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 04:29:30 +0800
From:	Yuyang Du <yuyang.du@...el.com>
To:	Dirk Brandewie <dirk.brandewie@...il.com>
Cc:	Peter Zijlstra <peterz@...radead.org>,
	"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 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. Actually, silicon supports indepdent non-Turbo pstate, but just not enabled.
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 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.
> 
> 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.

Thanks,
Yuyang
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