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Date:   Sun, 20 May 2018 10:55:58 +0300
From:   Vladimir Davydov <vdavydov.dev@gmail.com>
To:     Kirill Tkhai <ktkhai@virtuozzo.com>
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Subject: Re: [PATCH v6 12/17] mm: Set bit in memcg shrinker bitmap on first
 list_lru item apearance
Message-ID: <20180520075558.6ls4yzrkou63orkb@esperanza>
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On Fri, May 18, 2018 at 11:43:42AM +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@virtuozzo.com>
> ---
>  include/linux/memcontrol.h |   14 ++++++++++++++
>  mm/list_lru.c              |   22 ++++++++++++++++++++--
>  2 files changed, 34 insertions(+), 2 deletions(-)
> 
> diff --git a/include/linux/memcontrol.h b/include/linux/memcontrol.h
> index e51c6e953d7a..7ae1b94becf3 100644
> --- a/include/linux/memcontrol.h
> +++ b/include/linux/memcontrol.h
> @@ -1275,6 +1275,18 @@ static inline int memcg_cache_id(struct mem_cgroup *memcg)
>  
>  extern int memcg_expand_shrinker_maps(int new_id);
>  
> +static inline void memcg_set_shrinker_bit(struct mem_cgroup *memcg,
> +					  int nid, int shrinker_id)
> +{

> +	if (shrinker_id >= 0 && memcg && memcg != root_mem_cgroup) {

Nit: I'd remove these checks from this function and require the caller
to check that shrinker_id >= 0 and memcg != NULL or root_mem_cgroup.
See below how the call sites would look then.

> +		struct memcg_shrinker_map *map;
> +
> +		rcu_read_lock();
> +		map = rcu_dereference(memcg->nodeinfo[nid]->shrinker_map);
> +		set_bit(shrinker_id, map->map);
> +		rcu_read_unlock();
> +	}
> +}
>  #else
>  #define for_each_memcg_cache_index(_idx)	\
>  	for (; NULL; )
> @@ -1297,6 +1309,8 @@ static inline void memcg_put_cache_ids(void)
>  {
>  }
>  
> +static inline void memcg_set_shrinker_bit(struct mem_cgroup *memcg,
> +					  int nid, int shrinker_id) { }
>  #endif /* CONFIG_MEMCG_KMEM */
>  
>  #endif /* _LINUX_MEMCONTROL_H */
> diff --git a/mm/list_lru.c b/mm/list_lru.c
> index cab8fad7f7e2..7df71ab0de1c 100644
> --- a/mm/list_lru.c
> +++ b/mm/list_lru.c
> @@ -31,6 +31,11 @@ static void list_lru_unregister(struct list_lru *lru)
>  	mutex_unlock(&list_lrus_mutex);
>  }
>  
> +static int lru_shrinker_id(struct list_lru *lru)
> +{
> +	return lru->shrinker_id;
> +}
> +
>  static inline bool list_lru_memcg_aware(struct list_lru *lru)
>  {
>  	/*
> @@ -94,6 +99,11 @@ static void list_lru_unregister(struct list_lru *lru)
>  {
>  }
>  
> +static int lru_shrinker_id(struct list_lru *lru)
> +{
> +	return -1;
> +}
> +
>  static inline bool list_lru_memcg_aware(struct list_lru *lru)
>  {
>  	return false;
> @@ -119,13 +129,17 @@ bool list_lru_add(struct list_lru *lru, struct list_head *item)
>  {
>  	int nid = page_to_nid(virt_to_page(item));
>  	struct list_lru_node *nlru = &lru->node[nid];
> +	struct mem_cgroup *memcg;
>  	struct list_lru_one *l;
>  
>  	spin_lock(&nlru->lock);
>  	if (list_empty(item)) {
> -		l = list_lru_from_kmem(nlru, item, NULL);
> +		l = list_lru_from_kmem(nlru, item, &memcg);
>  		list_add_tail(item, &l->list);
> -		l->nr_items++;
> +		/* Set shrinker bit if the first element was added */
> +		if (!l->nr_items++)
> +			memcg_set_shrinker_bit(memcg, nid,
> +					       lru_shrinker_id(lru));

This would turn into

	if (!l->nr_items++ && memcg)
		memcg_set_shrinker_bit(memcg, nid, lru_shrinker_id(lru));

Note, you don't need to check that lru_shrinker_id(lru) is >= 0 here as
the fact that memcg != NULL guarantees that. Also, memcg can't be
root_mem_cgroup here as kmem objects allocated for the root cgroup go
unaccounted.

>  		nlru->nr_items++;
>  		spin_unlock(&nlru->lock);
>  		return true;
> @@ -520,6 +534,7 @@ static void memcg_drain_list_lru_node(struct list_lru *lru, int nid,
>  	struct list_lru_node *nlru = &lru->node[nid];
>  	int dst_idx = dst_memcg->kmemcg_id;
>  	struct list_lru_one *src, *dst;
> +	bool set;
>  
>  	/*
>  	 * Since list_lru_{add,del} may be called under an IRQ-safe lock,
> @@ -531,7 +546,10 @@ static void memcg_drain_list_lru_node(struct list_lru *lru, int nid,
>  	dst = list_lru_from_memcg_idx(nlru, dst_idx);
>  
>  	list_splice_init(&src->list, &dst->list);
> +	set = (!dst->nr_items && src->nr_items);
>  	dst->nr_items += src->nr_items;
> +	if (set)
> +		memcg_set_shrinker_bit(dst_memcg, nid, lru_shrinker_id(lru));

This would turn into

	if (set && dst_idx >= 0)
		memcg_set_shrinker_bit(dst_memcg, nid, lru_shrinker_id(lru));

Again, the shrinker is guaranteed to be memcg aware in this function and
dst_memcg != NULL.

IMHO such a change would make the code a bit more straightforward.

>  	src->nr_items = 0;
>  
>  	spin_unlock_irq(&nlru->lock);
>