Re: [PATCH-mm v2] mm/list_lru: Optimize memcg_reparent_list_lru_node()

[Roman Gushchin <roman.gushchin@linux.dev>]

On Tue, Mar 08, 2022 at 08:18:24PM -0500, Waiman Long wrote:
> Since commit 2c80cd57c743 ("mm/list_lru.c: fix list_lru_count_node()
> to be race free"), we are tracking the total number of lru
> entries in a list_lru_node in its nr_items field.  In the case of
> memcg_reparent_list_lru_node(), there is nothing to be done if nr_items
> is 0.  We don't even need to take the nlru->lock as no new lru entry
> could be added by a racing list_lru_add() to the draining src_idx memcg
> at this point.
> 
> Signed-off-by: Waiman Long <longman@redhat.com>
> ---
>  mm/list_lru.c | 6 ++++++
>  1 file changed, 6 insertions(+)
> 
> diff --git a/mm/list_lru.c b/mm/list_lru.c
> index ba76428ceece..c669d87001a6 100644
> --- a/mm/list_lru.c
> +++ b/mm/list_lru.c
> @@ -394,6 +394,12 @@ static void memcg_reparent_list_lru_node(struct list_lru *lru, int nid,
>  	int dst_idx = dst_memcg->kmemcg_id;
>  	struct list_lru_one *src, *dst;
>  
> +	/*
> +	 * If there is no lru entry in this nlru, we can skip it immediately.
> +	 */
> +	if (!READ_ONCE(nlru->nr_items))
> +		return;

This is a per-node counter, not a per-memcg, right?
If so, do we optimize for the case when all lru items belong to one node and
others are empty?