linux-kernel - Re: [RFC PATCH 18/19] cpufreq: remove transition

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Date:	Tue, 19 Jan 2016 20:17:34 +0100
From:	Peter Zijlstra <peterz@...radead.org>
To:	Juri Lelli <juri.lelli@....com>
Cc:	Michael Turquette <mturquette@...libre.com>,
	Viresh Kumar <viresh.kumar@...aro.org>,
	linux-kernel@...r.kernel.org, linux-pm@...r.kernel.org,
	rjw@...ysocki.net, steve.muckle@...aro.org,
	vincent.guittot@...aro.org, morten.rasmussen@....com,
	dietmar.eggemann@....com
Subject: Re: [RFC PATCH 18/19] cpufreq: remove transition_lock

On Tue, Jan 19, 2016 at 04:01:55PM +0000, Juri Lelli wrote:
> Right, read path is fast, but write path still requires some sort of
> locking (malloc, copy and update). So, I'm wondering if this still pays
> off for a structure that gets written a lot.

No, not at all.

struct cpufreq_driver *driver;

void sched_util_change(unsigned int util)
{
	struct my_per_cpu_data *foo;

	rcu_read_lock();
	foo = __this_cpu_ptr(rcu_dereference(driver)->data);
	if (foo) {
		if (abs(util - foo->last_util) > 10) {
			foo->last_util = util;
			foo->set_util(util);
		}
	}
	rcu_read_unlock();
}


struct cpufreq_driver *cpufreq_flip_driver(struct cpufreq_driver *new_driver)
{
	struct cpufreq_driver *old_driver;

	mutex_lock(&cpufreq_driver_lock);
	old_driver = driver;
	rcu_assign_driver(driver, new_driver);
	if (old_driver)
		synchronize_rcu();
	mutex_unlock(&cpufreq_driver_lock);

	return old_driver;
}