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SubjectRe: [PATCH v5 11/13] mm: Iterate only over charged shrinkers during memcg shrink_slab()
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On 15.05.2018 08:44, Vladimir Davydov wrote:
> On Thu, May 10, 2018 at 12:53:55PM +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>
>> ---
>> include/linux/memcontrol.h | 1 +
>> mm/vmscan.c | 70 ++++++++++++++++++++++++++++++++++++++------
>> 2 files changed, 62 insertions(+), 9 deletions(-)
>>
>> diff --git a/include/linux/memcontrol.h b/include/linux/memcontrol.h
>> index 82f892e77637..436691a66500 100644
>> --- a/include/linux/memcontrol.h
>> +++ b/include/linux/memcontrol.h
>> @@ -760,6 +760,7 @@ void mem_cgroup_split_huge_fixup(struct page *head);
>> #define MEM_CGROUP_ID_MAX 0
>>
>> struct mem_cgroup;
>> +#define root_mem_cgroup NULL
>
> Let's instead export mem_cgroup_is_root(). In case if MEMCG is disabled
> it will always return false.

export == move to header file

>>
>> static inline bool mem_cgroup_disabled(void)
>> {
>> diff --git a/mm/vmscan.c b/mm/vmscan.c
>> index d8a2870710e0..a2e38e05adb5 100644
>> --- a/mm/vmscan.c
>> +++ b/mm/vmscan.c
>> @@ -376,6 +376,7 @@ int prealloc_shrinker(struct shrinker *shrinker)
>> goto free_deferred;
>> }
>>
>> + INIT_LIST_HEAD(&shrinker->list);
>
> IMO this shouldn't be here, see my comment below.
>
>> return 0;
>>
>> free_deferred:
>> @@ -547,6 +548,63 @@ static unsigned long do_shrink_slab(struct shrink_control *shrinkctl,
>> return freed;
>> }
>>
>> +#ifdef CONFIG_MEMCG_SHRINKER
>> +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.
>
> ^^
> Nit: space missing here :-)

I don't understand what you mean here. Please, clarify...

>> + */
>> + map = rcu_dereference_protected(MEMCG_SHRINKER_MAP(memcg, nid), 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 (!shrinker) {
>> + clear_bit(i, map->map);
>> + continue;
>> + }
>
> The shrinker must be memcg aware so please add
>
> BUG_ON((shrinker->flags & SHRINKER_MEMCG_AWARE) == 0);
>
>> + if (list_empty(&shrinker->list))
>> + continue;
>
> I don't like using shrinker->list as an indicator that the shrinker has
> been initialized. IMO if you do need such a check, you should split
> shrinker_idr registration in two steps - allocate a slot in 'prealloc'
> and set the pointer in 'register'. However, can we really encounter an
> unregistered shrinker here? AFAIU a bit can be set in the shrinker map
> only after the corresponding shrinker has been initialized, no?

1)No, it's not so. Here is a race:
cpu#0 cpu#1 cpu#2
prealloc_shrinker()
prealloc_shrinker()
memcg_expand_shrinker_maps()
memcg_expand_one_shrinker_map()
memset(&new->map, 0xff);
do_shrink_slab() (on uninitialized LRUs)
init LRUs
register_shrinker_prepared()

So, the check is needed.

2)Assigning NULL pointer can't be used here, since NULL pointer is already used
to clear unregistered shrinkers from the map. See the check right after idr_find().

list_empty() is used since it's the already existing indicator, which does not
require additional member in struct shrinker.

>> +
>> + 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_SHRINKER */
>> +static unsigned long shrink_slab_memcg(gfp_t gfp_mask, int nid,
>> + struct mem_cgroup *memcg, int priority)
>> +{
>> + return 0;
>> +}
>> +#endif /* CONFIG_MEMCG_SHRINKER */
>> +
>> /**
>> * shrink_slab - shrink slab caches
>> * @gfp_mask: allocation context
>> @@ -576,8 +634,8 @@ 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 && memcg != root_mem_cgroup)
>
> if (!mem_cgroup_is_root(memcg))
>
>> + return shrink_slab_memcg(gfp_mask, nid, memcg, priority);
>>
>> if (!down_read_trylock(&shrinker_rwsem))
>> goto out;
>> @@ -589,13 +647,7 @@ 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;
>
> I want this check gone. It's easy to achieve, actually - just remove the
> following lines from shrink_node()
>
> if (global_reclaim(sc))
> shrink_slab(sc->gfp_mask, pgdat->node_id, NULL,
> sc->priority);
>
>>
>> if (!(shrinker->flags & SHRINKER_NUMA_AWARE))
>>

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