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

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Message-ID: <20140418125344.GX11182@twins.programming.kicks-ass.net>
Date:	Fri, 18 Apr 2014 14:53:44 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	Daniel Lezcano <daniel.lezcano@...aro.org>
Cc:	Nicolas Pitre <nicolas.pitre@...aro.org>,
	linux-kernel@...r.kernel.org, mingo@...e.hu, rjw@...ysocki.net,
	linux-pm@...r.kernel.org, alex.shi@...aro.org,
	vincent.guittot@...aro.org, morten.rasmussen@....com
Subject: Re: [RFC PATCHC 3/3] sched/fair: use the idle state info to choose
 the idlest cpu

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.
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