• Mel Gorman's avatar
    mm: compaction: avoid 100% CPU usage during compaction when a task is killed · c7e8100a
    Mel Gorman authored
    commit 670105a2 upstream.
    
    "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's avatarMel Gorman <mgorman@techsingularity.net>
    Reviewed-by: default avatarVlastimil Babka <vbabka@suse.cz>
    Cc: <stable@vger.kernel.org>	[5.1+]
    Signed-off-by: default avatarAndrew Morton <akpm@linux-foundation.org>
    Signed-off-by: default avatarLinus Torvalds <torvalds@linux-foundation.org>
    Signed-off-by: default avatarGreg Kroah-Hartman <gregkh@linuxfoundation.org>
    c7e8100a
compaction.c 75.1 KB