Re: [resend] Timer broadcast question

From: Andy Lutomirski
Date: Tue Feb 19 2013 - 17:46:18 EST

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?

To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@xxxxxxxxxxxxxxx
More majordomo info at
Please read the FAQ at