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Date:   Tue, 3 Jul 2018 13:58:13 -0700
From:   Andrew Morton <akpm@linux-foundation.org>
To:     Kirill Tkhai <ktkhai@virtuozzo.com>
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Subject: Re: [PATCH v8 14/17] mm: Iterate only over charged shrinkers during
 memcg shrink_slab()
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On Tue, 03 Jul 2018 18:11:06 +0300 Kirill Tkhai <ktkhai@virtuozzo.com> wrote:

> Using the preparations made in previous patches, in case of memcg
> shrink, we may avoid shrinkers, which are not set in memcg's shrinkers
> bitmap. To do that, we separate iterations over memcg-aware and
> !memcg-aware shrinkers, and memcg-aware shrinkers are chosen
> via for_each_set_bit() from the bitmap. In case of big nodes,
> having many isolated environments, this gives significant
> performance growth. See next patches for the details.
> 
> Note, that the patch does not respect to empty memcg shrinkers,
> since we never clear the bitmap bits after we set it once.
> Their shrinkers will be called again, with no shrinked objects
> as result. This functionality is provided by next patches.
> 
> ...
>
> @@ -541,6 +555,67 @@ static unsigned long do_shrink_slab(struct shrink_control *shrinkctl,
>  	return freed;
>  }
>  
> +#ifdef CONFIG_MEMCG_KMEM
> +static unsigned long shrink_slab_memcg(gfp_t gfp_mask, int nid,
> +			struct mem_cgroup *memcg, int priority)
> +{
> +	struct memcg_shrinker_map *map;
> +	unsigned long freed = 0;
> +	int ret, i;
> +
> +	if (!memcg_kmem_enabled() || !mem_cgroup_online(memcg))
> +		return 0;
> +
> +	if (!down_read_trylock(&shrinker_rwsem))
> +		return 0;

Why trylock?  Presumably some other code path is known to hold the lock
for long periods?  Dunno.  Comment it, please.

> +	/*
> +	 * 1) Caller passes only alive memcg, so map can't be NULL.
> +	 * 2) shrinker_rwsem protects from maps expanding.
> +	 */
> +	map = rcu_dereference_protected(memcg->nodeinfo[nid]->shrinker_map,
> +					true);
> +	BUG_ON(!map);
> +
> +	for_each_set_bit(i, map->map, shrinker_nr_max) {
> +		struct shrink_control sc = {
> +			.gfp_mask = gfp_mask,
> +			.nid = nid,
> +			.memcg = memcg,
> +		};
> +		struct shrinker *shrinker;
> +
> +		shrinker = idr_find(&shrinker_idr, i);
> +		if (unlikely(!shrinker)) {
> +			clear_bit(i, map->map);
> +			continue;
> +		}
> +		BUG_ON(!(shrinker->flags & SHRINKER_MEMCG_AWARE));

Fair enough as a development-time sanity check but we shouldn't need
this in production code.  Or make it VM_BUG_ON(), at least.

> +		/* See comment in prealloc_shrinker() */
> +		if (unlikely(list_empty(&shrinker->list)))
> +			continue;
> +
> +		ret = do_shrink_slab(&sc, shrinker, priority);
> +		freed += ret;
> +
> +		if (rwsem_is_contended(&shrinker_rwsem)) {
> +			freed = freed ? : 1;
> +			break;
> +		}
> +	}
> +
> +	up_read(&shrinker_rwsem);
> +	return freed;
> +}