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From:   Shakeel Butt <shakeelb@google.com>
Date:   Fri, 24 Mar 2023 21:46:05 -0700
Message-ID: <CALvZod41ecuCKmuFBNtAjoKJjQgWYzoe4_B8zRK37HYk-rYDkA@mail.gmail.com>
Subject: Re: [RFC PATCH 1/7] cgroup: rstat: only disable interrupts for the
 percpu lock
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>,
        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 Fri, Mar 24, 2023 at 9:37=E2=80=AFPM Yosry Ahmed <yosryahmed@google.com>=
 wrote:
>
> On Fri, Mar 24, 2023 at 9:31=E2=80=AFPM Shakeel Butt <shakeelb@google.com=
> wrote:
> >
> > On Fri, Mar 24, 2023 at 7:18=E2=80=AFPM Yosry Ahmed <yosryahmed@google.=
com> wrote:
> > >
> > [...]
> > > Any ideas here are welcome!
> > >
> >
> > Let's move forward. It seems like we are not going to reach an
> > agreement on making cgroup_rstat_lock a non-irq lock. However there is
> > agreement on the memcg code of not flushing in irq context and the
> > cleanup Johannes has requested. Let's proceed with those for now. We
> > can come back to cgroup_rstat_lock later if we still see issues in
> > production.
>
> Even if we do not flush from irq context, we still flush from atomic
> contexts that will currently hold the lock with irqs disabled
> throughout the entire flush sequence. A primary purpose of this reason
> is to avoid that.
>
> We can either:
> (a) Proceed with the following approach of making cgroup_rstat_lock a
> non-irq lock.
> (b) Proceed with Tejun's suggestion of always releasing and
> reacquiring the lock at CPU boundaries, even for atomic flushes (if
> the spinlock needs a break ofc).
> (c) Something else.

(d) keep the status quo regarding cgroup_rstat_lock
(e) decouple the discussion of cgroup_rstat_lock from the agreed
improvements. Send the patches for the agreed ones and continue
discussing cgroup_rstat_lock.