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From:   Shakeel Butt <shakeelb@google.com>
Date:   Tue, 28 Mar 2023 15:22:45 -0700
Message-ID: <CALvZod66XSX0Y-xRXE4MQ6cjC4j8q-Et4jz0q405RDG6AarS2g@mail.gmail.com>
Subject: Re: [PATCH v2 5/9] memcg: replace stats_flush_lock with an atomic
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>,
        Johannes Weiner <hannes@cmpxchg.org>,
        Michal Hocko <mhocko@kernel.org>,
        Roman Gushchin <roman.gushchin@linux.dev>,
        Muchun Song <muchun.song@linux.dev>,
        Andrew Morton <akpm@linux-foundation.org>,
        =?UTF-8?Q?Michal_Koutn=C3=BD?= <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
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On Tue, Mar 28, 2023 at 3:17=E2=80=AFPM Yosry Ahmed <yosryahmed@google.com>=
 wrote:
>
> As Johannes notes in [1], stats_flush_lock is currently used to:
> (a) Protect updated to stats_flush_threshold.
> (b) Protect updates to flush_next_time.
> (c) Serializes calls to cgroup_rstat_flush() based on those ratelimits.
>
> However:
>
> 1. stats_flush_threshold is already an atomic
>
> 2. flush_next_time is not atomic. The writer is locked, but the reader
>    is lockless. If the reader races with a flush, you could see this:
>
>                                         if (time_after(jiffies, flush_nex=
t_time))
>         spin_trylock()
>         flush_next_time =3D now + delay
>         flush()
>         spin_unlock()
>                                         spin_trylock()
>                                         flush_next_time =3D now + delay
>                                         flush()
>                                         spin_unlock()
>
>    which means we already can get flushes at a higher frequency than
>    FLUSH_TIME during races. But it isn't really a problem.
>
>    The reader could also see garbled partial updates, so it needs at
>    least READ_ONCE and WRITE_ONCE protection.
>
> 3. Serializing cgroup_rstat_flush() calls against the ratelimit
>    factors is currently broken because of the race in 2. But the race
>    is actually harmless, all we might get is the occasional earlier
>    flush. If there is no delta, the flush won't do much. And if there
>    is, the flush is justified.
>
> So the lock can be removed all together. However, the lock also served
> the purpose of preventing a thundering herd problem for concurrent
> flushers, see [2]. Use an atomic instead to serve the purpose of
> unifying concurrent flushers.
>
> [1]https://lore.kernel.org/lkml/20230323172732.GE739026@cmpxchg.org/
> [2]https://lore.kernel.org/lkml/20210716212137.1391164-2-shakeelb@google.=
com/
>
> Signed-off-by: Yosry Ahmed <yosryahmed@google.com>
> Acked-by: Johannes Weiner <hannes@cmpxchg.org>

Acked-by: Shakeel Butt <shakeelb@google.com>