Re: [RFC PATCHC 3/3] sched/fair: use the idle state info to choose the idlest cpu

[Daniel Lezcano]

On 04/18/2014 02:53 PM, Peter Zijlstra wrote:
> On Fri, Apr 18, 2014 at 02:13:48PM +0200, Daniel Lezcano wrote:
> > On 04/18/2014 11:38 AM, Peter Zijlstra wrote:
> > > On Thu, Apr 17, 2014 at 12:21:28PM -0400, Nicolas Pitre wrote:
> > > > CPU topology is needed to properly describe scheduling domains.  Whether
> > > > we balance across domains or pack using as few domains as possible is a
> > > > separate issue.  In other words, you shouldn't have to care in this
> > > > patch series.
> > > >
> > > > And IMHO coupled C-state is a low-level mechanism that should remain
> > > > private to cpuidle which the scheduler shouldn't be aware of.
> > >
> > > I'm confused.. why wouldn't you want to expose these?
> >
> > The couple C-state is used as a mechanism for cpuidle to sync the cpus when
> > entering a specific c-state. This mechanism is usually used to handle the
> > cluster power down. It is only used for a two drivers (soon three) but it is
> > not the only mechanism used for syncing the cpus. There are also the MCPM
> > (tc2), the hand made sync when the hardware allows it (ux500), and an
> > abstraction from the firmware (mwait), transparent to the kernel.
> >
> > Taking into account the couple c-state only does not make sense because of
> > the other mechanisms above. This is why it should stay inside the cpuidle
> > framework.
> >
> > The extension of the cpu topology will provide a generic way to describe and
> > abstracting such dependencies.
> >
> > Does it answer your question ?
>
> I suppose so; its still a bit like we won't but we will :-)
>
> So we _will_ actually expose coupled C states through the topology bits,
> that's good.

Ah, ok. I think I understood where the confusion is coming from.

A couple of definitions for the same thing :)

1. Coupled C-states : *mechanism* implemented in the cpuidle framework: 
drivers/cpuidle/coupled.c

2. Coupled C-states : *constraint* to reach a cluster power down state, 
will be described through the topology and could be implemented by 
different mechanism (MCPM, handmade sync, cpuidle-coupled-c-state, 
firmware).

We want to expose 2. not 1. to the scheduler.



--
 <http://www.linaro.org/> Linaro.org â Open source software for ARM SoCs

Follow Linaro:  <http://www.facebook.com/pages/Linaro> Facebook |
<http://twitter.com/#!/linaroorg> Twitter |
<http://www.linaro.org/linaro-blog/> Blog

--
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@xxxxxxxxxxxxxxx
More majordomo info at  http://vger.kernel.org/majordomo-info.html
Please read the FAQ at  http://www.tux.org/lkml/