linux-kernel - Re: [resend] Timer broadcast question

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Message-ID: <5123C299.3080005@linaro.org>
Date:	Tue, 19 Feb 2013 19:21:13 +0100
From:	Daniel Lezcano <daniel.lezcano@...aro.org>
To:	Thomas Gleixner <tglx@...utronix.de>
CC:	John Stultz <john.stultz@...aro.org>,
	Frederic Weisbecker <fweisbec@...il.com>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	"linaro-dev >> Lists Linaro-dev" <linaro-dev@...ts.linaro.org>
Subject: Re: [resend] Timer broadcast question

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.

  -- Daniel

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