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Re: [PATCH 06/11] mm, compaction: distinguish between full and partial COMPACT_COMPLETE

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On Mon 11-04-16 15:42:21, Vlastimil Babka wrote:
> On 04/11/2016 03:27 PM, Michal Hocko wrote:
> >On Mon 11-04-16 14:53:36, Vlastimil Babka wrote:
> >>On 04/11/2016 02:46 PM, Michal Hocko wrote:
> >>
> >>The racy part is negligible but I didn't realize the sync/async migrate
> >>scanner part until now. So yeah, free_pfn could have got to middle of zone
> >>when it was in the async mode. But that also means that the async mode
> >>recently used up all free pages in the second half of the zone. WRT free
> >>pages isolation, async mode is not trying less than sync, so it shouldn't be
> >>a considerable missed opportunity if we don't rescan the it, though.
> >
> >I am not really sure I understand. The primary intention of this patch
> >is to distinguish where we have scanned basically whole zones from cases
> >where a new scan started off previous mark and so it was just unlucky to
> >see only tiny bit of the zone where we would clearly give up too early.
> >FWIU this shouldn't be the case if we start scanning from the beginning
> >of the zone even if we raced on the other end of the zone because the
> >missed part would be negligible. Is that understanding correct?
> 
> Yes, it should be less unlucky than seeing a tiny bit of the zone. Just
> wanted to point out that you might still not see the whole zone in one
> compaction attempt. E.g. async compaction is first, advances the free
> scanner and caches its position when it bails out due to being contended.
> Then direct reclaim frees some pages behind the cached position. Sync
> compaction attempts starts migration scanner from start_pfn, but picks up
> the cached free scanner pfn. The result is missing some free pages and the
> scanners meeting somewhat earlier than they otherwise would. Probably not
> critical even for OOM decisions, as that's also racy anyway.

OK, I see now. I agree this shouldn't be critical and thanks for the
clarification.
-- 
Michal Hocko
SUSE Labs

