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SubjectRe: [PATCH v8 14/17] mm: Iterate only over charged shrinkers during memcg shrink_slab()
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On 03.07.2018 23:58, Andrew Morton wrote:
> 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.

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;
>> +}
>

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