [ 1209.853514]  xfs_buf_get_map+0x2a/0x480 [xfs]
> [ 1209.855831]  xfs_buf_read_map+0x2c/0x400 [xfs]
> [ 1209.857388]  ? free_debug_processing+0x27d/0x2af
> [ 1209.859037]  xfs_trans_read_buf_map+0x186/0x830 [xfs]
> [ 1209.860707]  xfs_read_agf+0xc8/0x2b0 [xfs]
> [ 1209.862184]  xfs_alloc_read_agf+0x7a/0x300 [xfs]
> [ 1209.863728]  ? xfs_alloc_space_available+0x7b/0x120 [xfs]
> [ 1209.865385]  xfs_alloc_fix_freelist+0x3bc/0x490 [xfs]
> [ 1209.866974]  ? __radix_tree_lookup+0x84/0xf0
> [ 1209.868374]  ? xfs_perag_get+0x1a0/0x310 [xfs]
> [ 1209.869798]  ? xfs_perag_get+0x5/0x310 [xfs]
> [ 1209.871288]  xfs_alloc_vextent+0x161/0xda0 [xfs]
> [ 1209.872757]  xfs_bmap_btalloc+0x46c/0x8b0 [xfs]
> [ 1209.874182]  ? save_stack_trace+0x1b/0x20
> [ 1209.875542]  xfs_bmap_alloc+0x17/0x30 [xfs]
> [ 1209.876847]  xfs_bmapi_write+0x74e/0x11d0 [xfs]
> [ 1209.878190]  xfs_iomap_write_allocate+0x199/0x3a0 [xfs]
> [ 1209.879632]  xfs_map_blocks+0x2cc/0x5a0 [xfs]
> [ 1209.880909]  xfs_do_writepage+0x215/0x920 [xfs]
> [ 1209.882255]  ? clear_page_dirty_for_io+0xb4/0x310
> [ 1209.883598]  xfs_vm_writepage+0x3b/0x70 [xfs]
> [ 1209.884841]  pageout.isra.54+0x1a4/0x460
> [ 1209.886210]  shrink_page_list+0xa86/0xcf0
> [ 1209.887441]  shrink_inactive_list+0x1c5/0x660
> [ 1209.888682]  shrink_node_memcg+0x535/0x7f0
> [ 1209.889975]  ? mem_cgroup_iter+0x14d/0x720
> [ 1209.891197]  shrink_node+0xe1/0x310
> [ 1209.892288]  kswapd+0x362/0x9b0
> [ 1209.893308]  kthread+0x10f/0x150
> [ 1209.894383]  ? mem_cgroup_shrink_node+0x3b0/0x3b0
> [ 1209.895703]  ? kthread_create_on_node+0x70/0x70
> [ 1209.896956]  ret_from_fork+0x31/0x40
> ----------
> 
>   (2) Both GFP_IO and GFP_KERNEL allocations are stuck at
>       too_many_isolated() loop.
> 
> ----------
> [ 1209.898117] MemAlloc: systemd-journal(526) flags=0x400900 switches=33248 seq=121659 gfp=0x14201ca(GFP_HIGHUSER_MOVABLE|__GFP_COLD) order=0 delay=52772 uninterruptible
> [ 1209.902154] systemd-journal D11240   526      1 0x00000000
> [ 1209.903642] Call Trace:
> [ 1209.904574]  __schedule+0x336/0xe00
> [ 1209.905734]  schedule+0x3d/0x90
> [ 1209.906817]  schedule_timeout+0x20d/0x510
> [ 1209.908025]  ? prepare_to_wait+0x2b/0xc0
> [ 1209.909268]  ? lock_timer_base+0xa0/0xa0
> [ 1209.910460]  io_schedule_timeout+0x1e/0x50
> [ 1209.911681]  congestion_wait+0x86/0x260
> [ 1209.912853]  ? remove_wait_queue+0x60/0x60
> [ 1209.914115]  shrink_inactive_list+0x5b4/0x660
> [ 1209.915385]  ? __list_lru_count_one.isra.2+0x22/0x80
> [ 1209.916768]  shrink_node_memcg+0x535/0x7f0
> [ 1209.918173]  shrink_node+0xe1/0x310
> [ 1209.919288]  do_try_to_free_pages+0xe1/0x300
> [ 1209.920548]  try_to_free_pages+0x131/0x3f0
> [ 1209.921827]  __alloc_pages_slowpath+0x3ec/0xd95
> [ 1209.923137]  __alloc_pages_nodemask+0x3e4/0x460
> [ 1209.924454]  ? __radix_tree_lookup+0x84/0xf0
> [ 1209.925790]  alloc_pages_current+0x97/0x1b0
> [ 1209.927021]  ? find_get_entry+0x5/0x300
> [ 1209.928189]  __page_cache_alloc+0x15d/0x1a0
> [ 1209.929471]  ? pagecache_get_page+0x2c/0x2b0
> [ 1209.930716]  filemap_fault+0x4df/0x8b0
> [ 1209.931867]  ? filemap_fault+0x373/0x8b0
> [ 1209.933111]  ? xfs_ilock+0x22c/0x360 [xfs]
> [ 1209.934510]  ? xfs_filemap_fault+0x64/0x1e0 [xfs]
> [ 1209.935857]  ? down_read_nested+0x7b/0xc0
> [ 1209.937123]  ? xfs_ilock+0x22c/0x360 [xfs]
> [ 1209.938373]  xfs_filemap_fault+0x6c/0x1e0 [xfs]
> [ 1209.939691]  __do_fault+0x1e/0xa0
> [ 1209.940807]  ? _raw_spin_unlock+0x27/0x40
> [ 1209.942002]  __handle_mm_fault+0xbb1/0xf40
> [ 1209.943228]  ? mutex_unlock+0x12/0x20
> [ 1209.944410]  ? devkmsg_read+0x15c/0x330
> [ 1209.945912]  handle_mm_fault+0x16b/0x390
> [ 1209.947297]  ? handle_mm_fault+0x49/0x390
> [ 1209.948868]  __do_page_fault+0x24a/0x530
> [ 1209.950351]  do_page_fault+0x30/0x80
