• Vlastimil Babka's avatar
    mm, compaction: simplify contended compaction handling · c3486f53
    Vlastimil Babka authored
    Async compaction detects contention either due to failing trylock on
    zone->lock or lru_lock, or by need_resched().  Since 1f9efdef ("mm,
    compaction: khugepaged should not give up due to need_resched()") the
    code got quite complicated to distinguish these two up to the
    __alloc_pages_slowpath() level, so different decisions could be taken
    for khugepaged allocations.
    
    After the recent changes, khugepaged allocations don't check for
    contended compaction anymore, so we again don't need to distinguish lock
    and sched contention, and simplify the current convoluted code a lot.
    
    However, I believe it's also possible to simplify even more and
    completely remove the check for contended compaction after the initial
    async compaction for costly orders, which was originally aimed at THP
    page fault allocations.  There are several reasons why this can be done
    now:
    
    - with the new defaults, THP page faults no longer do reclaim/compaction at
      all, unless the system admin has overridden the default, or application has
      indicated via madvise that it can benefit from THP's. In both cases, it
      means that the potential extra latency is expected and worth the benefits.
    - even if reclaim/compaction proceeds after this patch where it previously
      wouldn't, the second compaction attempt is still async and will detect the
      contention and back off, if the contention persists
    - there are still heuristics like deferred compaction and pageblock skip bits
      in place that prevent excessive THP page fault latencies
    
    Link: http://lkml.kernel.org/r/20160721073614.24395-9-vbabka@suse.czSigned-off-by: 's avatarVlastimil Babka <vbabka@suse.cz>
    Acked-by: 's avatarMichal Hocko <mhocko@suse.com>
    Acked-by: 's avatarMel Gorman <mgorman@techsingularity.net>
    Signed-off-by: 's avatarAndrew Morton <akpm@linux-foundation.org>
    Signed-off-by: 's avatarLinus Torvalds <torvalds@linux-foundation.org>
    c3486f53
compaction.h 6.05 KB