Re: [PATCH V4 2/3] tick/cpuidle: Initialize hrtimer mode of broadcast

[Daniel Lezcano]

On 02/11/2014 04:58 PM, Thomas Gleixner wrote:
> On Tue, 11 Feb 2014, Daniel Lezcano wrote:
> > On 02/07/2014 09:06 AM, Preeti U Murthy wrote:
> > Setting the smp affinity on the earliest timer should be handled automatically
> > with the CLOCK_EVT_FEAT_DYNIRQ flag. Did you look at using this flag ?
>
> How should this flag help? Not at all, because the hrtimer based
> broadcast device cannot assign affinities.
>
> > Another comment is the overall approach. We enter the cpuidle idle framework
> > with a specific state to go to and it is the tick framework telling us we
> > mustn't go to this state. IMO the logic is wrong, the decision to not enter
> > this state should be moved somewhere else.
> >
> > Why don't you create a cpuidle driver with the shallow idle states assigned to
> > a cpu (let's say cpu0) and another one with all the deeper idle states for the
> > rest of the cpus ? Using the multiple cpuidle driver support makes it
> > possible. The timer won't be moving around and a cpu will be dedicated to act
> > as the broadcast timer.
> >
> > Wouldn't make sense and be less intrusive than the patchset you proposed ?
>
> How do you arm the broadcast timer on CPU0 from CPU1? You can't!
>
> You cannot access the cpu local timer on a different cpu. So you would
> have to send an IPI over to CPU0 so that it can reevaluate and
> schedule the broadcast. That's even more backwards than telling the
> cpuidle code that the CPU is not in a state to go deep.

Indeed :)

Thanks for the clarification.

  -- Daniel

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