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Date:   Fri, 24 Nov 2017 08:50:06 +0100
From:   Michal Hocko <mhocko@...nel.org>
To:     Minchan Kim <minchan@...nel.org>
Cc:     Andrew Morton <akpm@...ux-foundation.org>, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org, kernel-team <kernel-team@....com>,
        Tetsuo Handa <penguin-kernel@...ove.SAKURA.ne.jp>,
        Shakeel Butt <shakeelb@...gle.com>,
        Johannes Weiner <hannes@...xchg.org>
Subject: Re: [PATCH] mm: Do not stall register_shrinker

On Fri 24-11-17 09:04:59, Minchan Kim wrote:
> Shakeel Butt reported, he have observed in production system that
> the job loader gets stuck for 10s of seconds while doing mount
> operation. It turns out that it was stuck in register_shrinker()
> and some unrelated job was under memory pressure and spending time
> in shrink_slab(). Machines have a lot of shrinkers registered and
> jobs under memory pressure has to traverse all of those memcg-aware
> shrinkers and do affect unrelated jobs which want to register their
> own shrinkers.
> 
> To solve the issue, this patch simply bails out slab shrinking
> once it found someone want to register shrinker in parallel.
> A downside is it could cause unfair shrinking between shrinkers.
> However, it should be rare and we can add compilcated logic once
> we found it's not enough.
> 
> Link: http://lkml.kernel.org/r/20171115005602.GB23810@bbox
> Cc: Michal Hocko <mhocko@...e.com>
> Cc: Tetsuo Handa <penguin-kernel@...ove.SAKURA.ne.jp>
> Acked-by: Johannes Weiner <hannes@...xchg.org>
> Reported-and-tested-by: Shakeel Butt <shakeelb@...gle.com>
> Signed-off-by: Shakeel Butt <shakeelb@...gle.com>
> Signed-off-by: Minchan Kim <minchan@...nel.org>

Acked-by: Michal Hocko <mhocko@...e.com>

> ---
>  mm/vmscan.c | 8 ++++++++
>  1 file changed, 8 insertions(+)
> 
> diff --git a/mm/vmscan.c b/mm/vmscan.c
> index 6a5a72baccd5..6698001787bd 100644
> --- a/mm/vmscan.c
> +++ b/mm/vmscan.c
> @@ -486,6 +486,14 @@ static unsigned long shrink_slab(gfp_t gfp_mask, int nid,
>  			sc.nid = 0;
>  
>  		freed += do_shrink_slab(&sc, shrinker, priority);
> +		/*
> +		 * bail out if someone want to register a new shrinker to
> +		 * prevent long time stall by parallel ongoing shrinking.
> +		 */
> +		if (rwsem_is_contended(&shrinker_rwsem)) {
> +			freed = freed ? : 1;
> +			break;
> +		}
>  	}
>  
>  	up_read(&shrinker_rwsem);
> -- 
> 2.7.4
> 

-- 
Michal Hocko
SUSE Labs

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