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

From: Rafael J. Wysocki
Date: Thu Mar 31 2016 - 08:32:53 EST

On Thu, Mar 31, 2016 at 2:24 PM, Peter Zijlstra <peterz@xxxxxxxxxxxxx> 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.