linux-kernel - Re: power-efficient scheduling design

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Message-ID: <20130624100732.GB14982@arm.com>
Date:	Mon, 24 Jun 2013 11:07:32 +0100
From:	Catalin Marinas <catalin.marinas@....com>
To:	Benjamin Herrenschmidt <benh@...nel.crashing.org>
Cc:	Arjan van de Ven <arjan@...ux.intel.com>,
	Morten Rasmussen <Morten.Rasmussen@....com>,
	David Lang <david@...g.hm>,
	"len.brown@...el.com" <len.brown@...el.com>,
	"alex.shi@...el.com" <alex.shi@...el.com>,
	"corbet@....net" <corbet@....net>,
	"peterz@...radead.org" <peterz@...radead.org>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	"efault@....de" <efault@....de>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"linaro-kernel@...ts.linaro.org" <linaro-kernel@...ts.linaro.org>,
	"preeti@...ux.vnet.ibm.com" <preeti@...ux.vnet.ibm.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	"pjt@...gle.com" <pjt@...gle.com>, Ingo Molnar <mingo@...nel.org>
Subject: Re: power-efficient scheduling design

On Mon, Jun 24, 2013 at 12:32:00AM +0100, Benjamin Herrenschmidt wrote:
> On Fri, 2013-06-21 at 14:34 -0700, Arjan van de Ven wrote:
> > On 6/21/2013 2:23 PM, Catalin Marinas wrote:
> > >>
> > >> oops sorry I misread your mail (lack of early coffee I suppose)
> > >>
> > >> I can see your point of having a thing for "did we ask for all the performance
> > >> we could ask for" prior to doing a load balance (although, for power efficiency,
> > >> if you have two tasks that could run in parallel, it's usually better to
> > >> run them in parallel... so likely we should balance anyway)
> > >
> > > Not necessarily, especially if parallel running implies powering up a
> > > full cluster just for one CPU (it depends on the hardware but for
> > > example a cluster may not be able to go in deeper sleep states unless
> > > all the CPUs in that cluster are idle).
> > 
> > I guess it depends on the system
> 
> Sort-of. We have something similar with threads on ppc. IE, the core can
> only really stop if all threads are. From a Linux persepctive it's a
> matter of how we define the scope of that 'cluster' Catalin is talking
> about. I'm sure you do too.
> 
> Then there is the package, which adds MC etc...

I think we can say cluster == package so that we use some common
terminology. On a big.little configuration (TC2), we have 3xA7 in one
package and 2xA15 in the other. So to efficiently stop an entire package
(cluster, multi-core etc.) we need to stop all the CPUs it has.

-- 
Catalin
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