linux-kernel - Re: [PATCH 2/5] mm/compaction: do not call suitable_migration

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Date:	Mon, 10 Feb 2014 09:41:22 +0900
From:	Joonsoo Kim <iamjoonsoo.kim@....com>
To:	Vlastimil Babka <vbabka@...e.cz>
Cc:	Andrew Morton <akpm@...ux-foundation.org>,
	Mel Gorman <mgorman@...e.de>, Rik van Riel <riel@...hat.com>,
	linux-mm@...ck.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 2/5] mm/compaction: do not call
 suitable_migration_target() on every page

On Fri, Feb 07, 2014 at 10:36:13AM +0100, Vlastimil Babka wrote:
> On 02/07/2014 06:08 AM, Joonsoo Kim wrote:
> > suitable_migration_target() checks that pageblock is suitable for
> > migration target. In isolate_freepages_block(), it is called on every
> > page and this is inefficient. So make it called once per pageblock.
> 
> Hmm but in sync compaction, compact_checklock_irqsave() may drop the zone->lock,
> reschedule and reacquire it and thus possibly invalidate your previous check. Async
> compaction is ok as that will quit immediately. So you could probably communicate that
> this happened and invalidate checked_pageblock in such case. Or maybe this would not
> happen too enough to worry about rare suboptimal migrations?

So, the result of previous check can be changed only if *this* pageblock's migratetype
is changed while we drop the lock. I guess that this is really rare event, and,
in this case, this pageblock already has mixed migratetype pages, so it has
no serious problem.

Thanks.
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