> [ 1209.951615]  page_fault+0x28/0x30
> 
> [ 1210.538496] MemAlloc: kworker/3:0(6345) flags=0x4208860 switches=10134 seq=22 gfp=0x1400000(GFP_NOIO) order=0 delay=45953 uninterruptible
> [ 1210.541487] kworker/3:0     D12560  6345      2 0x00000080
> [ 1210.542991] Workqueue: events_freezable_power_ disk_events_workfn
> [ 1210.544577] Call Trace:
> [ 1210.545468]  __schedule+0x336/0xe00
> [ 1210.546606]  schedule+0x3d/0x90
> [ 1210.547616]  schedule_timeout+0x20d/0x510
> [ 1210.548778]  ? prepare_to_wait+0x2b/0xc0
> [ 1210.550013]  ? lock_timer_base+0xa0/0xa0
> [ 1210.551208]  io_schedule_timeout+0x1e/0x50
> [ 1210.552519]  congestion_wait+0x86/0x260
> [ 1210.553650]  ? remove_wait_queue+0x60/0x60
> [ 1210.554900]  shrink_inactive_list+0x5b4/0x660
> [ 1210.556119]  ? __list_lru_count_one.isra.2+0x22/0x80
> [ 1210.557447]  shrink_node_memcg+0x535/0x7f0
> [ 1210.558714]  shrink_node+0xe1/0x310
> [ 1210.559803]  do_try_to_free_pages+0xe1/0x300
> [ 1210.561009]  try_to_free_pages+0x131/0x3f0
> [ 1210.562250]  __alloc_pages_slowpath+0x3ec/0xd95
> [ 1210.563506]  __alloc_pages_nodemask+0x3e4/0x460
> [ 1210.564777]  alloc_pages_current+0x97/0x1b0
> [ 1210.566017]  bio_copy_kern+0xc9/0x180
> [ 1210.567116]  blk_rq_map_kern+0x70/0x140
> [ 1210.568356]  __scsi_execute.isra.22+0x13a/0x1e0
> [ 1210.569839]  scsi_execute_req_flags+0x94/0x100
> [ 1210.571218]  sr_check_events+0xbf/0x2b0 [sr_mod]
> [ 1210.572500]  cdrom_check_events+0x18/0x30 [cdrom]
> [ 1210.573934]  sr_block_check_events+0x2a/0x30 [sr_mod]
> [ 1210.575335]  disk_check_events+0x60/0x170
> [ 1210.576509]  disk_events_workfn+0x1c/0x20
> [ 1210.577744]  process_one_work+0x22b/0x760
> [ 1210.578934]  ? process_one_work+0x194/0x760
> [ 1210.580147]  worker_thread+0x137/0x4b0
> [ 1210.581336]  kthread+0x10f/0x150
> [ 1210.582365]  ? process_one_work+0x760/0x760
> [ 1210.583603]  ? kthread_create_on_node+0x70/0x70
> [ 1210.584961]  ? do_syscall_64+0x6c/0x200
> [ 1210.586343]  ret_from_fork+0x31/0x40
> ----------
> 
>   (3) Number of stalling threads does not decrease over time and
>       number of out_of_memory() calls does not increase over time.
> 
> ----------
> [ 1209.781787] MemAlloc-Info: stalling=32 dying=1 exiting=0 victim=1 oom_count=45896
> [ 1212.195351] MemAlloc-Info: stalling=32 dying=1 exiting=0 victim=1 oom_count=45896
> [ 1242.551629] MemAlloc-Info: stalling=36 dying=1 exiting=0 victim=1 oom_count=45896
> [ 1245.149165] MemAlloc-Info: stalling=36 dying=1 exiting=0 victim=1 oom_count=45896
> [ 1275.319189] MemAlloc-Info: stalling=40 dying=1 exiting=0 victim=1 oom_count=45896
> [ 1278.241813] MemAlloc-Info: stalling=40 dying=1 exiting=0 victim=1 oom_count=45896
> ----------
> 
> Regarding [2], Alexander, Brian and Dave can explain it better than I. But
> I think that "check threads which cannot make progress" principle is same.
> 
> 
> 
> > (top-posting repaired - please don't do that)
> 
> Hmm, I don't see anything wrong with mail headers of replied one that can
> cause top-posting. I think something wired occurred outside of my control.
> 
>   Subject: Re: [PATCH v7] mm: Add memory allocation watchdog kernel thread.
>   From: Tetsuo Handa <penguin-kernel@I-love.SAKURA.ne.jp>
>   References: <1488244908-57586-1-git-send-email-penguin-kernel@I-love.SAKURA.ne.jp>
>   In-Reply-To: <1488244908-57586-1-git-send-email-penguin-kernel@I-love.SAKURA.ne.jp>
>   Message-Id: <201703091946.GDC21885.OQFFOtJHSOFVML@I-love.SAKURA.ne.jp>
>   Date: Thu, 9 Mar 2017 19:46:14 +0900
> 
> --
> To unsubscribe, send a message with 'unsubscribe linux-mm' in
> the body to majordomo@kvack.org.  For more info on Linux MM,
> see: http://www.linux-mm.org/ .
> Don't email: <a href=mailto:"dont@kvack.org"> email@kvack.org </a>

