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SubjectRe: [PATCH v6 14/17] mm: Iterate only over charged shrinkers during memcg shrink_slab()
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On 20.05.2018 11:00, Vladimir Davydov wrote:
> On Fri, May 18, 2018 at 11:44:01AM +0300, Kirill Tkhai 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.
>>
>> Signed-off-by: Kirill Tkhai <ktkhai@virtuozzo.com>
>> ---
>> mm/vmscan.c | 87 +++++++++++++++++++++++++++++++++++++++++++++++++++++------
>> 1 file changed, 78 insertions(+), 9 deletions(-)
>>
>> diff --git a/mm/vmscan.c b/mm/vmscan.c
>> index f09ea20d7270..2fbf3b476601 100644
>> --- a/mm/vmscan.c
>> +++ b/mm/vmscan.c
>> @@ -373,6 +373,20 @@ int prealloc_shrinker(struct shrinker *shrinker)
>> goto free_deferred;
>> }
>>
>> + /*
>> + * There is a window between prealloc_shrinker()
>> + * and register_shrinker_prepared(). We don't want
>> + * to clear bit of a shrinker in such the state
>> + * in shrink_slab_memcg(), since this will impose
>> + * restrictions on a code registering a shrinker
>> + * (they would have to guarantee, their LRU lists
>> + * are empty till shrinker is completely registered).
>> + * So, we differ the situation, when 1)a shrinker
>> + * is semi-registered (id is assigned, but it has
>> + * not yet linked to shrinker_list) and 2)shrinker
>> + * is not registered (id is not assigned).
>> + */
>> + INIT_LIST_HEAD(&shrinker->list);
>> return 0;
>>
>> free_deferred:
>> @@ -544,6 +558,67 @@ 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;
>> +
>> + /*
>> + * 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, memcg_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)) {
>
> Nit: I don't think 'unlikely' is required here as this is definitely not
> a hot path.

In case of big machines with many containers and overcommit, shrink_slab()
in general is very hot path. See the patchset description. There are configurations,
when only shrink_slab() is executing and occupies cpu for 100%, it's the reason
of this patchset is made for.

Here is the place we are absolutely sure shrinker is NULL in case if race with parallel
registering, so I don't see anything wrong to give compiler some information about branch
prediction.

>> + clear_bit(i, map->map);
>> + continue;
>> + }
>> + BUG_ON(!(shrinker->flags & SHRINKER_MEMCG_AWARE));
>> +
>> + /* See comment in prealloc_shrinker() */
>> + if (unlikely(list_empty(&shrinker->list)))
>
> Ditto.
>
>> + 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;
>> +}
>> +#else /* CONFIG_MEMCG_KMEM */
>> +static unsigned long shrink_slab_memcg(gfp_t gfp_mask, int nid,
>> + struct mem_cgroup *memcg, int priority)
>> +{
>> + return 0;
>> +}
>> +#endif /* CONFIG_MEMCG_KMEM */
>> +
>> /**
>> * shrink_slab - shrink slab caches
>> * @gfp_mask: allocation context
>> @@ -573,8 +648,8 @@ static unsigned long shrink_slab(gfp_t gfp_mask, int nid,
>> struct shrinker *shrinker;
>> unsigned long freed = 0;
>>
>> - if (memcg && (!memcg_kmem_enabled() || !mem_cgroup_online(memcg)))
>> - return 0;
>> + if (memcg && !mem_cgroup_is_root(memcg))
>> + return shrink_slab_memcg(gfp_mask, nid, memcg, priority);
>>
>> if (!down_read_trylock(&shrinker_rwsem))
>> goto out;
>> @@ -586,13 +661,7 @@ static unsigned long shrink_slab(gfp_t gfp_mask, int nid,
>> .memcg = memcg,
>> };
>>
>> - /*
>> - * If kernel memory accounting is disabled, we ignore
>> - * SHRINKER_MEMCG_AWARE flag and call all shrinkers
>> - * passing NULL for memcg.
>> - */
>> - if (memcg_kmem_enabled() &&
>> - !!memcg != !!(shrinker->flags & SHRINKER_MEMCG_AWARE))
>> + if (!!memcg != !!(shrinker->flags & SHRINKER_MEMCG_AWARE))
>> continue;
>>
>> if (!(shrinker->flags & SHRINKER_NUMA_AWARE))
>>

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