linux-kernel - Re: [RFC PATCH 4/7] memcg: sleep during flushing stats in safe contexts

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Date:   Thu, 23 Mar 2023 11:56:13 -0400
From:   Johannes Weiner <hannes@...xchg.org>
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>,
        Michal Hocko <mhocko@...nel.org>,
        Roman Gushchin <roman.gushchin@...ux.dev>,
        Shakeel Butt <shakeelb@...gle.com>,
        Muchun Song <muchun.song@...ux.dev>,
        Andrew Morton <akpm@...ux-foundation.org>,
        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: [RFC PATCH 4/7] memcg: sleep during flushing stats in safe
 contexts

On Thu, Mar 23, 2023 at 04:00:34AM +0000, Yosry Ahmed wrote:
> @@ -644,26 +644,26 @@ static void __mem_cgroup_flush_stats(void)
>  		return;
>  
>  	flush_next_time = jiffies_64 + 2*FLUSH_TIME;
> -	cgroup_rstat_flush(root_mem_cgroup->css.cgroup, false);
> +	cgroup_rstat_flush(root_mem_cgroup->css.cgroup, may_sleep);

How is it safe to call this with may_sleep=true when it's holding the
stats_flush_lock?

>  	atomic_set(&stats_flush_threshold, 0);
>  	spin_unlock(&stats_flush_lock);
>  }