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

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Date:	Fri, 6 Jun 2014 10:05:43 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	Yuyang Du <yuyang.du@...el.com>
Cc:	Dirk Brandewie <dirk.brandewie@...il.com>,
	"Rafael J. Wysocki" <rjw@...ysocki.net>,
	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 Fri, Jun 06, 2014 at 04:29:30AM +0800, Yuyang Du wrote:
> On Thu, Jun 05, 2014 at 08:03:15AM -0700, Dirk Brandewie wrote:
> > 
> > You can request a P state per core but the package does coordination at
> > a package level for the P state that will be used based on all requests.
> > This is due to the fact that most SKUs have a single VR and PLL. So
> > the highest P state wins.  When a core goes idle it loses it's vote
> > for the current package P state and that cores clock it turned off.
> > 
> 
> You need to differentiate Turbo and non-Turbo. The highest P state wins? Not
> really.

*sigh* and here we go again.. someone please, write something coherent
and have all intel people sign off on it and stop saying different
things.

> Actually, silicon supports indepdent non-Turbo pstate, but just not enabled.

Then it doesn't exist, so no point in mentioning it.

> For Turbo, it basically depends on power budget of both core and gfx (because
> they share) for each core to get which Turbo point.

And RAPL controls can give preference of which gfx/core gets most,
right?

> > intel_pstate tries to keep the core P state as low as possible to satisfy
> > the given load, so when various cores go idle the package P state can be
> > as low as possible.  The big power win is a core going idle.
> > 
> 
> In terms of prediction, it is definitely can't be 100% right. But the
> performance of most workloads does scale with pstate (frequency), may not be
> linearly. So it is to some point predictable FWIW. And this is all governors
> and Intel_pstate's basic assumption.

So frequency isn't _that_ interesting, voltage is. And while
predictability it might be their assumption, is it actually true? I
mean, there's really nothing else except to assume that, if its not you
can't do anything at all, so you _have_ to assume this.

But again, is the assumption true? Or just happy thoughts in an attempt
to do something.

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