From 670105a25608affe01cb0ccdc2a1f4bd2327172b Mon Sep 17 00:00:00 2001 From: Mel Gorman Date: Fri, 2 Aug 2019 21:48:51 -0700 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 commit 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. 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 Reviewed-by: Vlastimil Babka Cc: [5.1+] Signed-off-by: Andrew Morton Signed-off-by: Linus Torvalds --- mm/compaction.c | 11 +++++++---- 1 file changed, 7 insertions(+), 4 deletions(-) (limited to 'mm/compaction.c') diff --git a/mm/compaction.c b/mm/compaction.c index 9e1b9acb116b..952dc2fb24e5 100644 --- a/mm/compaction.c +++ b/mm/compaction.c @@ -842,13 +842,15 @@ isolate_migratepages_block(struct compact_control *cc, unsigned long low_pfn, /* * 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); -- cgit v1.2.3