Re: [v3 PATCH 10/11] mm: memcontrol: reparent nr_deferred when memcg offline

[Kirill Tkhai <ktkhai@virtuozzo.com>]

On 06.01.2021 01:58, Yang Shi wrote:
> Now shrinker's nr_deferred is per memcg for memcg aware shrinkers, add to parent's
> corresponding nr_deferred when memcg offline.
> 
> Signed-off-by: Yang Shi <shy828301@gmail.com>
> ---
>  include/linux/memcontrol.h |  1 +
>  mm/memcontrol.c            |  1 +
>  mm/vmscan.c                | 29 +++++++++++++++++++++++++++++
>  3 files changed, 31 insertions(+)
> 
> diff --git a/include/linux/memcontrol.h b/include/linux/memcontrol.h
> index 5599082df623..d1e52e916cc2 100644
> --- a/include/linux/memcontrol.h
> +++ b/include/linux/memcontrol.h
> @@ -1586,6 +1586,7 @@ extern int memcg_alloc_shrinker_info(struct mem_cgroup *memcg);
>  extern void memcg_free_shrinker_info(struct mem_cgroup *memcg);
>  extern void memcg_set_shrinker_bit(struct mem_cgroup *memcg,
>  				   int nid, int shrinker_id);
> +extern void memcg_reparent_shrinker_deferred(struct mem_cgroup *memcg);
>  #else
>  #define mem_cgroup_sockets_enabled 0
>  static inline void mem_cgroup_sk_alloc(struct sock *sk) { };
> diff --git a/mm/memcontrol.c b/mm/memcontrol.c
> index 126f1fd550c8..19e555675582 100644
> --- a/mm/memcontrol.c
> +++ b/mm/memcontrol.c
> @@ -5284,6 +5284,7 @@ static void mem_cgroup_css_offline(struct cgroup_subsys_state *css)
>  	page_counter_set_low(&memcg->memory, 0);
>  
>  	memcg_offline_kmem(memcg);
> +	memcg_reparent_shrinker_deferred(memcg);
>  	wb_memcg_offline(memcg);
>  
>  	drain_all_stock(memcg);
> diff --git a/mm/vmscan.c b/mm/vmscan.c
> index d9795fb0f1c5..71056057d26d 100644
> --- a/mm/vmscan.c
> +++ b/mm/vmscan.c
> @@ -396,6 +396,35 @@ static long set_nr_deferred_memcg(long nr, int nid, struct shrinker *shrinker,
>  	return atomic_long_add_return(nr, &info->nr_deferred[shrinker->id]);
>  }
>  
> +void memcg_reparent_shrinker_deferred(struct mem_cgroup *memcg)
> +{
> +	int i, nid;
> +	long nr;
> +	struct mem_cgroup *parent;
> +	struct memcg_shrinker_info *child_info, *parent_info;
> +
> +	parent = parent_mem_cgroup(memcg);
> +	if (!parent)
> +		parent = root_mem_cgroup;
> +
> +	/* Prevent from concurrent shrinker_info expand */
> +	down_read(&shrinker_rwsem);
> +	for_each_node(nid) {
> +		child_info = rcu_dereference_protected(
> +					memcg->nodeinfo[nid]->shrinker_info,
> +					true);
> +		parent_info = rcu_dereference_protected(
> +					parent->nodeinfo[nid]->shrinker_info,
> +					true);

Simple assignment can't take such lots of space, we have to do something with that.

Number of these

	rcu_dereference_protected(memcg->nodeinfo[nid]->shrinker_info, true)

became too big, and we can't allow every of them takes 3 lines.

We should introduce a short helper to dereferrence this, so we will be able to give
out attention to really difficult logic instead of wasting it on parsing this.

		child_info = memcg_shrinker_info(memcg, nid);
or
		child_info = memcg_shrinker_info_protected(memcg, nid);

Both of them fit in single line.

struct memcg_shrinker_info *memcg_shrinker_info_protected(
					struct mem_cgroup *memcg, int nid)
{
	return rcu_dereference_protected(memcg->nodeinfo[nid]->shrinker_info,
					 lockdep_assert_held(&shrinker_rwsem));
}


> +		for (i = 0; i < shrinker_nr_max; i++) {
> +			nr = atomic_long_read(&child_info->nr_deferred[i]);
> +			atomic_long_add(nr,
> +					&parent_info->nr_deferred[i]);

Why new line is here? In case of you merge it up, it will be even shorter then previous line.

> +		}
> +	}
> +	up_read(&shrinker_rwsem);
> +}
> +
>  static bool cgroup_reclaim(struct scan_control *sc)
>  {
>  	return sc->target_mem_cgroup;
>