linux-kernel - Re: [PATCH 3/3] compaction accouting fix

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Message-ID: <20110831145651.GA2198@barrios-desktop>
Date:	Wed, 31 Aug 2011 23:56:51 +0900
From:	Minchan Kim <minchan.kim@...il.com>
To:	Johannes Weiner <jweiner@...hat.com>
Cc:	Andrew Morton <akpm@...ux-foundation.org>,
	linux-mm <linux-mm@...ck.org>,
	LKML <linux-kernel@...r.kernel.org>,
	Mel Gorman <mgorman@...e.de>, Rik van Riel <riel@...hat.com>
Subject: Re: [PATCH 3/3] compaction accouting fix

On Wed, Aug 31, 2011 at 01:37:10PM +0200, Johannes Weiner wrote:
> On Sun, Nov 13, 2011 at 01:37:43AM +0900, Minchan Kim wrote:
> > I saw the following accouting of compaction during test of the series.
> > 
> > compact_blocks_moved 251
> > compact_pages_moved 44
> > 
> > It's very awkward to me although it's possbile because it means we try to compact 251 blocks
> > but it just migrated 44 pages. As further investigation, I found isolate_migratepages doesn't
> > isolate any pages but it returns ISOLATE_SUCCESS and then, it just increases compact_blocks_moved
> > but doesn't increased compact_pages_moved.
> > 
> > This patch makes accouting of compaction works only in case of success of isolation.
> > 
> > CC: Mel Gorman <mgorman@...e.de>
> > CC: Johannes Weiner <jweiner@...hat.com>
> > CC: Rik van Riel <riel@...hat.com>
> > Signed-off-by: Minchan Kim <minchan.kim@...il.com>
> 
> Acked-by: Johannes Weiner <jweiner@...hat.com>

Thanks, Hannes.

> 
> It's a teensy-bit awkward that isolate_migratepages() can return
> success without actually isolating any new pages, just because there
> are still some pages left from a previous run (cc->nr_migratepages is
> maintained across isolation calls).

If migrate_pages fails, we reset cc->nr_migratepages to zero in compact_zone.
Am I missing something?

> 
> Maybe isolate_migratepages() should just return an error if compaction
> should really be aborted and 0 otherwise, and have compact_zone()
> always check for cc->nr_migratepages itself?
> 
> 	if (isolate_migratepages(zone, cc) < 0) {
> 		ret = COMPACT_PARTIAL;
> 		goto out;
> 	}
> 
> 	if (!cc->nr_migratepages)
> 		continue;
> 
> 	...
> 
> Just a nit-pick, though.  If you don't agree, just leave it as is.

-- 
Kind regards,
Minchan Kim
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