Re: [PATCH v7] mm: Add memory allocation watchdog kernel thread.

[Michal Hocko]

On Fri 10-03-17 20:19:58, Tetsuo Handa wrote:
[...]
> Or, did you mean "some additional examples of how this new code was used
> to understand and improve real-world kernel problems" as how this patch
> helped in [2] [3] ?
> 
> Regarding [3] (now continued as
> http://lkml.kernel.org/r/201703031948.CHJ81278.VOHSFFFOOLJQMt@I-love.SAKURA.ne.jp ),
> I used this patch for confirming the following things.
> 
>   (1) kswapd cannot make progress waiting for lock.
> 
> ----------
> [  518.900038] MemAlloc: kswapd0(69) flags=0xa40840 switches=23883 uninterruptible
> [  518.902095] kswapd0         D10776    69      2 0x00000000
> [  518.903784] Call Trace:
[...]
> 
> [ 1095.632984] MemAlloc: kswapd0(69) flags=0xa40840 switches=23883 uninterruptible
> [ 1095.632985] kswapd0         D10776    69      2 0x00000000
> [ 1095.632988] Call Trace:
[...]
> ----------
> 
>   (2) All WQ_MEM_RECLAIM threads shown by show_workqueue_state()
>       cannot make progress waiting for memory allocation.
> 
> ----------
> [ 1095.633625] MemAlloc: xfs-data/sda1(451) flags=0x4228060 switches=45509 seq=1 gfp=0x1604240(GFP_NOFS|__GFP_NOWARN|__GFP_COMP|__GFP_NOTRACK) order=0 delay=652073
> [ 1095.633626] xfs-data/sda1   R  running task    12696   451      2 0x00000000
> [ 1095.633663] Workqueue: xfs-data/sda1 xfs_end_io [xfs]
> [ 1095.633665] Call Trace:
[...]

We can get all this from the sysrq+t output, right? Well except for the
gfp mask, oder and delay which is something that would give us a hint
that something is going on with the allocation because all those tasks
are in the page allocation obviously from the Call trace.
> ----------
> 
>   (3) order-0 GFP_NOIO allocation request cannot make progress
>       waiting for memory allocation.

which is completely irrelevant to the issue...
 
>   (4) Number of stalling threads does not decrease over time while
>       number of out_of_memory() calls increases over time.

You would see the same from the allocation stall warning. Even when it
is not really ideal (we are getting there though) you would see that
some allocation simply do not make a forward progress.

> ----------
> [  518.090012] MemAlloc-Info: stalling=184 dying=1 exiting=0 victim=1 oom_count=8441307
> [  553.070829] MemAlloc-Info: stalling=184 dying=1 exiting=0 victim=1 oom_count=10318507
> [  616.394649] MemAlloc-Info: stalling=186 dying=1 exiting=0 victim=1 oom_count=13908219
> [  642.266252] MemAlloc-Info: stalling=186 dying=1 exiting=0 victim=1 oom_count=15180673
> [  702.412189] MemAlloc-Info: stalling=187 dying=1 exiting=0 victim=1 oom_count=18732529
> [  736.787879] MemAlloc-Info: stalling=187 dying=1 exiting=0 victim=1 oom_count=20565244
> [  800.715759] MemAlloc-Info: stalling=188 dying=1 exiting=0 victim=1 oom_count=24411576
> [  837.571405] MemAlloc-Info: stalling=188 dying=1 exiting=0 victim=1 oom_count=26463562
> [  899.021495] MemAlloc-Info: stalling=189 dying=1 exiting=0 victim=1 oom_count=30144879
> [  936.282709] MemAlloc-Info: stalling=189 dying=1 exiting=0 victim=1 oom_count=32129234
> [  997.328119] MemAlloc-Info: stalling=190 dying=1 exiting=0 victim=1 oom_count=35657983
> [ 1033.977265] MemAlloc-Info: stalling=190 dying=1 exiting=0 victim=1 oom_count=37659912
> [ 1095.630961] MemAlloc-Info: stalling=190 dying=1 exiting=0 victim=1 oom_count=40639677
> [ 1095.821248] MemAlloc-Info: stalling=190 dying=1 exiting=0 victim=1 oom_count=40646791

yeah, this is more than you can find out right now but I am not really
sure how much more this will tell you from what we already know above.
Do not take me wrong this is all nice and dandy but existing debugging
tools would give us a good clue what is going on even without that.
 
[...]

