linux-kernel - Re: [PATCH v13 2/6] sched/core: uclamp: Propagate parent clamps

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Date:   Thu, 8 Aug 2019 19:16:21 +0200
From:   Michal Koutný <mkoutny@...e.com>
To:     Patrick Bellasi <patrick.bellasi@....com>
Cc:     Alessio Balsini <balsini@...roid.com>,
        Dietmar Eggemann <dietmar.eggemann@....com>,
        Morten Rasmussen <morten.rasmussen@....com>,
        Quentin Perret <quentin.perret@....com>,
        Joel Fernandes <joelaf@...gle.com>,
        Paul Turner <pjt@...gle.com>,
        Steve Muckle <smuckle@...gle.com>,
        Suren Baghdasaryan <surenb@...gle.com>,
        Todd Kjos <tkjos@...gle.com>,
        Peter Zijlstra <peterz@...radead.org>,
        "Rafael J . Wysocki" <rafael.j.wysocki@...el.com>,
        Tejun Heo <tj@...nel.org>,
        VincentGuittot <vincent.guittot@...aro.org>,
        Viresh Kumar <viresh.kumar@...aro.org>,
        Juri Lelli <juri.lelli@...hat.com>,
        Ingo Molnar <mingo@...hat.com>, cgroups@...r.kernel.org,
        linux-api@...r.kernel.org, linux-kernel@...r.kernel.org,
        linux-pm@...r.kernel.org
Subject: Re: [PATCH v13 2/6] sched/core: uclamp: Propagate parent clamps

On Thu, Aug 08, 2019 at 04:08:10PM +0100, Patrick Bellasi <patrick.bellasi@....com> wrote:
> Well, if I've got correctly your comment in the previous message, I
> would say that at this stage we don't need RCU looks at all.
Agreed.

> Reason being that cpu_util_update_eff() gets called only from
> cpu_uclamp_write() which is from an ongoing write operation on a cgroup
> attribute and thus granted to be available.
> 
> We will eventually need to move the RCU look only down the stack when
> uclamp_update_active_tasks() gets called to update the RUNNABLE tasks on
> a RQ... or perhaps we don't need them since we already get the
> task_rq_lock() for each task we visit.
Unless you remove css_for_each_descendant_pre() in
cpu_util_update_eff(), the rcu_read_lock() cannot go below it.
(You'd be RCU-accessing other csses that aren't pinned in the write.)

Michal

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