linux-kernel - Re: IRQ affinity problem from virtio

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Date:   Wed, 16 Nov 2022 00:04:19 +0100
From:   Thomas Gleixner <tglx@...utronix.de>
To:     "Michael S. Tsirkin" <mst@...hat.com>
Cc:     Angus Chen <angus.chen@...uarmicro.com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        Ming Lei <ming.lei@...hat.com>,
        Jason Wang <jasowang@...hat.com>
Subject: Re: IRQ affinity problem from virtio_blk

On Tue, Nov 15 2022 at 17:44, Michael S. Tsirkin wrote:
> On Tue, Nov 15, 2022 at 11:19:47PM +0100, Thomas Gleixner wrote:
>> > We can see global_available drop from 15354 to 15273, is 81.
>> > And the total_allocated increase from 411 to 413. One config irq,and
>> > one vq irq.
>> 
>> Right. That's perfectly fine. At the point where you looking at it, the
>> matrix allocator has given out 2 vectors as can be seen via
>> total_allocated.
>> 
>> But then it also has another 79 vectors put aside for the other queues,
>
> What makes it put these vectors aside? pci_alloc_irq_vectors_affinity ?

init_vq() -> virtio_find_vqs() -> vp_find_vqs() ->
vp_request_msix_vectors() -> pci_alloc_irq_vectors_affinity()

init_vq() hands in a struct irq_affinity which means that
pci_alloc_irq_vectors_affinity() will spread out interrupts and have one
for config and one per queue if vp_request_msix_vectors() is invoked
with per_vq_vectors == true, which is what the first invocation in
vp_find_vqs() does.

Thanks,

        tglx