Received-SPF: pass (google.com: domain of linux-kernel-owner@vger.kernel.org designates 2620:137:e000::1:20 as permitted sender) client-ip=2620:137:e000::1:20;
MIME-Version: 1.0
References: <20230328061638.203420-1-yosryahmed@google.com>
 <20230328061638.203420-6-yosryahmed@google.com> <20230328141523.txyhl7wt7wtvssea@google.com>
 <CAJD7tkYo=CeXJPUi_KxjzC0QCxC2qd_J2_FQi_aXh7svD8u60A@mail.gmail.com>
 <CALvZod4Gsngc6MjXdk4s5+ePVjsgcVppdRmsQovN6gSrxzdbfA@mail.gmail.com> <CAJD7tkb_YA3fvo3LgCzR+X-b-r-AmAR68hNR=xT7B6TJfBa54A@mail.gmail.com>
In-Reply-To: <CAJD7tkb_YA3fvo3LgCzR+X-b-r-AmAR68hNR=xT7B6TJfBa54A@mail.gmail.com>
From:   Yosry Ahmed <yosryahmed@google.com>
Date:   Tue, 28 Mar 2023 12:42:30 -0700
Message-ID: <CAJD7tkYbZKikn8VEzuuCL9pPiD_PX1gXdK7r9v8q-skKPBYf2w@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
Content-Type: text/plain; charset="UTF-8"
Content-Transfer-Encoding: quoted-printable
Precedence: bulk

On Tue, Mar 28, 2023 at 12:34=E2=80=AFPM Yosry Ahmed <yosryahmed@google.com=
> wrote:
>
> On Tue, Mar 28, 2023 at 12:28=E2=80=AFPM Shakeel Butt <shakeelb@google.co=
m> wrote:
> >
> > On Tue, Mar 28, 2023 at 11:53=E2=80=AFAM Yosry Ahmed <yosryahmed@google=
.com> wrote:
> > >
> > [...]
> > > > > +     if (atomic_xchg(&stats_flush_ongoing, 1))
> > > >
> > > > Have you profiled this? I wonder if we should replace the above wit=
h
> > > >
> > > >         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 notabl=
e
> > > difference between before and after the patch series -- but maybe som=
e
> > > specific access patterns cause a regression, not sure.
> > >
> > > Does an atomic_cmpxchg() satisfy the same purpose? it's easier to rea=
d
> > > / 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.

Anyway, perhaps it's better to follow what queued_spin_trylock() is
doing, even if only to avoid locking the cache line unnecessarily.

(Although now that I think about it, I wonder why atomic_cmpxchg
doesn't do this by default, food for thought)