From: Mel Gorman mgorman@suse.de Subject: mm: vmscan: remove deadlock due to throttling failing to make progress

A soft lockup bug in kcompactd was reported in a private bugzilla with the following visible in dmesg;

[15980.045209][ C33] watchdog: BUG: soft lockup - CPU#33 stuck for 26s! [kcompactd0:479] [16008.044989][ C33] watchdog: BUG: soft lockup - CPU#33 stuck for 52s! [kcompactd0:479] [16036.044768][ C33] watchdog: BUG: soft lockup - CPU#33 stuck for 78s! [kcompactd0:479] [16064.044548][ C33] watchdog: BUG: soft lockup - CPU#33 stuck for 104s! [kcompactd0:479]

The machine had 256G of RAM with no swap and an earlier failed allocation indicated that node 0 where kcompactd was run was potentially unreclaimable;

Node 0 active_anon:29355112kB inactive_anon:2913528kB active_file:0kB inactive_file:0kB unevictable:64kB isolated(anon):0kB isolated(file):0kB mapped:8kB dirty:0kB writeback:0kB shmem:26780kB shmem_thp: 0kB shmem_pmdmapped: 0kB anon_thp: 23480320kB writeback_tmp:0kB kernel_stack:2272kB pagetables:24500kB all_unreclaimable? yes

Vlastimil Babka investigated a crash dump and found that a task migrating pages was trying to drain PCP lists;

PID: 52922 TASK: ffff969f820e5000 CPU: 19 COMMAND: "kworker/u128:3" #0 [ffffaf4e4f4c3848] __schedule at ffffffffb840116d #1 [ffffaf4e4f4c3908] schedule at ffffffffb8401e81 #2 [ffffaf4e4f4c3918] schedule_timeout at ffffffffb84066e8 #3 [ffffaf4e4f4c3990] wait_for_completion at ffffffffb8403072 #4 [ffffaf4e4f4c39d0] __flush_work at ffffffffb7ac3e4d #5 [ffffaf4e4f4c3a48] __drain_all_pages at ffffffffb7cb707c #6 [ffffaf4e4f4c3a80] __alloc_pages_slowpath.constprop.114 at ffffffffb7cbd9dd #7 [ffffaf4e4f4c3b60] __alloc_pages at ffffffffb7cbe4f5 #8 [ffffaf4e4f4c3bc0] alloc_migration_target at ffffffffb7cf329c #9 [ffffaf4e4f4c3bf0] migrate_pages at ffffffffb7cf6d15 10 [ffffaf4e4f4c3cb0] migrate_to_node at ffffffffb7cdb5aa 11 [ffffaf4e4f4c3da8] do_migrate_pages at ffffffffb7cdcf26 12 [ffffaf4e4f4c3e88] cpuset_migrate_mm_workfn at ffffffffb7b859d2 13 [ffffaf4e4f4c3e98] process_one_work at ffffffffb7ac45f3 14 [ffffaf4e4f4c3ed8] worker_thread at ffffffffb7ac47fd 15 [ffffaf4e4f4c3f10] kthread at ffffffffb7acbdc6 16 [ffffaf4e4f4c3f50] ret_from_fork at ffffffffb7a047e2

This failure is specific to CONFIG_PREEMPT=n builds. The root of the problem is that kcompact0 is not rescheduling on a CPU while a task that has isolated a large number of the pages from the LRU is waiting on kcompact0 to reschedule so the pages can be released. While shrink_inactive_list() only loops once around too_many_isolated, reclaim can continue without rescheduling if sc->skipped_deactivate == 1 which could happen if there was no file LRU and the inactive anon list was not low.

Link: https://lkml.kernel.org/r/20220203100326.GD3301@suse.de Fixes: d818fca1cac3 ("mm/vmscan: throttle reclaim and compaction when too may pages are isolated") Signed-off-by: Mel Gorman mgorman@suse.de Debugged-by: Vlastimil Babka vbabka@suse.cz Reviewed-by: Vlastimil Babka vbabka@suse.cz Acked-by: Michal Hocko mhocko@suse.com Acked-by: David Rientjes rientjes@google.com Cc: Hugh Dickins hughd@google.com Cc: Michal Hocko mhocko@suse.com Cc: Rik van Riel riel@surriel.com Cc: stable@vger.kernel.org Signed-off-by: Andrew Morton akpm@linux-foundation.org ---

--- a/mm/vmscan.c~mm-vmscan-remove-deadlock-due-to-throttling-failing-to-make-progress +++ a/mm/vmscan.c @@ -1066,8 +1066,10 @@ void reclaim_throttle(pg_data_t *pgdat, * forward progress (e.g. journalling workqueues or kthreads). */ if (!current_is_kswapd() && - current->flags & (PF_IO_WORKER|PF_KTHREAD)) + current->flags & (PF_IO_WORKER|PF_KTHREAD)) { + cond_resched(); return; + }

/* * These figures are pulled out of thin air. _