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Date:   Thu, 8 Aug 2019 19:16:21 +0200
From:   Michal =?iso-8859-1?Q?Koutn=FD?= <mkoutny@suse.com>
To:     Patrick Bellasi <patrick.bellasi@arm.com>
Cc:     Alessio Balsini <balsini@android.com>,
        Dietmar Eggemann <dietmar.eggemann@arm.com>,
        Morten Rasmussen <morten.rasmussen@arm.com>,
        Quentin Perret <quentin.perret@arm.com>,
        Joel Fernandes <joelaf@google.com>,
        Paul Turner <pjt@google.com>,
        Steve Muckle <smuckle@google.com>,
        Suren Baghdasaryan <surenb@google.com>,
        Todd Kjos <tkjos@google.com>,
        Peter Zijlstra <peterz@infradead.org>,
        "Rafael J . Wysocki" <rafael.j.wysocki@intel.com>,
        Tejun Heo <tj@kernel.org>,
        VincentGuittot <vincent.guittot@linaro.org>,
        Viresh Kumar <viresh.kumar@linaro.org>,
        Juri Lelli <juri.lelli@redhat.com>,
        Ingo Molnar <mingo@redhat.com>, cgroups@vger.kernel.org,
        linux-api@vger.kernel.org, linux-kernel@vger.kernel.org,
        linux-pm@vger.kernel.org
Subject: Re: [PATCH v13 2/6] sched/core: uclamp: Propagate parent clamps
Message-ID: <20190808171620.GE8617@blackbody.suse.cz>
References: <20190802090853.4810-1-patrick.bellasi@arm.com>
 <20190802090853.4810-3-patrick.bellasi@arm.com>
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On Thu, Aug 08, 2019 at 04:08:10PM +0100, Patrick Bellasi <patrick.bellasi@=
arm.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.
>=20
> 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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