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[PATCH 00/11] oom detection rework v5 Michal Hocko <mhocko@kernel.org> - 2016-04-05 13:30 +0200
  [PATCH 10/11] mm, oom: protect !costly allocations some more Michal Hocko <mhocko@kernel.org> - 2016-04-05 13:30 +0200
    Re: [PATCH 10/11] mm, oom: protect !costly allocations some more Andrew Morton <akpm@linux-foundation.org> - 2016-04-06 02:10 +0200
      Re: [PATCH 10/11] mm, oom: protect !costly allocations some more Michal Hocko <mhocko@kernel.org> - 2016-04-06 11:30 +0200
    Re: [PATCH 10/11] mm, oom: protect !costly allocations some more Vlastimil Babka <vbabka@suse.cz> - 2016-04-11 16:50 +0200
  [PATCH 08/11] mm, compaction: Simplify __alloc_pages_direct_compact feedback interface Michal Hocko <mhocko@kernel.org> - 2016-04-05 13:30 +0200
    Re: [PATCH 08/11] mm, compaction: Simplify  __alloc_pages_direct_compact feedback interface Vlastimil Babka <vbabka@suse.cz> - 2016-04-11 16:00 +0200
  [PATCH 04/11] mm, compaction: cover all compaction mode in compact_zone Michal Hocko <mhocko@kernel.org> - 2016-04-05 13:30 +0200
  [PATCH 05/11] mm, compaction: distinguish COMPACT_DEFERRED from COMPACT_SKIPPED Michal Hocko <mhocko@kernel.org> - 2016-04-05 13:30 +0200
    Re: [PATCH 05/11] mm, compaction: distinguish COMPACT_DEFERRED from  COMPACT_SKIPPED Vlastimil Babka <vbabka@suse.cz> - 2016-04-11 13:10 +0200
      Re: [PATCH 05/11] mm, compaction: distinguish COMPACT_DEFERRED from  COMPACT_SKIPPED Michal Hocko <mhocko@kernel.org> - 2016-04-11 13:30 +0200
  [PATCH 07/11] mm, compaction: Update compaction_result ordering Michal Hocko <mhocko@kernel.org> - 2016-04-05 13:30 +0200
    Re: [PATCH 07/11] mm, compaction: Update compaction_result ordering Vlastimil Babka <vbabka@suse.cz> - 2016-04-11 14:20 +0200
  [PATCH 02/11] mm: throttle on IO only when there are too many dirty and writeback pages Michal Hocko <mhocko@kernel.org> - 2016-04-05 13:30 +0200
  [PATCH 01/11] mm, oom: rework oom detection Michal Hocko <mhocko@kernel.org> - 2016-04-05 13:30 +0200
  [PATCH 03/11] mm, compaction: change COMPACT_ constants into enum Michal Hocko <mhocko@kernel.org> - 2016-04-05 13:30 +0200
  [PATCH 06/11] mm, compaction: distinguish between full and partial COMPACT_COMPLETE Michal Hocko <mhocko@kernel.org> - 2016-04-05 13:30 +0200
    Re: [PATCH 06/11] mm, compaction: distinguish between full and  partial COMPACT_COMPLETE Vlastimil Babka <vbabka@suse.cz> - 2016-04-11 14:20 +0200
      Re: [PATCH 06/11] mm, compaction: distinguish between full and  partial COMPACT_COMPLETE Michal Hocko <mhocko@kernel.org> - 2016-04-11 14:50 +0200
        Re: [PATCH 06/11] mm, compaction: distinguish between full and  partial COMPACT_COMPLETE Vlastimil Babka <vbabka@suse.cz> - 2016-04-11 15:00 +0200
          Re: [PATCH 06/11] mm, compaction: distinguish between full and  partial COMPACT_COMPLETE Michal Hocko <mhocko@kernel.org> - 2016-04-11 15:30 +0200
            Re: [PATCH 06/11] mm, compaction: distinguish between full and  partial COMPACT_COMPLETE Vlastimil Babka <vbabka@suse.cz> - 2016-04-11 15:50 +0200
              Re: [PATCH 06/11] mm, compaction: distinguish between full and  partial COMPACT_COMPLETE Michal Hocko <mhocko@kernel.org> - 2016-04-11 15:50 +0200
  [PATCH 09/11] mm, compaction: Abstract compaction feedback to helpers Michal Hocko <mhocko@kernel.org> - 2016-04-05 13:30 +0200
    Re: [PATCH 09/11] mm, compaction: Abstract compaction feedback to  helpers Andrew Morton <akpm@linux-foundation.org> - 2016-04-06 02:00 +0200
      Re: [PATCH 09/11] mm, compaction: Abstract compaction feedback to  helpers Hugh Dickins <hughd@google.com> - 2016-04-06 03:00 +0200
        Re: [PATCH 09/11] mm, compaction: Abstract compaction feedback to  helpers Michal Hocko <mhocko@kernel.org> - 2016-04-06 11:30 +0200
        Re: [PATCH 09/11] mm, compaction: Abstract compaction feedback to  helpers Andrew Morton <akpm@linux-foundation.org> - 2016-04-06 19:50 +0200
    Re: [PATCH 09/11] mm, compaction: Abstract compaction feedback to  helpers Vlastimil Babka <vbabka@suse.cz> - 2016-04-11 16:40 +0200
      Re: [PATCH 09/11] mm, compaction: Abstract compaction feedback to  helpers Michal Hocko <mhocko@kernel.org> - 2016-04-11 17:20 +0200
        Re: [PATCH 09/11] mm, compaction: Abstract compaction feedback to  helpers Michal Hocko <mhocko@kernel.org> - 2016-04-11 17:40 +0200
        Re: [PATCH 09/11] mm, compaction: Abstract compaction feedback to  helpers Vlastimil Babka <vbabka@suse.cz> - 2016-04-12 14:00 +0200
          Re: [PATCH 09/11] mm, compaction: Abstract compaction feedback to  helpers Michal Hocko <mhocko@kernel.org> - 2016-04-12 14:30 +0200
    Re: [PATCH 09/11] mm, compaction: Abstract compaction feedback to  helpers Michal Hocko <mhocko@kernel.org> - 2016-04-11 18:00 +0200
      [RFC PATCH] mm: use compaction feedback for thp backoff conditions Michal Hocko <mhocko@kernel.org> - 2016-04-11 18:10 +0200
      Re: [PATCH 09/11] mm, compaction: Abstract compaction feedback to  helpers Vlastimil Babka <vbabka@suse.cz> - 2016-04-12 14:00 +0200
  [PATCH 11/11] mm: consider compaction feedback also for costly allocation Michal Hocko <mhocko@kernel.org> - 2016-04-05 13:30 +0200
    Re: [PATCH 11/11] mm: consider compaction feedback also for costly  allocation Michal Hocko <mhocko@kernel.org> - 2016-04-05 14:50 +0200
    Re: [PATCH 11/11] mm: consider compaction feedback also for costly  allocation Vlastimil Babka <vbabka@suse.cz> - 2016-04-11 17:10 +0200
  Re: [PATCH 00/11] oom detection rework v5 Michal Hocko <mhocko@kernel.org> - 2016-04-05 14:50 +0200

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