linux-kernel - Re: [PATCH v13 1/6] sched/core: uclamp: Extend CPU's cgroup controller

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Message-ID: <20190806161133.GA18532@blackbody.suse.cz>
Date:   Tue, 6 Aug 2019 18:11:34 +0200
From:   Michal Koutný <mkoutny@...e.com>
To:     Patrick Bellasi <patrick.bellasi@....com>
Cc:     linux-kernel@...r.kernel.org, linux-pm@...r.kernel.org,
        linux-api@...r.kernel.org, cgroups@...r.kernel.org,
        Ingo Molnar <mingo@...hat.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Tejun Heo <tj@...nel.org>,
        "Rafael J . Wysocki" <rafael.j.wysocki@...el.com>,
        Vincent Guittot <vincent.guittot@...aro.org>,
        Viresh Kumar <viresh.kumar@...aro.org>,
        Paul Turner <pjt@...gle.com>,
        Quentin Perret <quentin.perret@....com>,
        Dietmar Eggemann <dietmar.eggemann@....com>,
        Morten Rasmussen <morten.rasmussen@....com>,
        Juri Lelli <juri.lelli@...hat.com>,
        Todd Kjos <tkjos@...gle.com>,
        Joel Fernandes <joelaf@...gle.com>,
        Steve Muckle <smuckle@...gle.com>,
        Suren Baghdasaryan <surenb@...gle.com>,
        Alessio Balsini <balsini@...roid.com>
Subject: Re: [PATCH v13 1/6] sched/core: uclamp: Extend CPU's cgroup
 controller

On Fri, Aug 02, 2019 at 10:08:48AM +0100, Patrick Bellasi <patrick.bellasi@....com> wrote:
> +static ssize_t cpu_uclamp_write(struct kernfs_open_file *of, char *buf,
> +				size_t nbytes, loff_t off,
> +				enum uclamp_id clamp_id)
> +{
> +	struct uclamp_request req;
> +	struct task_group *tg;
> +
> +	req = capacity_from_percent(buf);
> +	if (req.ret)
> +		return req.ret;
> +
> +	rcu_read_lock();
This should be the uclamp_mutex.

(The compound results of the series is correct as the lock is introduced
in "sched/core: uclamp: Propagate parent clamps".
This is just for the happiness of cherry-pickers/bisectors.)

> +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();
Why is the rcu_read_lock() needed here? (I'm considering the comment in
of_css() that should apply here (and it seems that similar uses in other
seq_file handlers also skip this).)

> @@ -7369,6 +7506,20 @@ static struct cftype cpu_legacy_files[] = {
> [...]
> +		.name = "uclamp.min",
> [...]
> +		.name = "uclamp.max",
I don't see technical reasons why uclamp couldn't work on legacy
hierarchy and Tejun acked the series, despite that I'll ask -- should
the new attributes be exposed in v1 controller hierarchy (taking into
account the legacy API is sort of frozen and potential maintenance needs
spanning both hierarchies)?