Re: [PATCH 08/10] mm: vmscan: Have kswapd shrink slab only once per priority

[Michal Hocko <mhocko@suse.cz>]

On Sun 17-03-13 13:04:14, Mel Gorman wrote:
> If kswaps fails to make progress but continues to shrink slab then it'll
> either discard all of slab or consume CPU uselessly scanning shrinkers.
> This patch causes kswapd to only call the shrinkers once per priority.
> 
> Signed-off-by: Mel Gorman <mgorman@suse.de>

OK, looks good.
Reviewed-by: Michal Hocko <mhocko@suse.cz>

> ---
>  mm/vmscan.c | 28 +++++++++++++++++++++-------
>  1 file changed, 21 insertions(+), 7 deletions(-)
> 
> diff --git a/mm/vmscan.c b/mm/vmscan.c
> index 7d5a932..84375b2 100644
> --- a/mm/vmscan.c
> +++ b/mm/vmscan.c
> @@ -2661,9 +2661,10 @@ static bool prepare_kswapd_sleep(pg_data_t *pgdat, int order, long remaining,
>   */
>  static bool kswapd_shrink_zone(struct zone *zone,
>  			       struct scan_control *sc,
> -			       unsigned long lru_pages)
> +			       unsigned long lru_pages,
> +			       bool shrinking_slab)
>  {
> -	unsigned long nr_slab;
> +	unsigned long nr_slab = 0;
>  	struct reclaim_state *reclaim_state = current->reclaim_state;
>  	struct shrink_control shrink = {
>  		.gfp_mask = sc->gfp_mask,
> @@ -2673,9 +2674,15 @@ static bool kswapd_shrink_zone(struct zone *zone,
>  	sc->nr_to_reclaim = max(SWAP_CLUSTER_MAX, high_wmark_pages(zone));
>  	shrink_zone(zone, sc);
>  
> -	reclaim_state->reclaimed_slab = 0;
> -	nr_slab = shrink_slab(&shrink, sc->nr_scanned, lru_pages);
> -	sc->nr_reclaimed += reclaim_state->reclaimed_slab;
> +	/*
> +	 * Slabs are shrunk for each zone once per priority or if the zone
> +	 * being balanced is otherwise unreclaimable
> +	 */
> +	if (shrinking_slab || !zone_reclaimable(zone)) {
> +		reclaim_state->reclaimed_slab = 0;
> +		nr_slab = shrink_slab(&shrink, sc->nr_scanned, lru_pages);
> +		sc->nr_reclaimed += reclaim_state->reclaimed_slab;
> +	}
>  
>  	if (nr_slab == 0 && !zone_reclaimable(zone))
>  		zone->all_unreclaimable = 1;
> @@ -2713,6 +2720,7 @@ static unsigned long balance_pgdat(pg_data_t *pgdat, int order,
>  	int end_zone = 0;	/* Inclusive.  0 = ZONE_DMA */
>  	unsigned long nr_soft_reclaimed;
>  	unsigned long nr_soft_scanned;
> +	bool shrinking_slab = true;
>  	struct scan_control sc = {
>  		.gfp_mask = GFP_KERNEL,
>  		.priority = DEF_PRIORITY,
> @@ -2861,7 +2869,8 @@ static unsigned long balance_pgdat(pg_data_t *pgdat, int order,
>  				 * already being scanned that that high
>  				 * watermark would be met at 100% efficiency.
>  				 */
> -				if (kswapd_shrink_zone(zone, &sc, lru_pages))
> +				if (kswapd_shrink_zone(zone, &sc,
> +						lru_pages, shrinking_slab))
>  					raise_priority = false;
>  
>  				nr_to_reclaim += sc.nr_to_reclaim;
> @@ -2900,6 +2909,9 @@ static unsigned long balance_pgdat(pg_data_t *pgdat, int order,
>  				pfmemalloc_watermark_ok(pgdat))
>  			wake_up(&pgdat->pfmemalloc_wait);
>  
> +		/* Only shrink slab once per priority */
> +		shrinking_slab = false;
> +
>  		/*
>  		 * Fragmentation may mean that the system cannot be rebalanced
>  		 * for high-order allocations in all zones. If twice the
> @@ -2925,8 +2937,10 @@ static unsigned long balance_pgdat(pg_data_t *pgdat, int order,
>  		 * Raise priority if scanning rate is too low or there was no
>  		 * progress in reclaiming pages
>  		 */
> -		if (raise_priority || !this_reclaimed)
> +		if (raise_priority || !this_reclaimed) {
>  			sc.priority--;
> +			shrinking_slab = true;
> +		}
>  	} while (sc.priority >= 1 &&
>  		 !pgdat_balanced(pgdat, order, *classzone_idx));
>  
> -- 
> 1.8.1.4
> 

-- 
Michal Hocko
SUSE Labs