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

[Herton Ronaldo Krzesinski]

On Fri, Jun 17, 2011 at 11:06:52AM +0100, 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: Herton Ronaldo Krzesinski <herton.krzesinski at canonical.com>

>  
> -- 
> 1.7.0.4
> 
> 
> -- 
> kernel-team mailing list
> kernel-team at lists.ubuntu.com
> https://lists.ubuntu.com/mailman/listinfo/kernel-team
> 

-- 
[]'s
Herton