The patch titled Subject: mm: compaction: avoid 100% CPU usage during compaction when a task is killed has been added to the -mm tree. Its filename is mm-compaction-avoid-100%-cpu-usage-during-compaction-when-a-task-is-killed.patch

This patch should soon appear at http://ozlabs.org/~akpm/mmots/broken-out/mm-compaction-avoid-100%25-cpu-usag... and later at http://ozlabs.org/~akpm/mmotm/broken-out/mm-compaction-avoid-100%25-cpu-usag...

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------------------------------------------------------ From: Mel Gorman mgorman@techsingularity.net Subject: mm: compaction: avoid 100% CPU usage during compaction when a task is killed

"howaboutsynergy" reported via kernel buzilla number 204165 that compact_zone_order was consuming 100% CPU during a stress test for prolonged periods of time. Specifically the following command, which should exit in 10 seconds, was taking an excessive time to finish while the CPU was pegged at 100%.

stress -m 220 --vm-bytes 1000000000 --timeout 10

Tracing indicated a pattern as follows

stress-3923 [007] 519.106208: mm_compaction_isolate_migratepages: range=(0x70bb80 ~ 0x70bb80) nr_scanned=0 nr_taken=0 stress-3923 [007] 519.106212: mm_compaction_isolate_migratepages: range=(0x70bb80 ~ 0x70bb80) nr_scanned=0 nr_taken=0 stress-3923 [007] 519.106216: mm_compaction_isolate_migratepages: range=(0x70bb80 ~ 0x70bb80) nr_scanned=0 nr_taken=0 stress-3923 [007] 519.106219: mm_compaction_isolate_migratepages: range=(0x70bb80 ~ 0x70bb80) nr_scanned=0 nr_taken=0 stress-3923 [007] 519.106223: mm_compaction_isolate_migratepages: range=(0x70bb80 ~ 0x70bb80) nr_scanned=0 nr_taken=0 stress-3923 [007] 519.106227: mm_compaction_isolate_migratepages: range=(0x70bb80 ~ 0x70bb80) nr_scanned=0 nr_taken=0 stress-3923 [007] 519.106231: mm_compaction_isolate_migratepages: range=(0x70bb80 ~ 0x70bb80) nr_scanned=0 nr_taken=0 stress-3923 [007] 519.106235: mm_compaction_isolate_migratepages: range=(0x70bb80 ~ 0x70bb80) nr_scanned=0 nr_taken=0 stress-3923 [007] 519.106238: mm_compaction_isolate_migratepages: range=(0x70bb80 ~ 0x70bb80) nr_scanned=0 nr_taken=0 stress-3923 [007] 519.106242: mm_compaction_isolate_migratepages: range=(0x70bb80 ~ 0x70bb80) nr_scanned=0 nr_taken=0

Note that compaction is entered in rapid succession while scanning and isolating nothing. The problem is that when a task that is compacting receives a fatal signal, it retries indefinitely instead of exiting while making no progress as a fatal signal is pending.

It's not easy to trigger this condition although enabling zswap helps on the basis that the timing is altered. A very small window has to be hit for the problem to occur (signal delivered while compacting and isolating a PFN for migration that is not aligned to SWAP_CLUSTER_MAX).

This was reproduced locally -- 16G single socket system, 8G swap, 30% zswap configured, vm-bytes 22000000000 using Colin Kings stress-ng implementation from github running in a loop until the problem hits). Tracing recorded the problem occurring almost 200K times in a short window. With this patch, the problem hit 4 times but the task existed normally instead of consuming CPU.

This problem has existed for some time but it was made worse by cf66f0700c8f ("mm, compaction: do not consider a need to reschedule as contention"). Before that commit, if the same condition was hit then locks would be quickly contended and compaction would exit that way.

The reporter's real name is unknown. This was caught and repaired due to their testing and tracing.

Bugzilla: https://bugzilla.kernel.org/show_bug.cgi?id=204165 Link: http://lkml.kernel.org/r/20190718085708.GE24383@techsingularity.net Fixes: cf66f0700c8f ("mm, compaction: do not consider a need to reschedule as contention") Signed-off-by: Mel Gorman mgorman@techsingularity.net Reported-by: howaboutsynergy@protonmail.com Tested-by: howaboutsynergy@protonmail.com Cc: Vlastimil Babka vbabka@suse.cz Cc: stable@vger.kernel.org [5.1+] Signed-off-by: Andrew Morton akpm@linux-foundation.org ---

mm/compaction.c | 11 +++++++---- 1 file changed, 7 insertions(+), 4 deletions(-)

--- a/mm/compaction.c~mm-compaction-avoid-100%-cpu-usage-during-compaction-when-a-task-is-killed +++ a/mm/compaction.c @@ -842,13 +842,15 @@ isolate_migratepages_block(struct compac

/* * Periodically drop the lock (if held) regardless of its - * contention, to give chance to IRQs. Abort async compaction - * if contended. + * contention, to give chance to IRQs. Abort completely if + * a fatal signal is pending. */ if (!(low_pfn % SWAP_CLUSTER_MAX) && compact_unlock_should_abort(&pgdat->lru_lock, - flags, &locked, cc)) - break; + flags, &locked, cc)) { + low_pfn = 0; + goto fatal_pending; + }

if (!pfn_valid_within(low_pfn)) goto isolate_fail; @@ -1060,6 +1062,7 @@ isolate_abort: trace_mm_compaction_isolate_migratepages(start_pfn, low_pfn, nr_scanned, nr_isolated);

+fatal_pending: cc->total_migrate_scanned += nr_scanned; if (nr_isolated) count_compact_events(COMPACTISOLATED, nr_isolated); _

Patches currently in -mm which might be from mgorman@techsingularity.net are

mm-compaction-avoid-100%-cpu-usage-during-compaction-when-a-task-is-killed.patch