Re: power-efficient scheduling design

[Catalin Marinas <catalin.marinas@arm.com>]

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