Re: [resend] Timer broadcast question

[Andy Lutomirski <luto@amacapital.net>]

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?

--Andy