> Regarding "infinite too_many_isolated() loop" case
> ( http://lkml.kernel.org/r/201702212335.DJB30777.JOFMHSFtVLQOOF@I-love.SAKURA.ne.jp ),
> I used this patch for confirming the following things.
> 
>   (1) kswapd cannot make progress waiting for lock.
[...]
> 
>   (2) Both GFP_IO and GFP_KERNEL allocations are stuck at
>       too_many_isolated() loop.
[...]
> ----------
> 
>   (3) Number of stalling threads does not decrease over time and
>       number of out_of_memory() calls does not increase over time.
> 
> ----------
> [ 1209.781787] MemAlloc-Info: stalling=32 dying=1 exiting=0 victim=1 oom_count=45896
> [ 1212.195351] MemAlloc-Info: stalling=32 dying=1 exiting=0 victim=1 oom_count=45896
> [ 1242.551629] MemAlloc-Info: stalling=36 dying=1 exiting=0 victim=1 oom_count=45896
> [ 1245.149165] MemAlloc-Info: stalling=36 dying=1 exiting=0 victim=1 oom_count=45896
> [ 1275.319189] MemAlloc-Info: stalling=40 dying=1 exiting=0 victim=1 oom_count=45896
> [ 1278.241813] MemAlloc-Info: stalling=40 dying=1 exiting=0 victim=1 oom_count=45896
> ----------

OK, this is a better example but I would argue that multiple snapshots
when CPUs are basically idle while all memory allocations are sitting on
the congestion wait would signal this pretty clearly as well and we can
see this from the sysrq+t as well.

So, we have means to debug these issues. Some of them are rather coarse
and your watchdog can collect much more and maybe give us a clue much
quicker but we still have to judge whether all this is really needed
because it doesn't come for free. Have you considered this aspect?
-- 
Michal Hocko
SUSE Labs

Re: [PATCH v7] mm: Add memory allocation watchdog kernel thread.

[Tetsuo Handa]

Michal Hocko wrote:
> So, we have means to debug these issues. Some of them are rather coarse
> and your watchdog can collect much more and maybe give us a clue much
> quicker but we still have to judge whether all this is really needed
> because it doesn't come for free. Have you considered this aspect?

Sigh... You are ultimately ignoring the reality. Educating everybody to master
debugging tools does not come for free. If I liken your argumentation to
security modules, it looks like the following.

  "There is already SELinux. SELinux can do everything. Thus, AppArmor is not needed.
   I don't care about users/customers who cannot administrate SELinux."

The reality is different. We need tools which users/customers can afford using.
You had better getting away from existing debug tools which kernel developers
are using.

First of all, SysRq is an emergency tool and therefore it requires administrator's
intervention. Your argumentation sounds to me that "Give up debugging unless you
can sit on in front of console of Linux systems 24-7" which is already impossible.

SysRq-t cannot print seq= and delay= fields because information of in-flight allocation
request is not accessible from "struct task_struct", making extremely difficult to
judge whether progress is made when several SysRq-t snapshots are taken.

Also, year by year it is getting difficult to use vmcore for analysis because vmcore
might include sensitive data (even after filtering out user pages). I saw cases where
vmcore cannot be sent to support centers due to e.g. organization's information
control rules. Sometimes we have to analyze from only kernel messages. Some pieces of
information extracted by running scripts against /usr/bin/crash on cutomer's side
might be available, but in general we can't assume that the whole memory image which
includes whatever information is available.

In most cases, administrators can't capture even SysRq-t; let alone vmcore.
Therefore, automatic watchdog is highly appreciated. Have you considered this aspect?

Re: [PATCH v7] mm: Add memory allocation watchdog kernel thread.

[Michal Hocko]

On Sat 11-03-17 10:46:58, Tetsuo Handa wrote:
> Michal Hocko wrote:
> > So, we have means to debug these issues. Some of them are rather coarse
> > and your watchdog can collect much more and maybe give us a clue much
> > quicker but we still have to judge whether all this is really needed
> > because it doesn't come for free. Have you considered this aspect?
> 
> Sigh... You are ultimately ignoring the reality. Educating everybody to master
> debugging tools does not come for free. If I liken your argumentation to
> security modules, it looks like the following.
> 
>   "There is already SELinux. SELinux can do everything. Thus, AppArmor is not needed.
>    I don't care about users/customers who cannot administrate SELinux."
> 
> The reality is different. We need tools which users/customers can afford using.
> You had better getting away from existing debug tools which kernel developers
> are using.
> 
> First of all, SysRq is an emergency tool and therefore it requires administrator's
> intervention. Your argumentation sounds to me that "Give up debugging unless you
> can sit on in front of console of Linux systems 24-7" which is already impossible.

My experience also tells me that different soft/hard lockups tend to
generate quite non-trivial number of false positives and those are
reported as bugs. We simply tend to underestimate how easy it is to trigger
paths without scheduling or how easy it is to trigger hardlockups on
large machines just due to lock bouncing etc...

