linux-kernel - Re: [PATCH v2] mm: terminate shrink

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Date:   Fri, 8 Dec 2017 13:02:20 -0800 (PST)
From:   David Rientjes <rientjes@...gle.com>
To:     Suren Baghdasaryan <surenb@...gle.com>
cc:     akpm@...ux-foundation.org, mhocko@...e.com, hannes@...xchg.org,
        hillf.zj@...baba-inc.com, minchan@...nel.org,
        mgorman@...hsingularity.net, ying.huang@...el.com,
        linux-mm@...ck.org, linux-kernel@...r.kernel.org,
        timmurray@...gle.com, tkjos@...gle.com
Subject: Re: [PATCH v2] mm: terminate shrink_slab loop if signal is pending

On Thu, 7 Dec 2017, Suren Baghdasaryan wrote:

> Slab shrinkers can be quite time consuming and when signal
> is pending they can delay handling of the signal. If fatal
> signal is pending there is no point in shrinking that process
> since it will be killed anyway. This change checks for pending
> fatal signals inside shrink_slab loop and if one is detected
> terminates this loop early.
> 

I've proposed a similar patch in the past, but for a check on TIF_MEMDIE, 
which would today be a tsk_is_oom_victim(current), since we had observed 
lengthy stalls in reclaim that would have been prevented if the oom victim 
had exited out, returned back to the page allocator, allocated with 
ALLOC_NO_WATERMARKS, and proceeded to quickly exit.

I'm not sure that all fatal_signal_pending() tasks should get the same 
treatment, but I understand the point that the task is killed and should 
free memory when it fully exits.  How much memory is unknown.

 > Signed-off-by: Suren Baghdasaryan <surenb@...gle.com>
> 
> ---
> V2:
> Sergey Senozhatsky:
>   - Fix missing parentheses
> ---
>  mm/vmscan.c | 7 +++++++
>  1 file changed, 7 insertions(+)
> 
> diff --git a/mm/vmscan.c b/mm/vmscan.c
> index c02c850ea349..28e4bdc72c16 100644
> --- a/mm/vmscan.c
> +++ b/mm/vmscan.c
> @@ -486,6 +486,13 @@ static unsigned long shrink_slab(gfp_t gfp_mask, int nid,
>  			.memcg = memcg,
>  		};
>  
> +		/*
> +		 * We are about to die and free our memory.
> +		 * Stop shrinking which might delay signal handling.
> +		 */
> +		if (unlikely(fatal_signal_pending(current)))
> +			break;
> +
>  		/*
>  		 * If kernel memory accounting is disabled, we ignore
>  		 * SHRINKER_MEMCG_AWARE flag and call all shrinkers