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

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Message-ID: <20151022153559.GF26854@dhcp22.suse.cz>
Date:	Thu, 22 Oct 2015 17:35:59 +0200
From:	Michal Hocko <mhocko@...nel.org>
To:	Tejun Heo <htejun@...il.com>
Cc:	Christoph Lameter <cl@...ux.com>,
	Tetsuo Handa <penguin-kernel@...ove.SAKURA.ne.jp>,
	linux-mm@...ck.org, linux-kernel@...r.kernel.org,
	torvalds@...ux-foundation.org,
	David Rientjes <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

On Fri 23-10-15 00:15:28, Tejun Heo wrote:
> On Thu, Oct 22, 2015 at 05:06:23PM +0200, Michal Hocko wrote:
> > Do I get it right that if vmstat_update has its own workqueue with
> > WQ_MEM_RECLAIM then there is a _guarantee_ that the rescuer will always
> > be able to process vmstat_update work from the requested CPU?
> 
> Yeah.

Thanks for the confirmation.

> > That should be sufficient because vmstat_update doesn't sleep on
> > allocation. I agree that this would be a more appropriate fix.
> 
> The problem seems to be reclaim path busy looping waiting for
> vmstat_update and workqueue thinking that the work item must be making
> forward-progress and thus not starting the next work item.

But that shouldn't happen because the allocation path does cond_resched
even when nothing is really reclaimable (e.g. wait_iff_congested from
__alloc_pages_slowpath).
-- 
Michal Hocko
SUSE Labs
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