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From:   Suren Baghdasaryan <surenb@google.com>
Date:   Tue, 18 May 2021 11:23:04 -0700
Message-ID: <CAJuCfpHXhncetUziX=RS0kYTEQKHkeNXZA-hnz26bKkEJrHYzA@mail.gmail.com>
Subject: Re: [PATCH v2 1/1] cgroup: make per-cgroup pressure stall tracking configurable
To:     Shakeel Butt <shakeelb@google.com>
Cc:     Tejun Heo <tj@kernel.org>, Johannes Weiner <hannes@cmpxchg.org>,
        Zefan Li <lizefan.x@bytedance.com>,
        Ingo Molnar <mingo@redhat.com>,
        "Peter Zijlstra (Intel)" <peterz@infradead.org>,
        Juri Lelli <juri.lelli@redhat.com>,
        Vincent Guittot <vincent.guittot@linaro.org>,
        Dietmar Eggemann <dietmar.eggemann@arm.com>,
        Steven Rostedt <rostedt@goodmis.org>,
        Benjamin Segall <bsegall@google.com>,
        Mel Gorman <mgorman@suse.de>, Minchan Kim <minchan@kernel.org>,
        Jonathan Corbet <corbet@lwn.net>,
        Daniel Bristot de Oliveira <bristot@redhat.com>,
        "Paul E . McKenney" <paulmck@kernel.org>,
        Randy Dunlap <rdunlap@infradead.org>,
        Andrew Morton <akpm@linux-foundation.org>,
        Thomas Gleixner <tglx@linutronix.de>, macro@orcam.me.uk,
        Viresh Kumar <viresh.kumar@linaro.org>,
        Mike Kravetz <mike.kravetz@oracle.com>,
        linux-doc@vger.kernel.org, LKML <linux-kernel@vger.kernel.org>,
        Cgroups <cgroups@vger.kernel.org>,
        kernel-team <kernel-team@android.com>
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On Tue, May 18, 2021 at 11:08 AM Shakeel Butt <shakeelb@google.com> wrote:
>
> On Mon, May 17, 2021 at 7:02 PM Suren Baghdasaryan <surenb@google.com> wrote:
> >
> > PSI accounts stalls for each cgroup separately and aggregates it at each
> > level of the hierarchy. This causes additional overhead with psi_avgs_work
> > being called for each cgroup in the hierarchy. psi_avgs_work has been
> > highly optimized, however on systems with large number of cgroups the
> > overhead becomes noticeable.
> > Systems which use PSI only at the system level could avoid this overhead
> > if PSI can be configured to skip per-cgroup stall accounting.
> > Add "cgroup_disable=pressure" kernel command-line option to allow
> > requesting system-wide only pressure stall accounting. When set, it
> > keeps system-wide accounting under /proc/pressure/ but skips accounting
> > for individual cgroups and does not expose PSI nodes in cgroup hierarchy.
> >
> > Signed-off-by: Suren Baghdasaryan <surenb@google.com>
>
> I am assuming that this is for Android and at the moment Android is
> only interested in system level pressure. I am wondering if there is
> any plan for Android to have cgroup hierarchies with explicit limits
> in future?

Correct and yes, we would like to use memcgs to limit memory in the
future, however we do not plan on using per-cgroup psi so far.

>
> If yes, then I think we should follow up (this patch is fine
> independently) with making this feature more general by explicitly
> enabling psi for each cgroup level similar to how we enable
> controllers through cgroup.subtree_control.
>
> Something like:
>
> $ echo "+psi" > cgroup.subtree_control
>
> This definitely would be helpful for server use cases where jobs do
> sub-containers but might not be interested in psi but the admin is
> interested in the top level job's psi.

Haven't thought about it before but that makes sense to me.