Messages in this thread | | | Date | Wed, 20 Feb 2013 11:09:03 +0100 (CET) | From | Thomas Gleixner <> | Subject | Re: [resend] Timer broadcast question |
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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.
Thanks,
tglx
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