> SysRq-t cannot print seq= and delay= fields because information of in-flight allocation
> request is not accessible from "struct task_struct", making extremely difficult to
> judge whether progress is made when several SysRq-t snapshots are taken.
> 
> Also, year by year it is getting difficult to use vmcore for analysis because vmcore
> might include sensitive data (even after filtering out user pages). I saw cases where
> vmcore cannot be sent to support centers due to e.g. organization's information
> control rules. Sometimes we have to analyze from only kernel messages. Some pieces of
> information extracted by running scripts against /usr/bin/crash on cutomer's side
> might be available, but in general we can't assume that the whole memory image which
> includes whatever information is available.
> 
> In most cases, administrators can't capture even SysRq-t; let alone vmcore.
> Therefore, automatic watchdog is highly appreciated. Have you considered this aspect?

yes I have. I tend to work with our SUSE L3 and enterprise customer a
lot last 10 years. And what I claim is that adding more watchdog doesn't
necessarily mean we will get better bug reports. I do not have any exact
statistics but my perception is that allocation lockups tends to be less
than 1% of reported bugs. You seem to make a huge issue from this
particular class of issues basing your argumentation on "unknown
issues which might have been allocation lockups etc." I am not feeling
comfortable with this kind of arguing and making any decision on them.

So let me repeat (for the last time). I find your watchdog interesting
for stress testing but I am not convinced this is generally useful for
real workloads and the maintenance burden is worth it. I _might_ be
wrong here and that is why this is _no_ a NAK from me but I feel
uncomfortable how hard you are pushing this.

I expect this is my last word on this.
-- 
Michal Hocko
SUSE Labs

Re: [PATCH v7] mm: Add memory allocation watchdog kernel thread.

[Tetsuo Handa]

Michal Hocko wrote:
> On Sat 11-03-17 10:46:58, Tetsuo Handa wrote:
> > In most cases, administrators can't capture even SysRq-t; let alone vmcore.
> > Therefore, automatic watchdog is highly appreciated. Have you considered this aspect?
> 
> yes I have. I tend to work with our SUSE L3 and enterprise customer a
> lot last 10 years. And what I claim is that adding more watchdog doesn't
> necessarily mean we will get better bug reports. I do not have any exact
> statistics but my perception is that allocation lockups tends to be less
> than 1% of reported bugs. You seem to make a huge issue from this
> particular class of issues basing your argumentation on "unknown
> issues which might have been allocation lockups etc." I am not feeling
> comfortable with this kind of arguing and making any decision on them.

Allocation lockups might be less than 1% of _reported_ bugs.
What I'm talking about is that there will be _unreported_ (and therefore
unrecognized/unsolved) bugs caused by memory allocation behavior.
You are refusing to make an attempt to prove/verify/handle it.

> 
> So let me repeat (for the last time). I find your watchdog interesting
> for stress testing but I am not convinced this is generally useful for
> real workloads and the maintenance burden is worth it. I _might_ be
> wrong here and that is why this is _no_ a NAK from me but I feel
> uncomfortable how hard you are pushing this.

If you worry about false positives and/or side effects of watchdog, you can
disable it in your distribution (i.e. SUSE). There are developers/users/customers
who will be helped by it.

> 
> I expect this is my last word on this.

After all, there is no real objection. Andrew, what do you think?

Re: [PATCH v7] mm: Add memory allocation watchdog kernel thread.

[Andrew Morton]

On Tue, 28 Feb 2017 10:21:48 +0900 Tetsuo Handa <penguin-kernel@I-love.SAKURA.ne.jp> wrote:

> This patch adds a watchdog which periodically reports number of memory
> allocating tasks, dying tasks and OOM victim tasks when some task is
> spending too long time inside __alloc_pages_slowpath(). This patch also
> serves as a hook for obtaining additional information using SystemTap
> (e.g. examine other variables using printk(), capture a crash dump by
> calling panic()) by triggering a callback only when a stall is detected.
> Ability to take administrator-controlled actions based on some threshold
> is a big advantage gained by introducing a state tracking.
> 
> Commit 63f53dea0c9866e9 ("mm: warn about allocations which stall for
> too long") was a great step for reducing possibility of silent hang up
> problem caused by memory allocation stalls [1]. However, there are
> reports of long stalls (e.g. [2] is over 30 minutes!) and lockups (e.g.
> [3] is an "unable to invoke the OOM killer due to !__GFP_FS allocation"
> lockup problem) where this patch is more useful than that commit, for
> this patch can report possibly related tasks even if allocating tasks
> are unexpectedly blocked for so long. Regarding premature OOM killer
> invocation, tracepoints which can accumulate samples in short interval
> would be useful. But regarding too late to report allocation stalls,
> this patch which can capture all tasks (for reporting overall situation)
> in longer interval and act as a trigger (for accumulating short interval
> samples) would be useful.
> 
> ...
>
> +Build kernels with CONFIG_DETECT_HUNG_TASK=y and
> +CONFIG_DETECT_MEMALLOC_STALL_TASK=y.
> +
> +Default scan interval is configured by CONFIG_DEFAULT_MEMALLOC_TASK_TIMEOUT.
> +Scan interval can be changed at run time by writing timeout in seconds to
> +/proc/sys/kernel/memalloc_task_warning_secs. Writing 0 disables this scan.

"seconds" seems needlessly coarse.  Maybe milliseconds?

