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

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Message-ID: <20151023083316.GB2410@dhcp22.suse.cz>
Date:	Fri, 23 Oct 2015 10:33:16 +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 03:42:26, Tejun Heo wrote:
> On Thu, Oct 22, 2015 at 05:49:22PM +0200, Michal Hocko wrote:
> > I am confused. What makes rescuer to not run? Nothing seems to be
> > hogging CPUs, we are just out of workers which are loopin in the
> > allocator but that is preemptible context.
> 
> It's concurrency management.  Workqueue thinks that the pool is making
> positive forward progress and doesn't schedule anything else for
> execution while that work item is burning cpu cycles.

Ohh, OK I can see wq_worker_sleeping now. I've missed your point in
other email, sorry about that. But now I am wondering whether this
is an intended behavior. The documentation says:
  WQ_MEM_RECLAIM

        All wq which might be used in the memory reclaim paths _MUST_
        have this flag set.  The wq is guaranteed to have at least one
        execution context regardless of memory pressure.

Which doesn't seem to be true currently, right? Now I can see your patch
to introduce WQ_IMMEDIATE but I am wondering which WQ_MEM_RECLAIM users
could do without WQ_IMMEDIATE? I mean all current workers might be
looping in the page allocator and it seems possible that WQ_MEM_RECLAIM
work items might be waiting behind them so they cannot help to relieve
the memory pressure. This doesn't sound right to me. Or I am completely
confused and still fail to understand what is WQ_MEM_RECLAIM supposed to
be used for.
-- 
Michal Hocko
SUSE Labs
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