[Acked]Re: [PATCH 1/2] mm: vmscan: correctly check if reclaimer should schedule during shrink_slab

[Brad Figg]

On 06/17/2011 11:06 AM, Colin King wrote:
> From: Minchan Kim<minchan.kim at gmail.com>
>
> 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() remains in
> case one shrinker is particularly slow.
>
> [mgorman at suse.de: preserve call to cond_resched after each call into shrinker]
> Signed-off-by: Mel Gorman<mgorman at suse.de>
> Signed-off-by: Minchan Kim<minchan.kim at gmail.com>
> Cc: Rik van Riel<riel at redhat.com>
> Cc: Johannes Weiner<hannes at cmpxchg.org>
> Cc: Wu Fengguang<fengguang.wu at intel.com>
> Cc: James Bottomley<James.Bottomley at HansenPartnership.com>
> Tested-by: Colin King<colin.king at canonical.com>
> Cc: Raghavendra D Prabhu<raghu.prabhu13 at gmail.com>
> Cc: Jan Kara<jack at suse.cz>
> Cc: Chris Mason<chris.mason at oracle.com>
> Cc: Christoph Lameter<cl at linux.com>
> Cc: Pekka Enberg<penberg at kernel.org>
> Cc: Rik van Riel<riel at redhat.com>
> Cc:<stable at kernel.org>		[2.6.38+]
> Signed-off-by: Andrew Morton<akpm at linux-foundation.org>
> Signed-off-by: Linus Torvalds<torvalds at linux-foundation.org>
> ---
>   mm/vmscan.c |    9 +++++++--
>   1 files changed, 7 insertions(+), 2 deletions(-)
>
> diff --git a/mm/vmscan.c b/mm/vmscan.c
> index 0665520..648aab8 100644
> --- a/mm/vmscan.c
> +++ b/mm/vmscan.c
> @@ -230,8 +230,11 @@ unsigned long shrink_slab(unsigned long scanned, gfp_t gfp_mask,
>   	if (scanned == 0)
>   		scanned = SWAP_CLUSTER_MAX;
>
> -	if (!down_read_trylock(&shrinker_rwsem))
> -		return 1;	/* Assume we'll be able to shrink next time */
> +	if (!down_read_trylock(&shrinker_rwsem)) {
> +		/* Assume we'll be able to shrink next time */
> +		ret = 1;
> +		goto out;
> +	}
>
>   	list_for_each_entry(shrinker,&shrinker_list, list) {
>   		unsigned long long delta;
> @@ -282,6 +285,8 @@ unsigned long shrink_slab(unsigned long scanned, gfp_t gfp_mask,
>   		shrinker->nr += total_scan;
>   	}
>   	up_read(&shrinker_rwsem);
> +out:
> +	cond_resched();
>   	return ret;
>   }
>

Acked-by: Brad Figg <brad.figg at canonical.com>


-- 
Brad Figg brad.figg at canonical.com http://www.canonical.com