linux-kernel - Re: regression caused by 08f511fd41c3 ("cpufreq: Reduce cpufreq_update

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Message-ID: <20160617140353.GQ30154@twins.programming.kicks-ass.net>
Date:	Fri, 17 Jun 2016 16:03:53 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
Cc:	"Rafael J. Wysocki" <rafael@...nel.org>,
	Jisheng Zhang <jszhang@...vell.com>,
	"Rafael J. Wysocki" <rjw@...ysocki.net>,
	Viresh Kumar <viresh.kumar@...aro.org>,
	"linux-pm@...r.kernel.org" <linux-pm@...r.kernel.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: regression caused by 08f511fd41c3 ("cpufreq: Reduce
 cpufreq_update_util() overhead a bit")

On Fri, Jun 17, 2016 at 06:16:51AM -0700, Paul E. McKenney wrote:
> > Paul, Peter, any ideas about what may be going on here?
> 
> Looks to me like this commit moved some code from synchronize_rcu() to
> synchronize_sched().  Assuming that this is a CONFIG_PREEMPT=y system,
> might there have been a decrease in the wakeups from the rcu_preempt
> kthread?

The 'funny' thing is though; those synchronize thingies are only reached
when we change cpufreq policy, so things like:

  for i in /sys/devices/system/cpu/cpu*/cpufreq/scaling_governor ; do echo performance > $i ; done

Something which is hardly possible when idle. Weird.