linux-kernel - Re: [PATCH v1 5/9] memcg: replace stats_flush

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Message-ID: <CAJD7tkb_YA3fvo3LgCzR+X-b-r-AmAR68hNR=xT7B6TJfBa54A@mail.gmail.com>
Date:   Tue, 28 Mar 2023 12:34:15 -0700
From:   Yosry Ahmed <yosryahmed@...gle.com>
To:     Shakeel Butt <shakeelb@...gle.com>
Cc:     Tejun Heo <tj@...nel.org>, Josef Bacik <josef@...icpanda.com>,
        Jens Axboe <axboe@...nel.dk>,
        Zefan Li <lizefan.x@...edance.com>,
        Johannes Weiner <hannes@...xchg.org>,
        Michal Hocko <mhocko@...nel.org>,
        Roman Gushchin <roman.gushchin@...ux.dev>,
        Muchun Song <muchun.song@...ux.dev>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Michal Koutný <mkoutny@...e.com>,
        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: [PATCH v1 5/9] memcg: replace stats_flush_lock with an atomic

On Tue, Mar 28, 2023 at 12:28 PM Shakeel Butt <shakeelb@...gle.com> wrote:
>
> On Tue, Mar 28, 2023 at 11:53 AM Yosry Ahmed <yosryahmed@...gle.com> 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_flush_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 == 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.