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SubjectRe: [PATCH v5 11/13] mm: Iterate only over charged shrinkers during memcg shrink_slab()
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.

>
> 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 :-)

> + */
> + 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?

> +
> + 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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