Re: [PATCH v8 14/17] mm: Iterate only over charged shrinkers during memcg shrink_slab()

[Kirill Tkhai]

On 03.07.2018 23:58, Andrew Morton wrote:
> On Tue, 03 Jul 2018 18:11:06 +0300 Kirill Tkhai <ktkhai@xxxxxxxxxxxxx> 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.

We take shrinker_rwsem in prealloc_memcg_shrinker() and do memory allocation
there. It may result in reclaim under shrinker_rwsem write locked, so we use
down_read_trylock() to avoid deadlocks. The first versions of the patchset
contained different lock for this function, but it has gone in the process
of review.

>Comment it, please.

OK

>> +	/*
>> +	 * 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.

OK

>> +		/* 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;
>> +}
>