Re: [PATCH 08/10] mm: Set bit in memcg shrinker bitmap on first list_lru item apearance

[Vladimir Davydov]

On Wed, Mar 21, 2018 at 04:22:40PM +0300, Kirill Tkhai wrote:
> Introduce set_shrinker_bit() function to set shrinker-related
> bit in memcg shrinker bitmap, and set the bit after the first
> item is added and in case of reparenting destroyed memcg's items.
> 
> This will allow next patch to make shrinkers be called only,
> in case of they have charged objects at the moment, and
> to improve shrink_slab() performance.
> 
> Signed-off-by: Kirill Tkhai <ktkhai@xxxxxxxxxxxxx>
> ---
>  include/linux/shrinker.h |    7 +++++++
>  mm/list_lru.c            |   22 ++++++++++++++++++++--
>  mm/vmscan.c              |    7 +++++++
>  3 files changed, 34 insertions(+), 2 deletions(-)
> 
> diff --git a/include/linux/shrinker.h b/include/linux/shrinker.h
> index 738de8ef5246..24aeed1bc332 100644
> --- a/include/linux/shrinker.h
> +++ b/include/linux/shrinker.h
> @@ -78,4 +78,11 @@ struct shrinker {
>  
>  extern __must_check int register_shrinker(struct shrinker *);
>  extern void unregister_shrinker(struct shrinker *);
> +#if defined(CONFIG_MEMCG) && !defined(CONFIG_SLOB)
> +extern void set_shrinker_bit(struct mem_cgroup *, int, int);
> +#else
> +static inline void set_shrinker_bit(struct mem_cgroup *memcg, int node, int id)
> +{
> +}
> +#endif

IMO this function, as well as other shrinker bitmap manipulation
functions, should be defined in memcontrol.[hc] and have mem_cgroup_
prefix.

> diff --git a/mm/vmscan.c b/mm/vmscan.c
> index 9d1df5d90eca..265cf069b470 100644
> --- a/mm/vmscan.c
> +++ b/mm/vmscan.c
> @@ -378,6 +378,13 @@ static void del_shrinker(struct shrinker *shrinker)
>  	list_del(&shrinker->list);
>  	up_write(&shrinker_rwsem);
>  }
> +
> +void set_shrinker_bit(struct mem_cgroup *memcg, int nid, int nr)
> +{
> +	struct shrinkers_map *map = SHRINKERS_MAP(memcg);
> +
> +	set_bit(nr, map->map[nid]);
> +}

Shouldn't we use rcu_read_lock here? After all, the map can be
reallocated right from under our feet.