[PATCH] mm: vmscan: remove deadlock due to throttling failing to make progress

[Mel Gorman <mgorman@suse.de>]

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

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.

Debugged-by: Vlastimil Babka <vbabka@suse.cz>
Signed-off-by: Mel Gorman <mgorman@suse.de>
---
 mm/vmscan.c |    4 +++-
 1 file changed, 3 insertions(+), 1 deletion(-)

diff --git a/mm/vmscan.c b/mm/vmscan.c
index 090bfb605ecf..59b14e0d696c 100644
--- a/mm/vmscan.c
+++ b/mm/vmscan.c
@@ -1066,8 +1066,10 @@ void reclaim_throttle(pg_data_t *pgdat, enum vmscan_throttle_state reason)
 	 * 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.