Re: [PATCH 2/4] mm: vmscan: Do not apply pressure to slab if we are not applying pressure to zone

[KOSAKI Motohiro <kosaki.motohiro@jp.fujitsu.com>]

(2011/06/24 23:44), Mel Gorman wrote:
> During allocator-intensive workloads, kswapd will be woken frequently
> causing free memory to oscillate between the high and min watermark.
> This is expected behaviour.
> 
> When kswapd applies pressure to zones during node balancing, it checks
> if the zone is above a high+balance_gap threshold. If it is, it does
> not apply pressure but it unconditionally shrinks slab on a global
> basis which is excessive. In the event kswapd is being kept awake due to
> a high small unreclaimable zone, it skips zone shrinking but still
> calls shrink_slab().
> 
> Once pressure has been applied, the check for zone being unreclaimable
> is being made before the check is made if all_unreclaimable should be
> set. This miss of unreclaimable can cause has_under_min_watermark_zone
> to be set due to an unreclaimable zone preventing kswapd backing off
> on congestion_wait().
> 
> Reported-and-tested-by: PA!draig Brady <P@draigBrady.com>
> Signed-off-by: Mel Gorman <mgorman@suse.de>
> ---
>  mm/vmscan.c |   23 +++++++++++++----------
>  1 files changed, 13 insertions(+), 10 deletions(-)
> 
> diff --git a/mm/vmscan.c b/mm/vmscan.c
> index 841e3bf..9cebed1 100644
> --- a/mm/vmscan.c
> +++ b/mm/vmscan.c
> @@ -2507,18 +2507,18 @@ loop_again:
>  				KSWAPD_ZONE_BALANCE_GAP_RATIO);
>  			if (!zone_watermark_ok_safe(zone, order,
>  					high_wmark_pages(zone) + balance_gap,
> -					end_zone, 0))
> +					end_zone, 0)) {
>  				shrink_zone(priority, zone, &sc);
> -			reclaim_state->reclaimed_slab = 0;
> -			nr_slab = shrink_slab(&shrink, sc.nr_scanned, lru_pages);
> -			sc.nr_reclaimed += reclaim_state->reclaimed_slab;
> -			total_scanned += sc.nr_scanned;
>  
> -			if (zone->all_unreclaimable)
> -				continue;
> -			if (nr_slab == 0 &&
> -			    !zone_reclaimable(zone))
> -				zone->all_unreclaimable = 1;
> +				reclaim_state->reclaimed_slab = 0;
> +				nr_slab = shrink_slab(&shrink, sc.nr_scanned, lru_pages);
> +				sc.nr_reclaimed += reclaim_state->reclaimed_slab;
> +				total_scanned += sc.nr_scanned;
> +
> +				if (nr_slab == 0 && !zone_reclaimable(zone))
> +					zone->all_unreclaimable = 1;
> +			}
> +
>  			/*
>  			 * If we've done a decent amount of scanning and
>  			 * the reclaim ratio is low, start doing writepage
> @@ -2528,6 +2528,9 @@ loop_again:
>  			    total_scanned > sc.nr_reclaimed + sc.nr_reclaimed / 2)
>  				sc.may_writepage = 1;
>  
> +			if (zone->all_unreclaimable)
> +				continue;
> +
>  			if (!zone_watermark_ok_safe(zone, order,
>  					high_wmark_pages(zone), end_zone, 0)) {
>  				all_zones_ok = 0;

Reviewed-by: KOSAKI Motohiro <kosaki.motohiro@jp.fujitsu.com>


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