> +Even if you disable this scan, information about last memory allocation
> +request is kept. That is, you will get some hint for understanding
> +last-minute behavior of the kernel when you analyze vmcore (or memory
> +snapshot of a virtualized machine).
> 
> ...
>
> +struct memalloc_info {
> +	/*
> +	 * 0: not doing __GFP_RECLAIM allocation.
> +	 * 1: doing non-recursive __GFP_RECLAIM allocation.
> +	 * 2: doing recursive __GFP_RECLAIM allocation.
> +	 */
> +	u8 valid;
> +	/*
> +	 * bit 0: Will be reported as OOM victim.
> +	 * bit 1: Will be reported as dying task.
> +	 * bit 2: Will be reported as stalling task.
> +	 * bit 3: Will be reported as exiting task.
> +	 * bit 7: Will be reported unconditionally.

Create enums for these rather than hard-coding magic numbers?

These values don't seem to be used anyway - as far as I can tell this
could be a simple boolean.

> +	 */
> +	u8 type;
> +	/* Index used for memalloc_in_flight[] counter. */
> +	u8 idx;
> +	/* For progress monitoring. */
> +	unsigned int sequence;
> +	/* Started time in jiffies as of valid == 1. */
> +	unsigned long start;
> +	/* Requested order and gfp flags as of valid == 1. */
> +	unsigned int order;
> +	gfp_t gfp;
> +};
> 
> ...
>
> +#ifdef CONFIG_DETECT_MEMALLOC_STALL_TASK
> +/*
> + * Zero means infinite timeout - no checking done:
> + */
> +unsigned long __read_mostly sysctl_memalloc_task_warning_secs =
> +	CONFIG_DEFAULT_MEMALLOC_TASK_TIMEOUT;
> +static struct memalloc_info memalloc; /* Filled by is_stalling_task(). */

What locking protects `memalloc' from concurrent modifications and
holds it stable for readers?

> 
> ...
>
> +static noinline int check_memalloc_stalling_tasks(unsigned long timeout)
> +{
> +	char buf[256];
> +	struct task_struct *g, *p;
> +	unsigned long now;
> +	unsigned long expire;
> +	unsigned int sigkill_pending = 0;
> +	unsigned int exiting_tasks = 0;
> +	unsigned int memdie_pending = 0;
> +	unsigned int stalling_tasks = 0;
> +
> 
> ...
>
> +			goto restart_report;
> +	}
> +	rcu_read_unlock();
> +	preempt_enable_no_resched();
> +	cond_resched();

All the cond_resched()s in this function seem a bit random.

> +	/* Show memory information. (SysRq-m) */
> +	show_mem(0, NULL);
> +	/* Show workqueue state. */
> +	show_workqueue_state();
> +	/* Show lock information. (SysRq-d) */
> +	debug_show_all_locks();
> +	pr_warn("MemAlloc-Info: stalling=%u dying=%u exiting=%u victim=%u oom_count=%u\n",
> +		stalling_tasks, sigkill_pending, exiting_tasks, memdie_pending,
> +		out_of_memory_count);
> +	return stalling_tasks;
> +}
> +#endif /* CONFIG_DETECT_MEMALLOC_STALL_TASK */
> +
>  static void check_hung_task(struct task_struct *t, unsigned long timeout)
>  {
>  	unsigned long switch_count = t->nvcsw + t->nivcsw;
> @@ -228,20 +429,36 @@ void reset_hung_task_detector(void)
>  static int watchdog(void *dummy)
>  {
>  	unsigned long hung_last_checked = jiffies;
> +#ifdef CONFIG_DETECT_MEMALLOC_STALL_TASK
> +	unsigned long stall_last_checked = hung_last_checked;
> +#endif
>  
>  	set_user_nice(current, 0);
>  
>  	for ( ; ; ) {
>  		unsigned long timeout = sysctl_hung_task_timeout_secs;
>  		long t = hung_timeout_jiffies(hung_last_checked, timeout);
> -
> +#ifdef CONFIG_DETECT_MEMALLOC_STALL_TASK
> +		unsigned long timeout2 = sysctl_memalloc_task_warning_secs;
> +		long t2 = memalloc_timeout_jiffies(stall_last_checked,
> +						   timeout2);

Confused.  Shouldn't timeout2 be converted from seconds to jiffies
before being passed to memalloc_timeout_jiffies()?

> +		if (t2 <= 0) {
> +			if (memalloc_maybe_stalling())
> +				check_memalloc_stalling_tasks(timeout2);
> +			stall_last_checked = jiffies;
> +			continue;
> +		}
> +#else
> +		long t2 = t;
> +#endif
> 
> ...
>
> +bool memalloc_maybe_stalling(void)
> +{
> +	int cpu;
> +	int sum = 0;
> +	const u8 idx = memalloc_active_index ^ 1;
> +
> +	for_each_possible_cpu(cpu)

Do we really need to do this for offlined and not-present CPUs?

> +		sum += per_cpu(memalloc_in_flight[idx], cpu);
> +	if (sum)
> +		return true;
> +	memalloc_active_index ^= 1;
> +	return false;
> +}
> +
> 
> ...
>

Re: [PATCH v7] mm: Add memory allocation watchdog kernel thread.

