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SubjectRe: virtio-blk: should num_vqs be limited by num_possible_cpus()?
On Tue, Mar 12, 2019 at 10:22:46AM -0700, Dongli Zhang wrote:
> I observed that there is one msix vector for config and one shared vector
> for all queues in below qemu cmdline, when the num-queues for virtio-blk
> is more than the number of possible cpus:
>
> qemu: "-smp 4" while "-device virtio-blk-pci,drive=drive-0,id=virtblk0,num-queues=6"

So why do this?

> # cat /proc/interrupts
> CPU0 CPU1 CPU2 CPU3
> ... ...
> 24: 0 0 0 0 PCI-MSI 65536-edge virtio0-config
> 25: 0 0 0 59 PCI-MSI 65537-edge virtio0-virtqueues
> ... ...
>
>
> However, when num-queues is the same as number of possible cpus:
>
> qemu: "-smp 4" while "-device virtio-blk-pci,drive=drive-0,id=virtblk0,num-queues=4"
>
> # cat /proc/interrupts
> CPU0 CPU1 CPU2 CPU3
> ... ...
> 24: 0 0 0 0 PCI-MSI 65536-edge virtio0-config
> 25: 2 0 0 0 PCI-MSI 65537-edge virtio0-req.0
> 26: 0 35 0 0 PCI-MSI 65538-edge virtio0-req.1
> 27: 0 0 32 0 PCI-MSI 65539-edge virtio0-req.2
> 28: 0 0 0 0 PCI-MSI 65540-edge virtio0-req.3
> ... ...
>
> In above case, there is one msix vector per queue.
>
>
> This is because the max number of queues is not limited by the number of
> possible cpus.
>
> By default, nvme (regardless about write_queues and poll_queues) and
> xen-blkfront limit the number of queues with num_possible_cpus().
>
>
> Is this by design on purpose, or can we fix with below?
>
>
> diff --git a/drivers/block/virtio_blk.c b/drivers/block/virtio_blk.c
> index 4bc083b..df95ce3 100644
> --- a/drivers/block/virtio_blk.c
> +++ b/drivers/block/virtio_blk.c
> @@ -513,6 +513,8 @@ static int init_vq(struct virtio_blk *vblk)
> if (err)
> num_vqs = 1;
>
> + num_vqs = min(num_possible_cpus(), num_vqs);
> +
> vblk->vqs = kmalloc_array(num_vqs, sizeof(*vblk->vqs), GFP_KERNEL);
> if (!vblk->vqs)
> return -ENOMEM;
> --
>
>
> PS: The same issue is applicable to virtio-scsi as well.
>
> Thank you very much!
>
> Dongli Zhang

I don't think this will address the issue if there's vcpu hotplug though.
Because it's not about num_possible_cpus it's about the # of active VCPUs,
right? Does block hangle CPU hotplug generally?
We could maybe address that by switching vq to msi vector mapping in
a cpu hotplug notifier...

--
MST

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