linux-kernel - Re: [PATCH v14 1/6] sched/core: uclamp: Extend CPU's cgroup controller

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Message-ID: <20190903085248.GB8756@blackbody.suse.cz>
Date:   Tue, 3 Sep 2019 10:52:48 +0200
From:   Michal Koutný <mkoutny@...e.com>
To:     Suren Baghdasaryan <surenb@...gle.com>
Cc:     Patrick Bellasi <patrick.bellasi@....com>,
        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>,
        Todd Kjos <tkjos@...gle.com>,
        Peter Zijlstra <peterz@...radead.org>,
        "Rafael J . Wysocki" <rafael.j.wysocki@...el.com>,
        Tejun Heo <tj@...nel.org>,
        Vincent Guittot <vincent.guittot@...aro.org>,
        Viresh Kumar <viresh.kumar@...aro.org>,
        Juri Lelli <juri.lelli@...hat.com>,
        Ingo Molnar <mingo@...hat.com>,
        cgroups mailinglist <cgroups@...r.kernel.org>,
        linux-api@...r.kernel.org, LKML <linux-kernel@...r.kernel.org>,
        linux-pm@...r.kernel.org
Subject: Re: [PATCH v14 1/6] sched/core: uclamp: Extend CPU's cgroup
 controller

On Mon, Sep 02, 2019 at 04:02:57PM -0700, Suren Baghdasaryan <surenb@...gle.com> wrote:
> > +static inline void cpu_uclamp_print(struct seq_file *sf,
> > +                                   enum uclamp_id clamp_id)
> > [...]
> > +       rcu_read_lock();
> > +       tg = css_tg(seq_css(sf));
> > +       util_clamp = tg->uclamp_req[clamp_id].value;
> > +       rcu_read_unlock();
> > +
> > +       if (util_clamp == SCHED_CAPACITY_SCALE) {
> > +               seq_puts(sf, "max\n");
> > +               return;
> > +       }
> > +
> > +       percent = tg->uclamp_pct[clamp_id];
> 
> You are taking RCU lock when accessing tg->uclamp_req but not when
> accessing tg->uclamp_pct.
Good point.

> Is that intentional? Can tg be destroyed under you?
Actually, the rcu_read{,un}lock should be unnecessary in the context of
the kernfs file op handler -- the tg/css won't go away as long as its
kernfs file is being worked with.


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