Date: Wed, 25 Nov 2015 14:12:33 -0800
From: Andrew Morton <akpm@linux-foundation.org>
To: Michal Hocko <mhocko@kernel.org>
Cc: Tetsuo Handa <penguin-kernel@I-love.SAKURA.ne.jp>,
        Tejun Heo <tj@kernel.org>, Cristopher Lameter <clameter@sgi.com>,
        Arkadiusz =?UTF-8?Q?Mi=C5=9Bkiewicz?= <arekm@maven.pl>,
        linux-mm@kvack.org, LKML <linux-kernel@vger.kernel.org>,
        Joonsoo Kim <js1304@gmail.com>, Christoph Lameter <cl@linux.com>
Subject: Re: [PATCH] mm, vmstat: Allow WQ concurrency to discover memory
 reclaim doesn't make any progress
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On Wed, 25 Nov 2015 12:07:05 +0100 Michal Hocko <mhocko@kernel.org> wrote:

> On Tue 24-11-15 15:44:48, Andrew Morton wrote:
> [...]
> > > Even though we haven't seen bug reports in the past I would suggest
> > > backporting this to the stable trees. The issue is present since we have
> > > stopped useing congestion_wait in the retry loop because WQ concurrency
> > > is older as well as vmstat worqueue based refresh AFAICS.
> >
> > hm, I'm reluctant.  If the patch fixes something that real people are
> > really hurting from then yes.  But I suspect this is just one fly-swat
> > amongst many.
> 
> Arkadiusz was seeing reclaim issues [1] on 4.1 kernel. I didn't have
> time to look deeper in that report but vmstat counters seemed terribly
> outdated and the issue went away when this patch was used. The thing is
> that there were others in the bundle so it is not 100% clear whether the
> patch alone helped or it was just a part of the puzzle.
> 
> Anyway I think that the issue is not solely theoretical. WQ_MEM_RECLAIM
> is simply not working if the allocation path doesn't sleep currently and
> my understanding of what Tejun claims [2] is that that reimplementing WQ
> concurrency would be too intrusive and lacks sufficient justification
> because other kernel paths do sleep. This patch tries to reduce the
> sleep only to worker threads which should not cause any problems to
> regular tasks.
> 
> I am open to any other suggestions. I do not like artificial sleep as
> well but this sounds like the most practical way to go now.
> 
> [1] http://lkml.kernel.org/r/201511102313.36685.arekm@maven.pl
> [2] http://lkml.kernel.org/r/20151106001648.GA18183@mtj.duckdns.org

hmpf, OK, I stuck a cc:stable in there.  It looks like the current
changelog is sufficient to explain to Greg (and others) why we think
backporting is needed.

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