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

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Message-ID: <20151023083612.GC2410@dhcp22.suse.cz>
Date:	Fri, 23 Oct 2015 10:36:12 +0200
From:	Michal Hocko <mhocko@...nel.org>
To:	Tetsuo Handa <penguin-kernel@...ove.SAKURA.ne.jp>
Cc:	htejun@...il.com, cl@...ux.com, 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

On Fri 23-10-15 06:42:43, Tetsuo Handa wrote:
> 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.
> 
> Then, isn't below change easier to backport which will also alleviate
> needlessly burning CPU cycles?

This is quite obscure. If the vmstat_update fix needs workqueue tweaks
as well then I would vote for your original patch which is clear,
straightforward and easy to backport.

If WQ_MEM_RECLAIM can really guarantee one worker as described in the
documentation then I agree that fixing vmstat is a better fix. But that
doesn't seem to be the case currently.
 
> --- a/mm/page_alloc.c
> +++ b/mm/page_alloc.c
> @@ -3385,6 +3385,7 @@ retry:
>  	((gfp_mask & __GFP_REPEAT) && pages_reclaimed < (1 << order))) {
>  		/* Wait for some write requests to complete then retry */
>  		wait_iff_congested(ac->preferred_zone, BLK_RW_ASYNC, HZ/50);
> +		schedule_timeout_uninterruptible(1);
>  		goto retry;
>  	}
>  
> --
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-- 
Michal Hocko
SUSE Labs
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