linux-kernel - Re: [PATCH v2 2/2] irqchip: dw-apb-ictl: add irq_set

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Date:	Sat, 4 Jul 2015 10:25:46 +0100
From:	Russell King - ARM Linux <linux@....linux.org.uk>
To:	Jisheng Zhang <jszhang@...vell.com>
Cc:	tglx@...utronix.de, jason@...edaemon.net,
	linux-kernel@...r.kernel.org, linux-arm-kernel@...ts.infradead.org
Subject: Re: [PATCH v2 2/2] irqchip: dw-apb-ictl: add irq_set_affinity support

On Sat, Jul 04, 2015 at 04:50:07PM +0800, Jisheng Zhang wrote:
> Dear Russell,
> 
> On Sat, 4 Jul 2015 09:26:23 +0100
> Russell King - ARM Linux <linux@....linux.org.uk> wrote:
> 
> > On Sat, Jul 04, 2015 at 01:19:30PM +0800, Jisheng Zhang wrote:
> > > On Marvell Berlin SoCs, the cpu's local timer is shutdown when the cpu
> > > goes to a deep idle state, then the timer framework will be notified to
> > > use a broadcast timer instead. The broadcast timer uses dw-apb-ictl as
> > > interrupt chip, this patch adds irq_set_affinity support so that the
> > > going to deep idle state cpu can set the interrupt affinity of the
> > > broadcast interrupt to avoid unnecessary wakeups and IPIs.
> > 
> > NAK to this patch.
> > 
> > The real question is - if CPU0 is the CPU going offline, why is it
> > still receiving _any_ interrupts - all interrupts should be migrated
> > off it, including the chained interrupts.
> 
> I think it's due to broadcast timer interrupt. Let me describe the situation:
> 
> 1. cpu1 is going offline
> 2. cpuidle notify timer framework to use a broadcast timer instead due to localtimer
> is CLOCK_EVT_FEAT_C3STOP
> 3. when timer is expired, CPU0 will be waken up by the timer interrupt if it has
> gone offline
> 4. CPU0 sends broadcast timer IPI to CPU1

If CPU1 is going offline, then CPU1 should have no interrupts delivered to
it.  However, this is not the situation you're testing - in your results
below, and your "simple test" you never take CPU1 offline.

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