linux-kernel - Re: [PATCH 02/13] irq: Introduce IRQD_AFFINITY

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Date:	Wed, 22 Jun 2016 13:56:36 +0200
From:	Alexander Gordeev <agordeev@...hat.com>
To:	Keith Busch <keith.busch@...el.com>
Cc:	Bart Van Assche <bart.vanassche@...disk.com>,
	Christoph Hellwig <hch@....de>,
	"tglx@...utronix.de" <tglx@...utronix.de>,
	"axboe@...com" <axboe@...com>,
	"linux-block@...r.kernel.org" <linux-block@...r.kernel.org>,
	"linux-pci@...r.kernel.org" <linux-pci@...r.kernel.org>,
	"linux-nvme@...ts.infradead.org" <linux-nvme@...ts.infradead.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 02/13] irq: Introduce IRQD_AFFINITY_MANAGED flag

On Thu, Jun 16, 2016 at 11:19:51AM -0400, Keith Busch wrote:
> On Wed, Jun 15, 2016 at 10:50:53PM +0200, Bart Van Assche wrote:
> > Does it matter on x86 systems whether or not these interrupt vectors are
> > also associated with a CPU with a higher CPU number? Although multiple bits
> > can be set in /proc/irq/<n>/smp_affinity only the first bit counts on x86
> > platforms. In default_cpu_mask_to_apicid_and() it is easy to see that only
> > the first bit that has been set in that mask counts on x86 systems.
> 
> Wow, thanks for the information. I didn't know the apic wasn't using
> the full cpu mask, so this changes how I need to look at this, and will
> experiment with such a configuration.

I have vague memories of this, but you probably need to check PPC as well.
Its interrupt distribution is not straightforward as well, AFAIR.