Re: [PATCH 09/14] mm: use compaction feedback for thp backoff conditions

[Vlastimil Babka <vbabka@suse.cz>]

On 04/20/2016 09:47 PM, Michal Hocko wrote:
> From: Michal Hocko <mhocko@suse.com>
>
> THP requests skip the direct reclaim if the compaction is either
> deferred or contended to reduce stalls which wouldn't help the
> allocation success anyway. These checks are ignoring other potential
> feedback modes which we have available now.
>
> It clearly doesn't make much sense to go and reclaim few pages if the
> previous compaction has failed.
>
> We can also simplify the check by using compaction_withdrawn which
> checks for both COMPACT_CONTENDED and COMPACT_DEFERRED. This check
> is however covering more reasons why the compaction was withdrawn.
> None of them should be a problem for the THP case though.
>
> It is safe to back of if we see COMPACT_SKIPPED because that means
> that compaction_suitable failed and a single round of the reclaim is
> unlikely to make any difference here. We would have to be close to
> the low watermark to reclaim enough and even then there is no guarantee
> that the compaction would make any progress while the direct reclaim
> would have caused the stall.
>
> COMPACT_PARTIAL_SKIPPED is slightly different because that means that we
> have only seen a part of the zone so a retry would make some sense. But
> it would be a compaction retry not a reclaim retry to perform. We are
> not doing that and that might indeed lead to situations where THP fails
> but this should happen only rarely and it would be really hard to
> measure.
>
> Signed-off-by: Michal Hocko <mhocko@suse.com>

THP's don't compact by default in page fault path anymore, so we don't 
need to restrict them even more. And hopefully we'll replace the 
is_thp_gfp_mask() hack with something better soon, so this might be just 
extra code churn. But I don't feel strongly enough to nack it.

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