Re: [resend] Timer broadcast question

From: Thomas Gleixner <tglx@linutronix.de>
To: Daniel Lezcano <daniel.lezcano@linaro.org>
Cc: John Stultz <john.stultz@linaro.org>,
	Frederic Weisbecker <fweisbec@gmail.com>,
	Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
	"linaro-dev >> Lists Linaro-dev" <linaro-dev@lists.linaro.org>
Subject: Re: [resend] Timer broadcast question
Date: Tue, 19 Feb 2013 19:10:06 +0100 (CET)	[thread overview]
Message-ID: <alpine.LFD.2.02.1302191904560.22263@ionos> (raw)
In-Reply-To: <5123BE35.8070902@linaro.org>

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,

	tglx