linux-kernel - Re: [RFC 12/13] mm, compaction: more reliably increase direct compaction priority

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Date:	Mon, 16 May 2016 11:27:56 +0200
From:	Vlastimil Babka <vbabka@...e.cz>
To:	Michal Hocko <mhocko@...nel.org>
Cc:	linux-mm@...ck.org, Andrew Morton <akpm@...ux-foundation.org>,
	Joonsoo Kim <iamjoonsoo.kim@....com>,
	Rik van Riel <riel@...hat.com>,
	David Rientjes <rientjes@...gle.com>,
	Mel Gorman <mgorman@...hsingularity.net>,
	Johannes Weiner <hannes@...xchg.org>,
	Tetsuo Handa <penguin-kernel@...ove.sakura.ne.jp>,
	linux-kernel@...r.kernel.org,
	Linus Torvalds <torvalds@...ux-foundation.org>
Subject: Re: [RFC 12/13] mm, compaction: more reliably increase direct
 compaction priority

On 05/16/2016 10:14 AM, Michal Hocko wrote:
> On Mon 16-05-16 09:31:44, Vlastimil Babka wrote:
>>
>> Yeah it should work, my only worry was that this may get subtly wrong (as
>> experience shows us) and due to e.g. slightly different watermark checks
>> and/or a corner-case zone such as ZONE_DMA, should_reclaim_retry() would
>> keep returning true, even if reclaim couldn't/wouldn't help anything. Then
>> compaction would be needlessly kept at ineffective priority.
>
> watermark check for ZONE_DMA should always fail because it fails even
> when is completely free to the lowmem reserves. I had a subtle bug in
> the original code to check highzone_idx rather than classzone_idx but
> that should the fix has been posted recently:
> http://lkml.kernel.org/r/1463051677-29418-2-git-send-email-mhocko@kernel.org

Sure, but that just adds to the experience of being subtly wrong in this 
area :) But sure we can leave this part alone until proven wrong, I 
don't insist strongly.

>> Also my understanding of the initial compaction priorities is to lower the
>> latency if fragmentation is just light and there's enough memory. Once we
>> start struggling, I don't see much point in not switching to the full
>> compaction priority quickly.
>
> That is true but why to compact when there are high order pages and they
> are just hidden by the watermark check.

Compaction should skip such zone regardless of priority.