Re: virtio-blk: should num_vqs be limited by num_possible_cpus()?

["Michael S. Tsirkin" <mst@redhat.com>]

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