linux-kernel - Re: [RFCv7 PATCH 03/10] sched: scheduler-driven cpu frequency selection

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Message-ID: <20160301131724.GU6357@twins.programming.kicks-ass.net>
Date:	Tue, 1 Mar 2016 14:17:24 +0100
From:	Peter Zijlstra <peterz@...radead.org>
To:	Steve Muckle <steve.muckle@...aro.org>
Cc:	"Rafael J. Wysocki" <rjw@...ysocki.net>,
	Ingo Molnar <mingo@...hat.com>,
	"Rafael J. Wysocki" <rafael@...nel.org>,
	linux-kernel@...r.kernel.org, linux-pm@...r.kernel.org,
	Vincent Guittot <vincent.guittot@...aro.org>,
	Morten Rasmussen <morten.rasmussen@....com>,
	Dietmar Eggemann <dietmar.eggemann@....com>,
	Juri Lelli <Juri.Lelli@....com>,
	Patrick Bellasi <patrick.bellasi@....com>,
	Michael Turquette <mturquette@...libre.com>,
	Ricky Liang <jcliang@...omium.org>
Subject: Re: [RFCv7 PATCH 03/10] sched: scheduler-driven cpu frequency
 selection

On Thu, Feb 25, 2016 at 04:34:23PM -0800, Steve Muckle wrote:
> >> +	/*
> >> +	 * Ensure that all CPUs currently part of this policy are out
> >> +	 * of the hot path so that if this policy exits we can free gd.
> >> +	 */
> >> +	preempt_disable();
> >> +	smp_call_function_many(policy->cpus, dummy, NULL, true);
> >> +	preempt_enable();
> > 
> > I'm not sure how this works, can you please tell me?
> 
> Peter correctly interpreted my intentions.
> 
> The RCU possibility also crossed my mind. They both seemed like a bit of
> a hack to me - this wouldn't really be doing any RCU per se, rather
> relying on its implementation. I'll switch to RCU since that seems to be
> preferred though. It's certainly cleaner to write.

RCU is widely used in this fashion. synchronize_sched() is explicitly
constructed to sync against preempt disable sections. This is not an
implementation detail.