linux-kernel - Re: [PATCH -mm v2 3/9] vmscan: shrink slab on memcg pressure

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Date:	Thu, 6 Nov 2014 10:21:35 -0500
From:	Johannes Weiner <hannes@...xchg.org>
To:	Vladimir Davydov <vdavydov@...allels.com>
Cc:	Andrew Morton <akpm@...ux-foundation.org>,
	Michal Hocko <mhocko@...e.cz>,
	Greg Thelen <gthelen@...gle.com>,
	Glauber Costa <glommer@...il.com>,
	Dave Chinner <david@...morbit.com>,
	Alexander Viro <viro@...iv.linux.org.uk>, linux-mm@...ck.org,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH -mm v2 3/9] vmscan: shrink slab on memcg pressure

On Fri, Oct 24, 2014 at 02:37:34PM +0400, Vladimir Davydov wrote:
> diff --git a/mm/vmscan.c b/mm/vmscan.c
> index a384339bf718..2cf6b04a4e0c 100644
> --- a/mm/vmscan.c
> +++ b/mm/vmscan.c
> @@ -339,6 +339,26 @@ shrink_slab_node(struct shrink_control *shrinkctl, struct shrinker *shrinker,
>  	return freed;
>  }
>  
> +static unsigned long
> +run_shrinker(struct shrink_control *shrinkctl, struct shrinker *shrinker,
> +	     unsigned long nr_pages_scanned, unsigned long lru_pages)
> +{
> +	unsigned long freed = 0;
> +
> +	if (!(shrinker->flags & SHRINKER_NUMA_AWARE)) {
> +		shrinkctl->nid = 0;
> +		return shrink_slab_node(shrinkctl, shrinker,
> +					nr_pages_scanned, lru_pages);
> +	}
> +
> +	for_each_node_mask(shrinkctl->nid, shrinkctl->nodes_to_scan) {
> +		if (node_online(shrinkctl->nid))
> +			freed += shrink_slab_node(shrinkctl, shrinker,
> +						  nr_pages_scanned, lru_pages);
> +	}
> +	return freed;
> +}

The slab shrinking logic accumulates the lru pages, as well as the
nodes_to_scan mask, when going over the zones, only to go over the
zones here again using the accumulated node information.  Why not just
invoke the thing per-zone instead in the first place?  Kswapd already
does that (although it could probably work with the per-zone lru_pages
and nr_scanned deltas) and direct reclaim should as well.  It would
simplify the existing code as well as your series a lot.

> +		/*
> +		 * For memcg-aware shrinkers iterate over the target memcg
> +		 * hierarchy and run the shrinker on each kmem-active memcg
> +		 * found in the hierarchy.
> +		 */
> +		shrinkctl->memcg = shrinkctl->target_mem_cgroup;
> +		do {
> +			if (!shrinkctl->memcg ||
> +			    memcg_kmem_is_active(shrinkctl->memcg))
> +				freed += run_shrinker(shrinkctl, shrinker,
>  						nr_pages_scanned, lru_pages);
> -
> -		}
> +		} while ((shrinkctl->memcg =
> +			  mem_cgroup_iter(shrinkctl->target_mem_cgroup,
> +					  shrinkctl->memcg, NULL)) != NULL);

More symptoms of the above.  This hierarchy walk is duplicative and
potentially quite expensive.

> @@ -2381,6 +2414,7 @@ static bool shrink_zones(struct zonelist *zonelist, struct scan_control *sc)
>  	gfp_t orig_mask;
>  	struct shrink_control shrink = {
>  		.gfp_mask = sc->gfp_mask,
> +		.target_mem_cgroup = sc->target_mem_cgroup,
>  	};
>  	enum zone_type requested_highidx = gfp_zone(sc->gfp_mask);
>  	bool reclaimable = false;
> @@ -2400,18 +2434,22 @@ static bool shrink_zones(struct zonelist *zonelist, struct scan_control *sc)
>  					gfp_zone(sc->gfp_mask), sc->nodemask) {
>  		if (!populated_zone(zone))
>  			continue;
> +
> +		if (global_reclaim(sc) &&
> +		    !cpuset_zone_allowed(zone, GFP_KERNEL | __GFP_HARDWALL))
> +			continue;
> +
> +		lru_pages += global_reclaim(sc) ?
> +				zone_reclaimable_pages(zone) :
> +				mem_cgroup_zone_reclaimable_pages(zone,
> +						sc->target_mem_cgroup);
> +		node_set(zone_to_nid(zone), shrink.nodes_to_scan);

And yet another costly hierarchy walk.

The reclaim code walks zonelists according to a nodemask, and within
each zone it walks lruvecs according to the memcg hierarchy.  The
shrinkers are wrong in making up an ad-hoc concept of NUMA nodes that
otherwise does not exist anywhere in the VM.  Please integrate them
properly instead of adding more duplication on top.

Thanks
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