linux-kernel - Re: [PATCH v6 7/7][Resend] cpufreq: schedutil: New governor based on scheduler utilization data

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Message-ID: <CAJZ5v0j6eDmvVSBphvygA2Xnk9nQXqracFR+dBYz3UOeyrb7qA@mail.gmail.com>
Date:	Thu, 31 Mar 2016 14:32:44 +0200
From:	"Rafael J. Wysocki" <rafael@...nel.org>
To:	Peter Zijlstra <peterz@...radead.org>
Cc:	Steve Muckle <steve.muckle@...aro.org>,
	"Rafael J. Wysocki" <rafael@...nel.org>,
	"Rafael J. Wysocki" <rjw@...ysocki.net>,
	Linux PM list <linux-pm@...r.kernel.org>,
	Juri Lelli <juri.lelli@....com>,
	ACPI Devel Maling List <linux-acpi@...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>,
	Vincent Guittot <vincent.guittot@...aro.org>,
	Michael Turquette <mturquette@...libre.com>,
	Ingo Molnar <mingo@...nel.org>
Subject: Re: [PATCH v6 7/7][Resend] cpufreq: schedutil: New governor based on
 scheduler utilization data

On Thu, Mar 31, 2016 at 2:24 PM, Peter Zijlstra <peterz@...radead.org> wrote:
> On Mon, Mar 28, 2016 at 11:17:44AM -0700, Steve Muckle wrote:
>> The scenario I'm contemplating is that while a CPU-intensive task is
>> running a thermal interrupt goes off. The driver for this thermal
>> interrupt responds by capping fmax. If this happens just after the tick,
>> it seems possible that we could wait a full tick before changing the
>> frequency. Given a 10ms tick it could be rather annoying for thermal
>> management algorithms on some platforms (I'm familiar with a few).
>
> So I'm blissfully unaware of all the thermal stuffs we have; but it
> looks like its somehow bolten onto cpufreq without feedback.
>
> The thing I worry about is thermal scaling the CPU back past where RT/DL
> tasks can still complete in time. It should not be able to do that, or
> rather, missing deadlines because thermal is about as useful as
> rebooting the device.

Right.  If thermal throttling kicks in, the game is pretty much over.

That's why ideas float about taking the thermal constraints into
account upfront, but that's a different discussion entirely.

> I guess I'm saying is, the whole cpufreq/thermal 'interface' needs work
> anyhow.

Yes, it does.