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

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Date:	Wed, 15 Jun 2016 22:50:53 +0200
From:	Bart Van Assche <bart.vanassche@...disk.com>
To:	Keith Busch <keith.busch@...el.com>
CC:	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 06/15/2016 10:12 PM, Keith Busch wrote:
> On Wed, Jun 15, 2016 at 04:06:55PM -0400, Keith Busch wrote:
>>>
>>> 0: A0 B0
>>> 1: A1 B1
>>> 2: A2 B2
>>> 3: A3 B3
>>> 4: A4 B4
>>> 5: A5 B5
>>> 6: A6 B6
>>> 7: A7 B7
>>> 8: (none)
>>> ...
>>> 31: (none)
>>
>> I'll need to look at the follow on patches do to confirm, but that's
>> not what this should do. All CPU's should have a vector assigned because
>> every CPU needs to be assigned a submission context using a vector. In
>> your example, every vector's affinity mask should be assigned to 4 CPUs:
>> vector '8' starts over with A0 B0, '9' gets A1 B1, and so on.
>
>   ^^^^^^
>
> Sorry, I meant "CPU '8'", not "vector '8'".

Hello Keith,

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.

Bart.