Subject: Re: [PATCH 02/14] mm: vmscan: simplify memcg vs. global shrinker invocation

From: Vladimir Davydov <[email-masked]>
Date: Sat, 14 Nov 2015 15:36:50 +0300

On Thu, Nov 12, 2015 at 06:41:21PM -0500, Johannes Weiner wrote:
...
> diff --git a/mm/vmscan.c b/mm/vmscan.c
> index a4507ec..e4f5b3c 100644
> --- a/mm/vmscan.c
> +++ b/mm/vmscan.c
> @@ -411,6 +411,10 @@ static unsigned long shrink_slab(gfp_t gfp_mask, int nid,
>  	struct shrinker *shrinker;
>  	unsigned long freed = 0;
>  
> +	/* Global shrinker mode */
> +	if (memcg == root_mem_cgroup)
> +		memcg = NULL;
> +
>  	if (memcg && !memcg_kmem_is_active(memcg))
>  		return 0;
>  
> @@ -2410,11 +2414,22 @@ static bool shrink_zone(struct zone *zone, struct scan_control *sc,
>  			shrink_lruvec(lruvec, swappiness, sc, &lru_pages);
>  			zone_lru_pages += lru_pages;
>  
> -			if (memcg && is_classzone)
> +			/*
> +			 * Shrink the slab caches in the same proportion that
> +			 * the eligible LRU pages were scanned.
> +			 */
> +			if (is_classzone) {
>  				shrink_slab(sc->gfp_mask, zone_to_nid(zone),
>  					    memcg, sc->nr_scanned - scanned,
>  					    lru_pages);
>  
> +				if (reclaim_state) {
> +					sc->nr_reclaimed +=
> +						reclaim_state->reclaimed_slab;
> +					reclaim_state->reclaimed_slab = 0;
> +				}
> +			}
> +
>  			/*
>  			 * Direct reclaim and kswapd have to scan all memory
>  			 * cgroups to fulfill the overall scan target for the
> @@ -2432,20 +2447,6 @@ static bool shrink_zone(struct zone *zone, struct scan_control *sc,
>  			}
>  		} while ((memcg = mem_cgroup_iter(root, memcg, &reclaim)));
>  
> -		/*
> -		 * Shrink the slab caches in the same proportion that
> -		 * the eligible LRU pages were scanned.
> -		 */
> -		if (global_reclaim(sc) && is_classzone)
> -			shrink_slab(sc->gfp_mask, zone_to_nid(zone), NULL,
> -				    sc->nr_scanned - nr_scanned,
> -				    zone_lru_pages);
> -
> -		if (reclaim_state) {
> -			sc->nr_reclaimed += reclaim_state->reclaimed_slab;
> -			reclaim_state->reclaimed_slab = 0;
> -		}
> -

AFAICS this patch deadly breaks memcg-unaware shrinkers vs LRU balance:
currently we scan (*total* LRU scanned / *total* LRU pages) of all such
objects; with this patch we'd use the numbers from the root cgroup
instead. If most processes reside in memory cgroups, the root cgroup
will have only a few LRU pages and hence the pressure exerted upon such
objects will be unfairly severe.

Thanks,
Vladimir

>  		vmpressure(sc->gfp_mask, sc->target_mem_cgroup,
>  			   sc->nr_scanned - nr_scanned,
>  			   sc->nr_reclaimed - nr_reclaimed);

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