linux-kernel - Re: [PATCH v3 13/17] mm, compaction: use correct watermark when checking allocation success

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Date:	Mon, 18 Jul 2016 11:23:38 +0200
From:	Vlastimil Babka <vbabka@...e.cz>
To:	Joonsoo Kim <iamjoonsoo.kim@....com>
Cc:	Andrew Morton <akpm@...ux-foundation.org>,
	linux-kernel@...r.kernel.org, linux-mm@...ck.org,
	Michal Hocko <mhocko@...nel.org>,
	Mel Gorman <mgorman@...hsingularity.net>,
	David Rientjes <rientjes@...gle.com>,
	Rik van Riel <riel@...hat.com>
Subject: Re: [PATCH v3 13/17] mm, compaction: use correct watermark when
 checking allocation success

On 07/06/2016 07:47 AM, Joonsoo Kim wrote:
> On Fri, Jun 24, 2016 at 11:54:33AM +0200, Vlastimil Babka wrote:
>> The __compact_finished() function uses low watermark in a check that has to
>> pass if the direct compaction is to finish and allocation should succeed. This
>> is too pessimistic, as the allocation will typically use min watermark. It may
>> happen that during compaction, we drop below the low watermark (due to parallel
>> activity), but still form the target high-order page. By checking against low
>> watermark, we might needlessly continue compaction.
>>
>> Similarly, __compaction_suitable() uses low watermark in a check whether
>> allocation can succeed without compaction. Again, this is unnecessarily
>> pessimistic.
>>
>> After this patch, these check will use direct compactor's alloc_flags to
>> determine the watermark, which is effectively the min watermark.
>>
>> Signed-off-by: Vlastimil Babka <vbabka@...e.cz>
>> Acked-by: Michal Hocko <mhocko@...e.com>
>> ---
>>  mm/compaction.c | 6 +++---
>>  1 file changed, 3 insertions(+), 3 deletions(-)
>>
>> diff --git a/mm/compaction.c b/mm/compaction.c
>> index 76897850c3c2..371760a85085 100644
>> --- a/mm/compaction.c
>> +++ b/mm/compaction.c
>> @@ -1320,7 +1320,7 @@ static enum compact_result __compact_finished(struct zone *zone, struct compact_
>>  		return COMPACT_CONTINUE;
>>
>>  	/* Compaction run is not finished if the watermark is not met */
>> -	watermark = low_wmark_pages(zone);
>> +	watermark = zone->watermark[cc->alloc_flags & ALLOC_WMARK_MASK];
>
> finish condition is changed. We have two more watermark checks in
> try_to_compact_pages() and kcompactd_do_work() and they should be
> changed too.

Ugh, I've completely missed them. Thanks for catching this, hopefully 
fixing that will improve the results.