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

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Date:	Sat, 4 Jul 2015 14:49:31 +0200 (CEST)
From:	Thomas Gleixner <tglx@...utronix.de>
To:	Russell King - ARM Linux <linux@....linux.org.uk>
cc:	Jisheng Zhang <jszhang@...vell.com>, 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, 4 Jul 2015, Russell King - ARM Linux wrote:
> On Sat, Jul 04, 2015 at 11:53:57AM +0200, Thomas Gleixner wrote:
> > On Sat, 4 Jul 2015, Russell King - ARM Linux 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.
> > > 
> > > Sounds like there's a bug in the migration code which needs further
> > > investigation, rather than hacking around the problem by introducing
> > > lots of driver code.
> > 
> > I think you misunderstood the changelog, which is horrible btw.
> > 
> > So the real reason to do this is to steer the broadcast interrupt to
> > the CPU which has the earliest expiry time. This avoids that another
> > cpu is woken from idle just to deliver the broadcast IPI to the other
> > cpu.
> 
> Unless I'm mistaken, the code does this by messing around with the parent
> interrupt affinity of a chained interrupt, which really isn't a good thing
> to do, because it migrates every interrupt on the child interrupt
> controller.

Fair enough, I missed that chained hackery.

For that powersaving scenario it's probably ok to move the all child
irqs around, but we should at least make that an opt-in behaviour and
not enabled by default.

Thanks,

	tglx
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