[Tetsuo Handa]

Andrew Morton wrote:
> On Tue, 28 Feb 2017 10:21:48 +0900 Tetsuo Handa <penguin-kernel@I-love.SAKURA.ne.jp> wrote:
> 
> > This patch adds a watchdog which periodically reports number of memory
> > allocating tasks, dying tasks and OOM victim tasks when some task is
> > spending too long time inside __alloc_pages_slowpath(). This patch also
> > serves as a hook for obtaining additional information using SystemTap
> > (e.g. examine other variables using printk(), capture a crash dump by
> > calling panic()) by triggering a callback only when a stall is detected.
> > Ability to take administrator-controlled actions based on some threshold
> > is a big advantage gained by introducing a state tracking.
> > 
> > Commit 63f53dea0c9866e9 ("mm: warn about allocations which stall for
> > too long") was a great step for reducing possibility of silent hang up
> > problem caused by memory allocation stalls [1]. However, there are
> > reports of long stalls (e.g. [2] is over 30 minutes!) and lockups (e.g.
> > [3] is an "unable to invoke the OOM killer due to !__GFP_FS allocation"
> > lockup problem) where this patch is more useful than that commit, for
> > this patch can report possibly related tasks even if allocating tasks
> > are unexpectedly blocked for so long. Regarding premature OOM killer
> > invocation, tracepoints which can accumulate samples in short interval
> > would be useful. But regarding too late to report allocation stalls,
> > this patch which can capture all tasks (for reporting overall situation)
> > in longer interval and act as a trigger (for accumulating short interval
> > samples) would be useful.
> > 
> > ...
> >
> > +Build kernels with CONFIG_DETECT_HUNG_TASK=y and
> > +CONFIG_DETECT_MEMALLOC_STALL_TASK=y.
> > +
> > +Default scan interval is configured by CONFIG_DEFAULT_MEMALLOC_TASK_TIMEOUT.
> > +Scan interval can be changed at run time by writing timeout in seconds to
> > +/proc/sys/kernel/memalloc_task_warning_secs. Writing 0 disables this scan.
> 
> "seconds" seems needlessly coarse.  Maybe milliseconds?

I can change it to milliseconds although I don't think someone wants to
set e.g. 3500 milliseconds. This timeout is "more than ... seconds" and
actual alert takes longer than this timeout, as with check_hung_task().

> 
> > +Even if you disable this scan, information about last memory allocation
> > +request is kept. That is, you will get some hint for understanding
> > +last-minute behavior of the kernel when you analyze vmcore (or memory
> > +snapshot of a virtualized machine).
> > 
> > ...
> >
> > +struct memalloc_info {
> > +	/*
> > +	 * 0: not doing __GFP_RECLAIM allocation.
> > +	 * 1: doing non-recursive __GFP_RECLAIM allocation.
> > +	 * 2: doing recursive __GFP_RECLAIM allocation.
> > +	 */
> > +	u8 valid;
> > +	/*
> > +	 * bit 0: Will be reported as OOM victim.
> > +	 * bit 1: Will be reported as dying task.
> > +	 * bit 2: Will be reported as stalling task.
> > +	 * bit 3: Will be reported as exiting task.
> > +	 * bit 7: Will be reported unconditionally.
> 
> Create enums for these rather than hard-coding magic numbers?

Sure.

> 
> These values don't seem to be used anyway - as far as I can tell this
> could be a simple boolean.

These values are used in the following switch.

+		u8 type = 0;
+
+		if (test_tsk_thread_flag(p, TIF_MEMDIE)) {
+			type |= 1;
+			memdie_pending++;
+		}
+		if (fatal_signal_pending(p)) {
+			type |= 2;
+			sigkill_pending++;
+		}
+		if ((p->flags & PF_EXITING) && p->state != TASK_DEAD) {
+			type |= 8;
+			exiting_tasks++;
+		}
+		if (is_stalling_task(p, expire)) {
+			type |= 4;
+			stalling_tasks++;
+		}
+		if (p->flags & PF_KSWAPD)
+			type |= 128;
+		p->memalloc.type = type;

Since writing logs to consoles stored by printk() is a slow operation,
we will need to add some delays for avoiding logs being dropped. But
we can't sleep while we are traversing the tasklist whereas tasks used
as a cursor can go away while we are sleeping with rcu_lock_break().
Thus, kmallocwd marks whether and how each thread should be reported
in the first round without sleeping, and reports threads which should be
reported in the second round. The second round sleeps and can complete
without duplicates/skips because p->memalloc.type remembers whether p
should be reported.

This approach can be as well used for check_hung_uninterruptible_tasks()
for avoiding RCU stall warnings, avoiding hung tasks being skipped due to
"goto unlock;" and eliminating the need to call touch_nmi_watchdog().
We can mark whether each thread should be reported as hung in the first
round, and report threads which should be reported as hung in the second
round. Such changes are outside of this patch's scope.

> 
> > +	 */
> > +	u8 type;
> > +	/* Index used for memalloc_in_flight[] counter. */
> > +	u8 idx;
> > +	/* For progress monitoring. */
> > +	unsigned int sequence;
> > +	/* Started time in jiffies as of valid == 1. */
> > +	unsigned long start;
> > +	/* Requested order and gfp flags as of valid == 1. */
> > +	unsigned int order;
> > +	gfp_t gfp;
> > +};
> > 
> > ...
> >
> > +#ifdef CONFIG_DETECT_MEMALLOC_STALL_TASK
> > +/*
> > + * Zero means infinite timeout - no checking done:
> > + */
> > +unsigned long __read_mostly sysctl_memalloc_task_warning_secs =
> > +	CONFIG_DEFAULT_MEMALLOC_TASK_TIMEOUT;
> > +static struct memalloc_info memalloc; /* Filled by is_stalling_task(). */
> 
> What locking protects `memalloc' from concurrent modifications and
> holds it stable for readers?

