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

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Date:	Fri, 26 Feb 2016 10:18:43 +0100
From:	Peter Zijlstra <peterz@...radead.org>
To:	"Rafael J. Wysocki" <rjw@...ysocki.net>
Cc:	Steve Muckle <steve.muckle@...aro.org>,
	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 10:08:48PM +0100, Rafael J. Wysocki wrote:
> On Thursday, February 25, 2016 10:28:37 AM Peter Zijlstra wrote:
> > Its vile though; one should not spray IPIs if one can avoid it. Such
> > things are much better done with RCU. Sure sync_sched() takes a little
> > longer, but this isn't a fast path by any measure.
> 
> I see, thanks!
> 
> BTW, when cpufreq_update_util() callbacks are removed, I use synchronize_rcu()
> to wait for the running ones, but would it be better to use synchronize_sched()
> in there instead?

So I think we only call the callback with rq->lock held, in which case
sync_sched() is good enough.

It would allow you to get rid of the rcu_read_{,un}lock() calls as well.

The down-side is that it all makes the code a little harder to get,
because you're relying on caller context to DTRT.