linux-kernel - Re: [RFC PATCH 06/16] arm: topology: Define TC2 sched energy and provide it to scheduler

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Date:	Thu, 5 Jun 2014 08:52:05 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	"Rafael J. Wysocki" <rjw@...ysocki.net>
Cc:	Morten Rasmussen <morten.rasmussen@....com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"linux-pm@...r.kernel.org" <linux-pm@...r.kernel.org>,
	"mingo@...nel.org" <mingo@...nel.org>,
	"vincent.guittot@...aro.org" <vincent.guittot@...aro.org>,
	"daniel.lezcano@...aro.org" <daniel.lezcano@...aro.org>,
	"preeti@...ux.vnet.ibm.com" <preeti@...ux.vnet.ibm.com>,
	Dietmar Eggemann <Dietmar.Eggemann@....com>,
	len.brown@...el.com
Subject: Re: [RFC PATCH 06/16] arm: topology: Define TC2 sched energy and
 provide it to scheduler

On Wed, Jun 04, 2014 at 11:56:55PM +0200, Rafael J. Wysocki wrote:
> On Wednesday, June 04, 2014 07:27:12 PM Peter Zijlstra wrote:

> > Well, we eventually want to go there I think. Although we still needed
> > to come up with something for Intel, because I'm not at all sure how all
> > that works.
> 
> Do you mean power numbers or how P-states work on Intel in general?

P-states, I'm still not at all sure how all that works on Intel and what
we can sanely do with them.

Supposedly Intel has a means of setting P-states (there's a driver after
all), but then is completely free to totally ignore it and do something
entirely different anyhow.

And while APERF/MPERF allows observing what it did, its afaik, nigh on
impossible to predict wtf its going to do, and therefore any such energy
computation is going to be a PRNG at best.

Now, given all that I'm not sure what we need that P-state driver for,
so supposedly I'm missing something.

Ideally Len (or someone equally in-the-know) would explain to me how
exactly all that works and what we can rely upon. All I've gotten so far
is, you can't rely on anything, and magik. Which is entirely useless.

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