linux-kernel - Re: [PATCH v2 9/9] memcg: do not modify rstat tree for zero updates

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Date:   Thu, 30 Mar 2023 09:43:42 +0200
From:   Michal Hocko <mhocko@...e.com>
To:     Yosry Ahmed <yosryahmed@...gle.com>
Cc:     Tejun Heo <tj@...nel.org>, Josef Bacik <josef@...icpanda.com>,
        Jens Axboe <axboe@...nel.dk>,
        Zefan Li <lizefan.x@...edance.com>,
        Johannes Weiner <hannes@...xchg.org>,
        Roman Gushchin <roman.gushchin@...ux.dev>,
        Shakeel Butt <shakeelb@...gle.com>,
        Muchun Song <muchun.song@...ux.dev>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Michal Koutný <mkoutny@...e.com>,
        Vasily Averin <vasily.averin@...ux.dev>,
        cgroups@...r.kernel.org, linux-block@...r.kernel.org,
        linux-kernel@...r.kernel.org, linux-mm@...ck.org,
        bpf@...r.kernel.org
Subject: Re: [PATCH v2 9/9] memcg: do not modify rstat tree for zero updates

On Tue 28-03-23 22:16:44, Yosry Ahmed wrote:
> In some situations, we may end up calling memcg_rstat_updated() with a
> value of 0, which means the stat was not actually updated. An example is
> if we fail to reclaim any pages in shrink_folio_list().
> 
> Do not add the cgroup to the rstat updated tree in this case, to avoid
> unnecessarily flushing it.
> 
> Signed-off-by: Yosry Ahmed <yosryahmed@...gle.com>
> Acked-by: Shakeel Butt <shakeelb@...gle.com>
> Acked-by: Johannes Weiner <hannes@...xchg.org>

Acked-by: Michal Hocko <mhocko@...e.com>

> ---
>  mm/memcontrol.c | 3 +++
>  1 file changed, 3 insertions(+)
> 
> diff --git a/mm/memcontrol.c b/mm/memcontrol.c
> index 361c0bbf7283..a63ee2efa780 100644
> --- a/mm/memcontrol.c
> +++ b/mm/memcontrol.c
> @@ -618,6 +618,9 @@ static inline void memcg_rstat_updated(struct mem_cgroup *memcg, int val)
>  {
>  	unsigned int x;
>  
> +	if (!val)
> +		return;
> +
>  	cgroup_rstat_updated(memcg->css.cgroup, smp_processor_id());
>  
>  	x = __this_cpu_add_return(stats_updates, abs(val));
> -- 
> 2.40.0.348.gf938b09366-goog

-- 
Michal Hocko
SUSE Labs