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From:   Shakeel Butt <shakeelb@google.com>
Date:   Tue, 4 Apr 2023 10:13:47 -0700
Message-ID: <CALvZod5Y+quOS1XQvVBTvv7FRs3455j_79f0GoR+FqCFzbwkuA@mail.gmail.com>
Subject: Re: [PATCH v3 4/8] memcg: replace stats_flush_lock with an atomic
To:     =?UTF-8?Q?Michal_Koutn=C3=BD?= <mkoutny@suse.com>
Cc:     Yosry Ahmed <yosryahmed@google.com>, 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>,
        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>
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On Tue, Apr 4, 2023 at 9:53=E2=80=AFAM Michal Koutn=C3=BD <mkoutny@suse.com=
> wrote:
>
> Hello.
>
> On Thu, Mar 30, 2023 at 07:17:57PM +0000, Yosry Ahmed <yosryahmed@google.=
com> wrote:
> >  static void __mem_cgroup_flush_stats(void)
> >  {
> > -     unsigned long flag;
> > -
> > -     if (!spin_trylock_irqsave(&stats_flush_lock, flag))
> > +     /*
> > +      * We always flush the entire tree, so concurrent flushers can ju=
st
> > +      * skip. This avoids a thundering herd problem on the rstat globa=
l lock
> > +      * from memcg flushers (e.g. reclaim, refault, etc).
> > +      */
> > +     if (atomic_read(&stats_flush_ongoing) ||
> > +         atomic_xchg(&stats_flush_ongoing, 1))
> >               return;
>
> I'm curious about why this instead of
>
>         if (atomic_xchg(&stats_flush_ongoing, 1))
>                 return;
>
> Is that some microarchitectural cleverness?
>

Yes indeed it is. Basically we want to avoid unconditional cache
dirtying. This pattern is also used at other places in the kernel like
qspinlock.