linux-kernel - Re: [PATCH v2 8/9] vmscan: memcg: sleep when flushing stats during reclaim

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Message-ID: <ZCU9ByZybEi5G5sl@dhcp22.suse.cz>
Date:   Thu, 30 Mar 2023 09:40:55 +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 8/9] vmscan: memcg: sleep when flushing stats during
 reclaim

On Tue 28-03-23 22:16:43, Yosry Ahmed wrote:
> Memory reclaim is a sleepable context. Allow sleeping when flushing
> memcg stats to avoid unnecessarily performing a lot of work without
> sleeping. This can slow down reclaim code if flushing stats is taking
> too long, but there is already multiple cond_resched()'s in reclaim
> code.

Why is this preferred? Memory reclaim is surely a slow path but what is
the advantage of calling mem_cgroup_flush_stats here?

> Signed-off-by: Yosry Ahmed <yosryahmed@...gle.com>
> Acked-by: Shakeel Butt <shakeelb@...gle.com>
> Acked-by: Johannes Weiner <hannes@...xchg.org>
> ---
>  mm/vmscan.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/mm/vmscan.c b/mm/vmscan.c
> index a9511ccb936f..9c1c5e8b24b8 100644
> --- a/mm/vmscan.c
> +++ b/mm/vmscan.c
> @@ -2845,7 +2845,7 @@ static void prepare_scan_count(pg_data_t *pgdat, struct scan_control *sc)
>  	 * Flush the memory cgroup stats, so that we read accurate per-memcg
>  	 * lruvec stats for heuristics.
>  	 */
> -	mem_cgroup_flush_stats_atomic();
> +	mem_cgroup_flush_stats();
>  
>  	/*
>  	 * Determine the scan balance between anon and file LRUs.
> -- 
> 2.40.0.348.gf938b09366-goog

-- 
Michal Hocko
SUSE Labs