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

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Message-ID: <20160119192111.GC6373@twins.programming.kicks-ass.net>
Date:	Tue, 19 Jan 2016 20:21:11 +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 08:17:34PM +0100, Peter Zijlstra wrote:
> 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();

That should obviously be:

	d = rcu_dereference(driver);
	if (d) {
		foo = __this_cpu_ptr(d->data);

> 		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;
> }
> 
> 
>