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 cf66f070 ("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: cf66f070 ("mm, compaction: do not consider a need to reschedule as contention") Signed-off-by:Mel Gorman <mgorman@techsingularity.net> Reviewed-by:
Vlastimil Babka <vbabka@suse.cz> Cc: <stable@vger.kernel.org> [5.1+] Signed-off-by:
Andrew Morton <akpm@linux-foundation.org> Signed-off-by:
Linus Torvalds <torvalds@linux-foundation.org>
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