linux-kernel - Re: [PATCH] memcg: avoid refill

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Message-ID: <Z9KOu9XTe5O7tqRd@tiehlicka>
Date: Thu, 13 Mar 2025 08:52:27 +0100
From: Michal Hocko <mhocko@...e.com>
To: Shakeel Butt <shakeel.butt@...ux.dev>
Cc: Andrew Morton <akpm@...ux-foundation.org>,
	Johannes Weiner <hannes@...xchg.org>,
	Roman Gushchin <roman.gushchin@...ux.dev>,
	Muchun Song <muchun.song@...ux.dev>, linux-mm@...ck.org,
	cgroups@...r.kernel.org, linux-kernel@...r.kernel.org,
	Meta kernel team <kernel-team@...a.com>
Subject: Re: [PATCH] memcg: avoid refill_stock for root memcg

On Wed 12-03-25 22:48:12, Shakeel Butt wrote:
> We never charge the page counters of root memcg, so there is no need to
> put root memcg in the memcg stock. At the moment, refill_stock() can be
> called from try_charge_memcg(), obj_cgroup_uncharge_pages() and
> mem_cgroup_uncharge_skmem().
> 
> The try_charge_memcg() and mem_cgroup_uncharge_skmem() are never called
> with root memcg, so those are fine. However obj_cgroup_uncharge_pages()
> can potentially call refill_stock() with root memcg if the objcg object
> has been reparented over to the root memcg. Let's just avoid
> refill_stock() from obj_cgroup_uncharge_pages() for root memcg.

Makes sense. It is just pointless draining of a different memcg.

> Signed-off-by: Shakeel Butt <shakeel.butt@...ux.dev>

Acked-by: Michal Hocko <mhockoc@...e.com>
Thanks!

> ---
>  mm/memcontrol.c | 3 ++-
>  1 file changed, 2 insertions(+), 1 deletion(-)
> 
> diff --git a/mm/memcontrol.c b/mm/memcontrol.c
> index e08ce52caabd..393b73aec6dd 100644
> --- a/mm/memcontrol.c
> +++ b/mm/memcontrol.c
> @@ -2643,7 +2643,8 @@ static void obj_cgroup_uncharge_pages(struct obj_cgroup *objcg,
>  
>  	mod_memcg_state(memcg, MEMCG_KMEM, -nr_pages);
>  	memcg1_account_kmem(memcg, -nr_pages);
> -	refill_stock(memcg, nr_pages);
> +	if (!mem_cgroup_is_root(memcg))
> +		refill_stock(memcg, nr_pages);
>  
>  	css_put(&memcg->css);
>  }
> -- 
> 2.47.1
> 

-- 
Michal Hocko
SUSE Labs