Re: [PATCH] mm: avoid blocking lock_page() in kcompactd

[Cong Wang]

On Tue, Jan 21, 2020 at 1:00 AM Michal Hocko <mhocko@xxxxxxxxxx> wrote:
>
> On Mon 20-01-20 14:48:05, Cong Wang wrote:
> > It got stuck somewhere along the call path of mem_cgroup_try_charge(),
> > and the trace events of mm_vmscan_lru_shrink_inactive() indicates this
> > too:
>
> So it seems that you are condending on the page lock. It is really
> unexpected that the reclaim would take that long though. Please try to
> enable more vmscan tracepoints to see where the time is spent.

Sorry for the delay. I have been trying to collect more data in one shot.

This is a a typical round of the loop after enabling all vmscan tracepoints:

           <...>-455450 [007] .... 4048595.842992:
mm_vmscan_memcg_reclaim_begin: order=0 may_writepage=1
gfp_flags=GFP_NOFS|__GFP_HIGHMEM|__GFP_HARDWALL|__GFP_MOVABLE
classzone_idx=
4
           <...>-455450 [007] d... 4048595.842995:
mm_vmscan_lru_isolate: isolate_mode=0 classzone=3 order=0
nr_requested=1 nr_scanned=1 nr_skipped=0 nr_taken=1 lru=inactive_file
           <...>-455450 [007] .... 4048595.842995:
mm_vmscan_lru_shrink_inactive: nid=0 nr_scanned=1 nr_reclaimed=0
nr_dirty=0 nr_writeback=0 nr_congested=0 nr_immediate=0 nr_activat
e=0 nr_ref_keep=0 nr_unmap_fail=0 priority=4
flags=RECLAIM_WB_FILE|RECLAIM_WB_ASYNC
           <...>-455450 [007] dN.. 4048595.842997:
mm_vmscan_lru_isolate: isolate_mode=0 classzone=3 order=0
nr_requested=1 nr_scanned=1 nr_skipped=0 nr_taken=1 lru=inactive_file
           <...>-455450 [007] .... 4048595.843001:
mm_vmscan_lru_shrink_inactive: nid=0 nr_scanned=1 nr_reclaimed=0
nr_dirty=0 nr_writeback=0 nr_congested=0 nr_immediate=0 nr_activat
e=0 nr_ref_keep=0 nr_unmap_fail=0 priority=3
flags=RECLAIM_WB_FILE|RECLAIM_WB_ASYNC
           <...>-455450 [007] d... 4048595.843002:
mm_vmscan_lru_isolate: isolate_mode=0 classzone=3 order=0
nr_requested=5 nr_scanned=5 nr_skipped=0 nr_taken=5 lru=inactive_file
           <...>-455450 [007] .... 4048595.843004:
mm_vmscan_lru_shrink_inactive: nid=0 nr_scanned=5 nr_reclaimed=0
nr_dirty=0 nr_writeback=0 nr_congested=0 nr_immediate=0 nr_activat
e=0 nr_ref_keep=0 nr_unmap_fail=0 priority=2
flags=RECLAIM_WB_FILE|RECLAIM_WB_ASYNC
           <...>-455450 [007] d... 4048595.843006:
mm_vmscan_lru_isolate: isolate_mode=0 classzone=3 order=0
nr_requested=9 nr_scanned=9 nr_skipped=0 nr_taken=9 lru=inactive_file
           <...>-455450 [007] .... 4048595.843007:
mm_vmscan_lru_shrink_inactive: nid=0 nr_scanned=9 nr_reclaimed=0
nr_dirty=0 nr_writeback=0 nr_congested=0 nr_immediate=0 nr_activat
e=0 nr_ref_keep=0 nr_unmap_fail=0 priority=1
flags=RECLAIM_WB_FILE|RECLAIM_WB_ASYNC
           <...>-455450 [007] d... 4048595.843009:
mm_vmscan_lru_isolate: isolate_mode=0 classzone=3 order=0
nr_requested=17 nr_scanned=15 nr_skipped=0 nr_taken=15
lru=inactive_file
           <...>-455450 [007] .... 4048595.843011:
mm_vmscan_lru_shrink_inactive: nid=0 nr_scanned=15 nr_reclaimed=0
nr_dirty=0 nr_writeback=0 nr_congested=0 nr_immediate=0 nr_activa
te=0 nr_ref_keep=0 nr_unmap_fail=0 priority=0
flags=RECLAIM_WB_FILE|RECLAIM_WB_ASYNC
           <...>-455450 [007] .... 4048595.843012:
mm_vmscan_memcg_reclaim_end: nr_reclaimed=0

The whole trace output is huge (33M), I can provide it on request.

I suspect the process gets stuck in the retry loop in try_charge(), as
the _shortest_ stacktrace of the perf samples indicated:

cycles:ppp:
        ffffffffa72963db mem_cgroup_iter
        ffffffffa72980ca mem_cgroup_oom_unlock
        ffffffffa7298c15 try_charge
        ffffffffa729a886 mem_cgroup_try_charge
        ffffffffa720ec03 __add_to_page_cache_locked
        ffffffffa720ee3a add_to_page_cache_lru
        ffffffffa7312ddb iomap_readpages_actor
        ffffffffa73133f7 iomap_apply
        ffffffffa73135da iomap_readpages
        ffffffffa722062e read_pages
        ffffffffa7220b3f __do_page_cache_readahead
        ffffffffa7210554 filemap_fault
        ffffffffc039e41f __xfs_filemap_fault
        ffffffffa724f5e7 __do_fault
        ffffffffa724c5f2 __handle_mm_fault
        ffffffffa724cbc6 handle_mm_fault
        ffffffffa70a313e __do_page_fault
        ffffffffa7a00dfe page_fault

But I don't see how it could be, the only possible case is when
mem_cgroup_oom() returns OOM_SUCCESS. However I can't
find any clue in dmesg pointing to OOM. These processes in the
same memcg are either running or sleeping (that is not exiting or
coredump'ing), I don't see how and why they could be selected as
a victim of OOM killer. I don't see any signal pending either from
their /proc/X/status.

Here is the status of the whole memcg:

$ sudo cat /sys/fs/cgroup/memory/system.slice/osqueryd.service/memory.oom_control
oom_kill_disable 0
under_oom 0
oom_kill 0

$ sudo cat /sys/fs/cgroup/memory/system.slice/osqueryd.service/memory.stat
cache 376832
rss 255864832
rss_huge 0
shmem 0
mapped_file 135168
dirty 135168
writeback 0
pgpgin 5327157
pgpgout 5264598
pgfault 4251687
pgmajfault 7491
inactive_anon 4096
active_anon 256184320
inactive_file 32768
active_file 0
unevictable 0
hierarchical_memory_limit 262144000
total_cache 376832
total_rss 255864832
total_rss_huge 0
total_shmem 0
total_mapped_file 135168
total_dirty 135168
total_writeback 0
total_pgpgin 5327157
total_pgpgout 5264598
total_pgfault 4251687
total_pgmajfault 7491
total_inactive_anon 4096
total_active_anon 256184320
total_inactive_file 86016
total_active_file 0
total_unevictable 0

Please let me know if I can provide anything else.

Thanks.