linux-kernel - Re: [PATCH V3 4/4] genirq/affinity: irq vector spread among online CPUs as far as possible

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Date:   Wed, 4 Apr 2018 00:00:05 +0800
From:   Ming Lei <ming.lei@...hat.com>
To:     Thomas Gleixner <tglx@...utronix.de>
Cc:     Jens Axboe <axboe@...nel.dk>,
        Christoph Hellwig <hch@...radead.org>,
        linux-kernel@...r.kernel.org, linux-block@...r.kernel.org,
        Laurence Oberman <loberman@...hat.com>
Subject: Re: [PATCH V3 4/4] genirq/affinity: irq vector spread among online
 CPUs as far as possible

On Tue, Apr 03, 2018 at 03:32:21PM +0200, Thomas Gleixner wrote:
> On Thu, 8 Mar 2018, Ming Lei wrote:
> > 1) before 84676c1f21 ("genirq/affinity: assign vectors to all possible CPUs")
> > 	irq 39, cpu list 0
> > 	irq 40, cpu list 1
> > 	irq 41, cpu list 2
> > 	irq 42, cpu list 3
> > 
> > 2) after 84676c1f21 ("genirq/affinity: assign vectors to all possible CPUs")
> > 	irq 39, cpu list 0-2
> > 	irq 40, cpu list 3-4,6
> > 	irq 41, cpu list 5
> > 	irq 42, cpu list 7
> > 
> > 3) after applying this patch against V4.15+:
> > 	irq 39, cpu list 0,4
> > 	irq 40, cpu list 1,6
> > 	irq 41, cpu list 2,5
> > 	irq 42, cpu list 3,7
> 
> That's more or less window dressing. If the device is already in use when
> the offline CPUs get hot plugged, then the interrupts still stay on cpu 0-3
> because the effective affinity of interrupts on X86 (and other
> architectures) is always a single CPU.
> 
> So this only might move interrupts to the hotplugged CPUs when the device
> is initialized after CPU hotplug and the actual vector allocation moves an
> interrupt out to the higher numbered CPUs if they have less vectors
> allocated than the lower numbered ones.

It works for blk-mq devices, such as NVMe.

Now NVMe driver creates num_possible_cpus() hw queues, and each
hw queue is assigned one msix irq vector.

Storage is Client/Server model, that means the interrupt is only
delivered to CPU after one IO request is submitted to hw queue and
it is completed by this hw queue.

When CPUs is hotplugged, and there will be IO submitted from these
CPUs, then finally IOs complete and irq events are generated from
hw queues, and notify these submission CPU by IRQ finally.

Thanks,
Ming