Re: [PATCH 7/7] ARM: tegra30: cpuidle: add LP2 driver for CPU0

[Peter De Schrijver <pdeschrijver-DDmLM1+adcrQT0dZR+AlfA@public.gmane.org>]

On Thu, Oct 18, 2012 at 11:24:40AM +0200, Peter De Schrijver wrote:
> On Tue, Oct 16, 2012 at 07:03:43PM +0200, Stephen Warren wrote:
> > On 10/16/2012 02:06 AM, Peter De Schrijver wrote:
> > >>> Even though we have plan to use coupled cpuidle, I still prefer to go
> > >>> with the LP2 driver first. Then adding one more patch to support coupled
> > >>> cpuidle based on LP2 driver. This is good for history. And if there is
> > >>> any issue, it's more easy to roll back to the stable one.
> > >>
> > >> I don't think that implementing it one way and then changing to a
> > >> different way will benefit history at all. It'll make the history more
> > >> complicated. What exactly is the problem with just using coupled cpuidle
> > >> from the start? If we did merge this implementation now, then switch to
> > >> coupled cpuidle later, when do you think the switch would happen?
> > > 
> > > Before we consider doing this, I think we should have some idea on how
> > > frequently we run into the situation where CPU0 is idle but a secondary
> > > core is not. Depending on that we can then decide how useful coupled cpuidle
> > > would be for us.
> > 
> > Would it not be 75% of the time where we have 1 of 4 CPUs active? At
> > least, that's assuming that all work is evenly distributed amongst CPUs,
> > and hence it's random which CPU is the last to go idle, but perhaps
> > that's not the case if CPU0 is somehow special workload-wise?
> > 
> 
> Depends, at least it used to be possible to tune the scheduler to prefer
> CPU0 if the workload can run on a single CPU.
> 

I just noticed https://lwn.net/Articles/518834/. If we can configure this to
prefer CPU0 in case all work can be done on a single core, we shouldn't
hit the case were a secondary CPU is the only active CPU for a significant
period of time.

Cheers,

Peter.