linux-kernel - Re: [patch v3 -mm 1/3] memcg: integrate soft reclaim tighter with zone shrinking code

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Date:	Wed, 29 May 2013 17:57:56 +0200
From:	Michal Hocko <mhocko@...e.cz>
To:	Johannes Weiner <hannes@...xchg.org>
Cc:	Andrew Morton <akpm@...ux-foundation.org>,
	KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>,
	Ying Han <yinghan@...gle.com>, Hugh Dickins <hughd@...gle.com>,
	Glauber Costa <glommer@...allels.com>,
	Michel Lespinasse <walken@...gle.com>,
	Greg Thelen <gthelen@...gle.com>, Tejun Heo <tj@...nel.org>,
	Balbir Singh <bsingharora@...il.com>, cgroups@...r.kernel.org,
	linux-kernel@...r.kernel.org, linux-mm@...ck.org
Subject: Re: [patch v3 -mm 1/3] memcg: integrate soft reclaim tighter with
 zone shrinking code

On Wed 29-05-13 15:05:38, Michal Hocko wrote:
> On Mon 27-05-13 19:13:08, Michal Hocko wrote:
> [...]
> > Nevertheless I have encountered an issue while testing the huge number
> > of groups scenario. And the issue is not limitted to only to this
> > scenario unfortunately. As memcg iterators use per node-zone-priority
> > cache to prevent from over reclaim it might quite easily happen that
> > the walk will not visit all groups and will terminate the loop either
> > prematurely or skip some groups. An example could be the direct reclaim
> > racing with kswapd. This might cause that the loop misses over limit
> > groups so no pages are scanned and so we will fall back to all groups
> > reclaim.
> 
> And after some more testing and head scratching it turned out that
> fallbacks to pass#2 I was seeing are caused by something else. It is
> not race between iterators but rather reclaiming from zone DMA which
> has troubles to scan anything despite there are pages on LRU and so we
> fall back. I have to look into that more but what-ever the issue is it
> shouldn't be related to the patch series.

Think I know what is going on. get_scan_count sees relatively small
amount of pages in the lists (around 2k). This means that get_scan_count
will tell us to scan nothing for DEF_PRIORITY (as the DMA32 is usually
~16M) then the DEF_PRIORITY is basically no-op and we have to wait and
fall down to a priority which actually let us scan something.

Hmm, maybe ignoring soft reclaim for DMA zone would help to reduce
one pointless loop over groups.
-- 
Michal Hocko
SUSE Labs
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