linux-kernel - Re: [RFC] mm: show deferred_compaction state in page alloc fail

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Date:	Thu, 25 Sep 2014 10:26:11 +0900
From:	Minchan Kim <minchan@...nel.org>
To:	Vlastimil Babka <vbabka@...e.cz>
Cc:	Andrew Morton <akpm@...ux-foundation.org>,
	linux-kernel@...r.kernel.org, linux-mm@...ck.org,
	Mel Gorman <mgorman@...e.de>,
	David Rientjes <rientjes@...gle.com>,
	Joonsoo Kim <iamjoonsoo.kim@....com>,
	Rik van Riel <riel@...hat.com>
Subject: Re: [RFC] mm: show deferred_compaction state in page alloc fail

On Wed, Sep 24, 2014 at 05:22:32PM +0200, Vlastimil Babka wrote:
> On 08/26/2014 09:30 AM, Minchan Kim wrote:
> >Recently, I saw several reports that high order allocation failed
> >although there were many freeable pages but it's hard to reproduce
> >so asking them to reproduce the problem several time is really painful.
> >
> >A culprit I doubt is compaction deferring logic which prevent
> >compaction for a while so high order allocation could be fail.
> 
> Could be that, but also the non-determinism of watermark checking,
> where compaction thinks allocation should succeed, but in the end it
> won't.
> 
> >It would be more clear if we can see the stat which can show
> >current zone's compaction deferred state when allocatil fail.
> >
> >It's a RFC and never test it. I just get an idea with
> >handling another strange high order allocation fail.
> >Any comments are welcome.
> 
> It's quite large patch. Maybe it could be much simpler if you did
> not print just true/false but:
> 
> 1) true/false based on zone->compact_considered < defer_limit, ignoring
>    zone->compact_order_failed
> 
> 2) zone->compact_order_failed value itself
> 
> Then you wouldn't need to pass the allocation order around like you do.
> The "allocation failed" message tells you the order which was
> attempted, and then it's easy for the user to compare with the
> reported
> zone->compact_order_failed and decide if the defer status actually
> applies or not.

Actually, I thought about it. The reason I avoid that approach
is I don't want to expose deferring logic internal but now that
I think about it, it was wrong conclusion because show_free_area
already have been exported lots of internal. IOW, without it,
there is not much to investigate the reason.

I will send it.
 
Thanks for the review, Vlastimil.
> 
> 
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-- 
Kind regards,
Minchan Kim
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