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

[Johannes Weiner <hannes@cmpxchg.org>]

On Sat, Nov 14, 2015 at 03:36:50PM +0300, Vladimir Davydov wrote:
> On Thu, Nov 12, 2015 at 06:41:21PM -0500, Johannes Weiner wrote:
> > @@ -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.

You're absolutely right, good catch.

Please disregard this patch. It's not necessary for this series after
v2, I just kept it because I thought it's a nice simplification that's
possible after making root_mem_cgroup public.

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