linux-kernel - Re: [PATCH] mm: vmscan: Correctly check if reclaimer should schedule during shrink

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Date:	Thu, 19 May 2011 09:03:44 +0900
From:	Minchan Kim <minchan.kim@...il.com>
To:	Mel Gorman <mgorman@...e.de>
Cc:	akpm@...ux-foundation.org,
	Colin Ian King <colin.king@...onical.com>,
	James Bottomley <James.Bottomley@...senpartnership.com>,
	KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com>,
	raghu.prabhu13@...il.com, jack@...e.cz, chris.mason@...cle.com,
	cl@...ux.com, penberg@...nel.org, riel@...hat.com,
	hannes@...xchg.org, linux-fsdevel@...r.kernel.org,
	linux-mm@...ck.org, linux-kernel@...r.kernel.org,
	linux-ext4@...r.kernel.org
Subject: Re: [PATCH] mm: vmscan: Correctly check if reclaimer should schedule
 during shrink_slab

On Wed, May 18, 2011 at 1:15 AM, Mel Gorman <mgorman@...e.de> wrote:
> It has been reported on some laptops that kswapd is consuming large
> amounts of CPU and not being scheduled when SLUB is enabled during
> large amounts of file copying. It is expected that this is due to
> kswapd missing every cond_resched() point because;
>
> shrink_page_list() calls cond_resched() if inactive pages were isolated
>        which in turn may not happen if all_unreclaimable is set in
>        shrink_zones(). If for whatver reason, all_unreclaimable is
>        set on all zones, we can miss calling cond_resched().
>
> balance_pgdat() only calls cond_resched if the zones are not
>        balanced. For a high-order allocation that is balanced, it
>        checks order-0 again. During that window, order-0 might have
>        become unbalanced so it loops again for order-0 and returns
>        that it was reclaiming for order-0 to kswapd(). It can then
>        find that a caller has rewoken kswapd for a high-order and
>        re-enters balance_pgdat() without ever calling cond_resched().
>
> shrink_slab only calls cond_resched() if we are reclaiming slab
>        pages. If there are a large number of direct reclaimers, the
>        shrinker_rwsem can be contended and prevent kswapd calling
>        cond_resched().
>
> This patch modifies the shrink_slab() case. If the semaphore is
> contended, the caller will still check cond_resched(). After each
> successful call into a shrinker, the check for cond_resched() is
> still necessary in case one shrinker call is particularly slow.
>
> This patch replaces
> mm-vmscan-if-kswapd-has-been-running-too-long-allow-it-to-sleep.patch
> in -mm.
>
> [mgorman@...e.de: Preserve call to cond_resched after each call into shrinker]
> From: Minchan Kim <minchan.kim@...il.com>
Signed-off-by: Minchan Kim <minchan.kim@...il.com>
> Signed-off-by: Mel Gorman <mgorman@...e.de>

-- 
Kind regards,
Minchan Kim
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