linux-kernel - Re: [PATCH 0/3] cpufreq: Replace timers with utilization update callbacks

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Message-ID: <20160212141009.GT6357@twins.programming.kicks-ass.net>
Date:	Fri, 12 Feb 2016 15:10:09 +0100
From:	Peter Zijlstra <peterz@...radead.org>
To:	Steve Muckle <steve.muckle@...aro.org>
Cc:	"Rafael J. Wysocki" <rjw@...ysocki.net>,
	"Rafael J. Wysocki" <rafael@...nel.org>,
	Linux PM list <linux-pm@...r.kernel.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Srinivas Pandruvada <srinivas.pandruvada@...ux.intel.com>,
	Viresh Kumar <viresh.kumar@...aro.org>,
	Juri Lelli <juri.lelli@....com>,
	Thomas Gleixner <tglx@...utronix.de>
Subject: Re: [PATCH 0/3] cpufreq: Replace timers with utilization update
 callbacks

On Thu, Feb 11, 2016 at 10:52:20AM -0800, Steve Muckle wrote:
> On 02/11/2016 09:30 AM, Peter Zijlstra wrote:
> >> My concern above is that pokes are guaranteed to keep occurring when
> >> > there is only RT or DL activity so nothing breaks.
> >
> > The hook in their respective tick handler should ensure stuff is called
> > sporadically and isn't stalled.
> 
> But that's only true if the RT/DL tasks happen to be running when the
> tick arrives right?
> 
> Couldn't we have RT/DL activity which doesn't overlap with the tick? And
> if no CFS tasks happen to be executing on that CPU, we'll never trigger
> the cpufreq update. This could go on for an arbitrarily long time
> depending on the periodicity of the work.

Possible yes, but why do we care? Such a CPU would be so much idle that
cpufreq doesn't matter one way or another, right?