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

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Message-Id: <20160617131651.GU3923@linux.vnet.ibm.com>
Date:	Fri, 17 Jun 2016 06:16:51 -0700
From:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:	"Rafael J. Wysocki" <rafael@...nel.org>
Cc:	Jisheng Zhang <jszhang@...vell.com>,
	Peter Zijlstra <peterz@...radead.org>,
	"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 03:09:36PM +0200, Rafael J. Wysocki wrote:
> On Fri, Jun 17, 2016 at 10:40 AM, Jisheng Zhang <jszhang@...vell.com> wrote:
> > Dear all,
> >
> > On Fri, 17 Jun 2016 16:30:23 +0800 Jisheng Zhang wrote:
> >
> >> Dear all,
> >>
> >> I found one regression: In an idle system, wakeups/s (reported by powertop)
> >> is increased a lot, e.g on a intel snb 4 core platform, the wakeup event
> >> number is increased from 8 wakeups/s to 24 wakeup/s. bisect points to
> >> this commit. I could send detailed bisect log if it's wanted.
> >>
> >
> > more information maybe useful: after the commit, the top two wakeup source
> > are
> >
> >         Process        [rcu_sched]
> >
> >         Timer          tick_sched_timer
> 
> And what was there before the commit?
> 
> Granted, I'm not seeing this on my systems.
> 
> 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?

							Thanx, Paul