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

[Ming Lei <ming.lei@redhat.com>]

On Wed, Apr 04, 2018 at 10:25:16AM +0200, Thomas Gleixner wrote:
> On Wed, 4 Apr 2018, Ming Lei wrote:
> > 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.
> 
> I'm aware how that hw-queue stuff works. But that only works if the
> spreading algorithm makes the interrupts affine to offline/not-present CPUs
> when the block device is initialized.
> 
> In the example above:
> 
> > > > 	irq 39, cpu list 0,4
> > > > 	irq 40, cpu list 1,6
> > > > 	irq 41, cpu list 2,5
> > > > 	irq 42, cpu list 3,7
> 
> and assumed that at driver init time only CPU 0-3 are online then the
> hotplug of CPU 4-7 will not result in any interrupt delivered to CPU 4-7.

Indeed, and I just tested this case, and found that no interrupts are
delivered to CPU 4-7.

In theory, the affinity has been assigned to these irq vectors, and
programmed to interrupt controller, I understand it should work.

Could you explain it a bit why interrupts aren't delivered to CPU 4-7?


Thanks,
Ming