Re: [PATCH 1/3] dynticks - implement no idle hz for x86

[Nish Aravamudan <nish.aravamudan@gmail.com>]

On 9/7/05, Bill Davidsen <davidsen@tmr.com> wrote:
> Srivatsa Vaddagiri wrote:
> > On Sun, Sep 04, 2005 at 10:48:13PM -0700, Nishanth Aravamudan wrote:
> >
> >>Admittedly, I don't think SMP ARM has been around all that long? Maybe
> >>the existing code just has not been extended.
> >
> >
> > Yeah, maybe ARM never cared for SMP. But we do care :)
> >
> >
> >>I'm not sure on this. It's going to be NULL for other architectures, or
> >>end up being called by the reprogram() call for the last CPU to go idle,
> >>right (presuming there isn't a separate TOD source, like in x86). I
> >>think it is better to be in the reprogram() interface.
> >
> >
> > Non-x86 could have it set to NULL, in which case it doesn't get called.
> > (I know the current code does not take care of this situation).
> > But having an explicit 'all_cpus_idle' interface may be good, since
> > Tony talked of idling some devices when all CPUs are idle. So it
> > probably has non-x86/PIT uses too.
> 
> If this is intended to reduce power, and it originally came from that
> root, then this is the time to put in a hook for transitions to<=>from
> the all-idle state. Various arch may have things other than the PIT
> which should (or at least can) be stopped, and which need to be restarted.

Hrm, got dropped from the Cc... :) Yes, the dynamic-tick generic
infrastructure being proposed, with the idle CPU mask and the
set_all_cpus_idle() tick_source hook, would allow exactly this in
arch-specific code.

Is there a generic location where the all-idle state is entered?
Currently, I think we can do it via the generic reprogram() routine
checking the mask and then calling set_all_cpus_idle(), if
appropriate, after reprogramming the last idle CPU.

Thanks,
Nish