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Date:   Thu, 30 Mar 2023 15:30:13 -0400
From:   Johannes Weiner <hannes@cmpxchg.org>
To:     Yosry Ahmed <yosryahmed@google.com>
Cc:     Tejun Heo <tj@kernel.org>, Josef Bacik <josef@toxicpanda.com>,
        Jens Axboe <axboe@kernel.dk>,
        Zefan Li <lizefan.x@bytedance.com>,
        Michal Hocko <mhocko@kernel.org>,
        Roman Gushchin <roman.gushchin@linux.dev>,
        Shakeel Butt <shakeelb@google.com>,
        Muchun Song <muchun.song@linux.dev>,
        Andrew Morton <akpm@linux-foundation.org>,
        Michal =?iso-8859-1?Q?Koutn=FD?= <mkoutny@suse.com>,
        Vasily Averin <vasily.averin@linux.dev>,
        cgroups@vger.kernel.org, linux-block@vger.kernel.org,
        linux-kernel@vger.kernel.org, linux-mm@kvack.org,
        bpf@vger.kernel.org, Michal Hocko <mhocko@suse.com>
Subject: Re: [PATCH v3 5/8] memcg: sleep during flushing stats in safe
 contexts
Message-ID: <ZCXjRYASA+Cjomlx@cmpxchg.org>
References: <20230330191801.1967435-1-yosryahmed@google.com>
 <20230330191801.1967435-6-yosryahmed@google.com>
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On Thu, Mar 30, 2023 at 07:17:58PM +0000, Yosry Ahmed wrote:
> Currently, all contexts that flush memcg stats do so with sleeping not
> allowed. Some of these contexts are perfectly safe to sleep in, such as
> reading cgroup files from userspace or the background periodic flusher.
> Flushing is an expensive operation that scales with the number of cpus
> and the number of cgroups in the system, so avoid doing it atomically
> where possible.
> 
> Refactor the code to make mem_cgroup_flush_stats() non-atomic (aka
> sleepable), and provide a separate atomic version. The atomic version is
> used in reclaim, refault, writeback, and in mem_cgroup_usage(). All
> other code paths are left to use the non-atomic version. This includes
> callbacks for userspace reads and the periodic flusher.
> 
> Since refault is the only caller of mem_cgroup_flush_stats_ratelimited(),
> change it to mem_cgroup_flush_stats_atomic_ratelimited(). Reclaim and
> refault code paths are modified to do non-atomic flushing in separate
> later patches -- so it will eventually be changed back to
> mem_cgroup_flush_stats_ratelimited().
> 
> Signed-off-by: Yosry Ahmed <yosryahmed@google.com>
> Acked-by: Shakeel Butt <shakeelb@google.com>
> Acked-by: Michal Hocko <mhocko@suse.com>

Acked-by: Johannes Weiner <hannes@cmpxchg.org>