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From:   Yosry Ahmed <yosryahmed@google.com>
Date:   Tue, 28 Mar 2023 12:34:15 -0700
Message-ID: <CAJD7tkb_YA3fvo3LgCzR+X-b-r-AmAR68hNR=xT7B6TJfBa54A@mail.gmail.com>
Subject: Re: [PATCH v1 5/9] memcg: replace stats_flush_lock with an atomic
To:     Shakeel Butt <shakeelb@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 12:28=E2=80=AFPM Shakeel Butt <shakeelb@google.com>=
 wrote:
>
> On Tue, Mar 28, 2023 at 11:53=E2=80=AFAM Yosry Ahmed <yosryahmed@google.c=
om> wrote:
> >
> [...]
> > > > +     if (atomic_xchg(&stats_flush_ongoing, 1))
> > >
> > > Have you profiled this? I wonder if we should replace the above with
> > >
> > >         if (atomic_read(&stats_flush_ongoing) || atomic_xchg(&stats_f=
lush_ongoing, 1))
> >
> > I profiled the entire series with perf and I haven't noticed a notable
> > difference between before and after the patch series -- but maybe some
> > specific access patterns cause a regression, not sure.
> >
> > Does an atomic_cmpxchg() satisfy the same purpose? it's easier to read
> > / more concise I guess.
> >
> > Something like
> >
> >     if (atomic_cmpxchg(&stats_flush_ongoing, 0, 1))
> >
> > WDYT?
> >
>
> No, I don't think cmpxchg will be any different from xchg(). On x86,
> the cmpxchg will always write to stats_flush_ongoing and depending on
> the comparison result, it will either be 0 or 1 here.
>
> If you see the implementation of queued_spin_trylock(), it does the
> same as well.

Interesting. I thought cmpxchg by definition will compare first and
only do the write if stats_flush_ongoing =3D=3D 0 in this case.

I thought queued_spin_trylock() was doing an atomic_read() first to
avoid the LOCK instruction unnecessarily the lock is held by someone
else.