linux-kernel - Re: [RFC PATCH 1/7] cgroup: rstat: only disable interrupts for the percpu lock

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Date:   Fri, 24 Mar 2023 21:46:05 -0700
From:   Shakeel Butt <shakeelb@...gle.com>
To:     Yosry Ahmed <yosryahmed@...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>,
        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: [RFC PATCH 1/7] cgroup: rstat: only disable interrupts for the
 percpu lock

On Fri, Mar 24, 2023 at 9:37 PM Yosry Ahmed <yosryahmed@...gle.com> wrote:
>
> On Fri, Mar 24, 2023 at 9:31 PM Shakeel Butt <shakeelb@...gle.com> wrote:
> >
> > On Fri, Mar 24, 2023 at 7:18 PM Yosry Ahmed <yosryahmed@...gle.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.