• Vlastimil Babka's avatar
    mm, compaction: khugepaged should not give up due to need_resched() · 1f9efdef
    Vlastimil Babka authored
    Async compaction aborts when it detects zone lock contention or
    need_resched() is true.  David Rientjes has reported that in practice,
    most direct async compactions for THP allocation abort due to
    need_resched().  This means that a second direct compaction is never
    attempted, which might be OK for a page fault, but khugepaged is intended
    to attempt a sync compaction in such case and in these cases it won't.
    
    This patch replaces "bool contended" in compact_control with an int that
    distinguishes between aborting due to need_resched() and aborting due to
    lock contention.  This allows propagating the abort through all compaction
    functions as before, but passing the abort reason up to
    __alloc_pages_slowpath() which decides when to continue with direct
    reclaim and another compaction attempt.
    
    Another problem is that try_to_compact_pages() did not act upon the
    reported contention (both need_resched() or lock contention) immediately
    and would proceed with another zone from the zonelist.  When
    need_resched() is true, that means initializing another zone compaction,
    only to check again need_resched() in isolate_migratepages() and aborting.
     For zone lock contention, the unintended consequence is that the lock
    contended status reported back to the allocator is detrmined from the last
    zone where compaction was attempted, which is rather arbitrary.
    
    This patch fixes the problem in the following way:
    - async compaction of a zone aborting due to need_resched() or fatal signal
      pending means that further zones should not be tried. We report
      COMPACT_CONTENDED_SCHED to the allocator.
    - aborting zone compaction due to lock contention means we can still try
      another zone, since it has different set of locks. We report back
      COMPACT_CONTENDED_LOCK only if *all* zones where compaction was attempted,
      it was aborted due to lock contention.
    
    As a result of these fixes, khugepaged will proceed with second sync
    compaction as intended, when the preceding async compaction aborted due to
    need_resched().  Page fault compactions aborting due to need_resched()
    will spare some cycles previously wasted by initializing another zone
    compaction only to abort again.  Lock contention will be reported only
    when compaction in all zones aborted due to lock contention, and therefore
    it's not a good idea to try again after reclaim.
    
    In stress-highalloc from mmtests configured to use __GFP_NO_KSWAPD, this
    has improved number of THP collapse allocations by 10%, which shows
    positive effect on khugepaged.  The benchmark's success rates are
    unchanged as it is not recognized as khugepaged.  Numbers of compact_stall
    and compact_fail events have however decreased by 20%, with
    compact_success still a bit improved, which is good.  With benchmark
    configured not to use __GFP_NO_KSWAPD, there is 6% improvement in THP
    collapse allocations, and only slight improvement in stalls and failures.
    
    [akpm@linux-foundation.org: fix warnings]
    Reported-by: default avatarDavid Rientjes <rientjes@google.com>
    Signed-off-by: default avatarVlastimil Babka <vbabka@suse.cz>
    Cc: Minchan Kim <minchan@kernel.org>
    Acked-by: default avatarMel Gorman <mgorman@suse.de>
    Cc: Joonsoo Kim <iamjoonsoo.kim@lge.com>
    Cc: Michal Nazarewicz <mina86@mina86.com>
    Cc: Naoya Horiguchi <n-horiguchi@ah.jp.nec.com>
    Cc: Christoph Lameter <cl@linux.com>
    Cc: Rik van Riel <riel@redhat.com>
    Signed-off-by: default avatarAndrew Morton <akpm@linux-foundation.org>
    Signed-off-by: default avatarLinus Torvalds <torvalds@linux-foundation.org>
    1f9efdef
compaction.c 38.6 KB