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

[Cornelia Huck]

On Wed, 13 Mar 2019 11:26:04 +0800
Dongli Zhang <dongli.zhang@xxxxxxxxxx> wrote:

> On 3/13/19 1:33 AM, Cornelia Huck wrote:
> > On Tue, 12 Mar 2019 10:22:46 -0700 (PDT)
> > Dongli Zhang <dongli.zhang@xxxxxxxxxx> 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"
> >>
> >> # 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.  
> > 
> > Please note that this is pci-specific...
> >   
> >>
> >>
> >> 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().  
> > 
> > ...and these are probably pci-specific as well.  
> 
> Not pci-specific, but per-cpu as well.

Ah, I meant that those are pci devices.

> 
> >   
> >>
> >>
> >> 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;  
> > 
> > virtio-blk, however, is not pci-specific.
> > 
> > If we are using the ccw transport on s390, a completely different
> > interrupt mechanism is in use ('floating' interrupts, which are not
> > per-cpu). A check like that should therefore not go into the generic
> > driver.
> >   
> 
> So far there seems two options.
> 
> The 1st option is to ask the qemu user to always specify "-num-queues" with the
> same number of vcpus when running x86 guest with pci for virtio-blk or
> virtio-scsi, in order to assign a vector for each queue.

That does seem like an extra burden for the user: IIUC, things work
even if you have too many queues, it's just not optimal. It sounds like
something that can be done by a management layer (e.g. libvirt), though.

> Or, is it fine for virtio folks to add a new hook to 'struct virtio_config_ops'
> so that different platforms (e.g., pci or ccw) would use different ways to limit
> the max number of queues in guest, with something like below?

That sounds better, as both transports and drivers can opt-in here.

However, maybe it would be even better to try to come up with a better
strategy of allocating msix vectors in virtio-pci. More vectors in the
num_queues > num_cpus case, even if they still need to be shared?
Individual vectors for n-1 cpus and then a shared one for the remaining
queues?

It might even be device-specific: Have some low-traffic status queues
share a vector, and provide an individual vector for high-traffic
queues. Would need some device<->transport interface, obviously.