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

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Message-ID: <CAJWu+opnNT3bQwe+SsdR0Q+PSt7DA=JAQ_5sbZ6h2DXd4ZqwHA@mail.gmail.com>
Date:   Tue, 3 Sep 2019 10:21:59 -0400
From:   Joel Fernandes <joelaf@...gle.com>
To:     Michal Koutný <mkoutny@...e.com>
Cc:     Suren Baghdasaryan <surenb@...gle.com>,
        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>,
        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 <linux-pm@...r.kernel.org>
Subject: Re: [PATCH v14 1/6] sched/core: uclamp: Extend CPU's cgroup controller

On Tue, Sep 3, 2019 at 4:53 AM Michal Koutný <mkoutny@...e.com> wrote:
>
> 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.
>

Also, add to that the fact that there is no rcu_dereference() call to
access any of the pointers in the reader or any of its callers. And, I
don't see any "wait for completion" type of pattern here so that
rcu_read_{lock, unlock}() pair does seem useless.

thanks,

 - Joel