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 _______________________________________________ Virtualization mailing list Virtualization@xxxxxxxxxxxxxxxxxxxxxxxxxx https://lists.linuxfoundation.org/mailman/listinfo/virtualization