No locking is needed because only khungtaskd kernel thread accesses
this variable.

> 
> > 
> > ...
> >
> > +static noinline int check_memalloc_stalling_tasks(unsigned long timeout)
> > +{
> > +	char buf[256];
> > +	struct task_struct *g, *p;
> > +	unsigned long now;
> > +	unsigned long expire;
> > +	unsigned int sigkill_pending = 0;
> > +	unsigned int exiting_tasks = 0;
> > +	unsigned int memdie_pending = 0;
> > +	unsigned int stalling_tasks = 0;
> > +
> > 
> > ...
> >
> > +			goto restart_report;
> > +	}
> > +	rcu_read_unlock();
> > +	preempt_enable_no_resched();
> > +	cond_resched();
> 
> All the cond_resched()s in this function seem a bit random.

This function is trying to be as safe as possible; try to yield CPU time
whenever possible because printing stall warnings is not so urgent enough
to block other high priority threads. Since khungtaskd kernel thread is
not SCHED_IDLE priority, we can expect that stall warnings will be
printed eventually (unless a runaway by realtime threads occurs).

> 
> > +	/* Show memory information. (SysRq-m) */
> > +	show_mem(0, NULL);
> > +	/* Show workqueue state. */
> > +	show_workqueue_state();
> > +	/* Show lock information. (SysRq-d) */
> > +	debug_show_all_locks();
> > +	pr_warn("MemAlloc-Info: stalling=%u dying=%u exiting=%u victim=%u oom_count=%u\n",
> > +		stalling_tasks, sigkill_pending, exiting_tasks, memdie_pending,
> > +		out_of_memory_count);
> > +	return stalling_tasks;
> > +}
> > +#endif /* CONFIG_DETECT_MEMALLOC_STALL_TASK */
> > +
> >  static void check_hung_task(struct task_struct *t, unsigned long timeout)
> >  {
> >  	unsigned long switch_count = t->nvcsw + t->nivcsw;
> > @@ -228,20 +429,36 @@ void reset_hung_task_detector(void)
> >  static int watchdog(void *dummy)
> >  {
> >  	unsigned long hung_last_checked = jiffies;
> > +#ifdef CONFIG_DETECT_MEMALLOC_STALL_TASK
> > +	unsigned long stall_last_checked = hung_last_checked;
> > +#endif
> >  
> >  	set_user_nice(current, 0);
> >  
> >  	for ( ; ; ) {
> >  		unsigned long timeout = sysctl_hung_task_timeout_secs;
> >  		long t = hung_timeout_jiffies(hung_last_checked, timeout);
> > -
> > +#ifdef CONFIG_DETECT_MEMALLOC_STALL_TASK
> > +		unsigned long timeout2 = sysctl_memalloc_task_warning_secs;
> > +		long t2 = memalloc_timeout_jiffies(stall_last_checked,
> > +						   timeout2);
> 
> Confused.  Shouldn't timeout2 be converted from seconds to jiffies
> before being passed to memalloc_timeout_jiffies()?

timeout2 is converted from seconds to jiffies by memalloc_timeout_jiffies()
as with timeout is converted from seconds to jiffies by hung_timeout_jiffies().

+static long memalloc_timeout_jiffies(unsigned long last_checked, long timeout)
+{
+	/* timeout of 0 will disable the watchdog */
+	return timeout ? last_checked - jiffies + timeout * HZ :
+		MAX_SCHEDULE_TIMEOUT;
+}

static long hung_timeout_jiffies(unsigned long last_checked,
                                 unsigned long timeout)
{
	/* timeout of 0 will disable the watchdog */
	return timeout ? last_checked - jiffies + timeout * HZ :
		MAX_SCHEDULE_TIMEOUT;
}

We can rename and share hung_timeout_jiffies() for both timeouts
if kmallocwd is acceptable.

> 
> > +		if (t2 <= 0) {
> > +			if (memalloc_maybe_stalling())
> > +				check_memalloc_stalling_tasks(timeout2);
> > +			stall_last_checked = jiffies;
> > +			continue;
> > +		}
> > +#else
> > +		long t2 = t;
> > +#endif
> > 
> > ...
> >
> > +bool memalloc_maybe_stalling(void)
> > +{
> > +	int cpu;
> > +	int sum = 0;
> > +	const u8 idx = memalloc_active_index ^ 1;
> > +
> > +	for_each_possible_cpu(cpu)
> 
> Do we really need to do this for offlined and not-present CPUs?

I just did not want to touch other files within this patch because
I'm not familiar with CPU online/offline handling and want to split
non-essential parts to followup patches.

I assume that a CPU can become offline when someone entered into
__alloc_pages_slowpath() using that CPU. We can update kmallocwd
not to check offlined and not-present CPUs by adding hooks for
resetting memalloc_in_flight counter of a CPU going offline.

> 
> > +		sum += per_cpu(memalloc_in_flight[idx], cpu);
> > +	if (sum)
> > +		return true;
> > +	memalloc_active_index ^= 1;
> > +	return false;
> > +}
> > +
> > 
> > ...
> >
> 
>