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

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Date:	Sat, 4 Jul 2015 17:35:33 +0800
From:	Jisheng Zhang <jszhang@...vell.com>
To:	Russell King - ARM Linux <linux@....linux.org.uk>,
	<tglx@...utronix.de>, <jason@...edaemon.net>
CC:	<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

Dear Russell,

On Sat, 4 Jul 2015 10:25:46 +0100
Russell King - ARM Linux <linux@....linux.org.uk> wrote:

> 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

"sleep $i" on CPU1 will make the timer framework to wake up it at current_time+$i
in the future then go offline, currently this is achieved by programming one
broadcast timer. In our case, the broadcast timer interrupt will be routed to
CPU0 by default, so CPU0 has to send broadcast timer IPI to CPU1.

> it.  However, this is not the situation you're testing - in your results
> below, and your "simple test" you never take CPU1 offline.
> 

when cpu1 executes "sleep $i", cpu1 will go to deepest cpuidle level, in our
case it will go offline.

Thanks a lot for your review,
Jisheng
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