Re: [resend] Timer broadcast question

From: Thomas Gleixner
Date: Wed Feb 20 2013 - 05:09:15 EST

On Tue, 19 Feb 2013, Andy Lutomirski wrote:
> On 02/19/2013 10:21 AM, Daniel Lezcano wrote:
> > On 02/19/2013 07:10 PM, Thomas Gleixner wrote:
> >> On Tue, 19 Feb 2013, Daniel Lezcano wrote:
> >>> I am working on identifying the different wakeup sources from the
> >>> interrupts and I have a question regarding the timer broadcast.
> >>>
> >>> The broadcast timer is setup to the next event and that will wake up any
> >>> idle cpu belonging to the "broadcast cpumask", right ?
> >>>
> >>> The cpu which has been woken up will look for each cpu the next-event
> >>> and send an IPI to wake it up.
> >>>
> >>> Although, it is possible the sender of this IPI may not be concerned by
> >>> the timer expiration and has been woken up just for sending the IPI, right ?
> >>
> >> Correct.
> >>
> >>> If this is correct, is it possible to setup the timer irq affinity to a
> >>> cpu which will be concerned by the timer expiration ? so we prevent an
> >>> unnecessary wake up for a cpu.
> >>
> >> It is possible, but we never implemented it.
> >>
> >> If we go there, we want to make that conditional on a property flag,
> >> because some interrupt controllers especially on x86 only allow to
> >> move the affinity from interrupt context, which is pointless.
> >
> > Thanks Thomas for your quick answer. I will write a RFC patchset.
> I'm curious what the use case is. I played with this code awhile ago,
> and AFAICT it's not used on sensible (i.e. modern) systems. Is there
> anything other than old x86 machines that needs it?

If the local apic timer is not affected by C-States, it's irrelevant,
but there are enough machines out there which do not have that. The
point is that we want a flag on the broadcast device which tells us
whether we should use dynamic affinity settings or not. On x86 we
would not set that flag ever.



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