linux-kernel - Re: [PATCH] mm,vmscan: Use accurate values for zone

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Message-Id: <201510220039.BIF64554.OMOVFLJFFOSQHt@I-love.SAKURA.ne.jp>
Date:	Thu, 22 Oct 2015 00:39:02 +0900
From:	Tetsuo Handa <penguin-kernel@...ove.SAKURA.ne.jp>
To:	mhocko@...nel.org, cl@...ux.com
Cc:	linux-mm@...ck.org, linux-kernel@...r.kernel.org,
	torvalds@...ux-foundation.org, rientjes@...gle.com,
	oleg@...hat.com, kwalker@...hat.com, akpm@...ux-foundation.org,
	hannes@...xchg.org, vdavydov@...allels.com, skozina@...hat.com,
	mgorman@...e.de, riel@...hat.com
Subject: Re: [PATCH] mm,vmscan: Use accurate values for zone_reclaimable() checks

Michal Hocko wrote:
> On Wed 21-10-15 09:49:07, Christoph Lameter wrote:
> > On Wed, 21 Oct 2015, Michal Hocko wrote:
> > 
> > > Because all the WQ workers are stuck somewhere, maybe in the memory
> > > allocation which cannot make any progress and the vmstat update work is
> > > queued behind them.

After invoking the OOM killer, we can easily observe that vmstat_update
cannot be processed due to memory allocation by disk_events_workfn stalls.
http://lkml.kernel.org/r/201509120019.BJI48986.OOSVMJtOLFQHFF@I-love.SAKURA.ne.jp

I worried that blocking forever from workqueue is an exclusive occupation of
workqueue. In fact, changing to GFP_ATOMIC avoids this problem.
http://lkml.kernel.org/r/201503012017.EAD00571.HOOJVOStMFLFQF@I-love.SAKURA.ne.jp

Now we realized that we are hitting this problem before invoking the OOM
killer. The situation is similar to the case after the OOM killer is
invoked; there are no reclaimable pages but vmstat_update cannot be
processed. We are caught by a small difference of vmstat counter values.

> > >
> > > At least this is my current understanding.
> > 
> > Eww. Maybe need a queue that does not do such evil things as memory
> > allocation?
> 
> I am not sure how to achieve that. Requiring non-sleeping worker would
> work out but do we have enough users to add such an API?

If a queue does not need to sleep, can't that queue be processed from
timer context (e.g. mod_timer()) ?
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