Re: [PATCH 00/10] vhost/qemu: thread per IO SCSI vq

[Mike Christie]

On 11/18/20 12:57 AM, Mike Christie wrote:
> On 11/17/20 11:17 PM, Jason Wang wrote:
>>
>> On 2020/11/18 上午12:40, Stefan Hajnoczi wrote:
>>> On Thu, Nov 12, 2020 at 05:18:59PM -0600, Mike Christie wrote:
>>>> The following kernel patches were made over Michael's vhost branch:
>>>>
>>>> https://urldefense.com/v3/__https://git.kernel.org/pub/scm/linux/kernel/git/mst/vhost.git/log/?h=vhost__;!!GqivPVa7Brio!MzCv3wdRfz5dltunazRWGCeUkMg91pPEOLpIivsebLX9vhYDSi_E1V36e9H0NoRys_hU$
>>>> and the vhost-scsi bug fix patchset:
>>>>
>>>> https://urldefense.com/v3/__https://lore.kernel.org/linux-scsi/20201112170008.GB1555653@stefanha-x1.localdomain/T/*t__;Iw!!GqivPVa7Brio!MzCv3wdRfz5dltunazRWGCeUkMg91pPEOLpIivsebLX9vhYDSi_E1V36e9H0NmuPE_m8$
>>>> And the qemu patch was made over the qemu master branch.
>>>>
>>>> vhost-scsi currently supports multiple queues with the num_queues
>>>> setting, but we end up with a setup where the guest's scsi/block
>>>> layer can do a queue per vCPU and the layers below vhost can do
>>>> a queue per CPU. vhost-scsi will then do a num_queue virtqueues,
>>>> but all IO gets set on and completed on a single vhost-scsi thread.
>>>> After 2 - 4 vqs this becomes a bottleneck.
>>>>
>>>> This patchset allows us to create a worker thread per IO vq, so we
>>>> can better utilize multiple CPUs with the multiple queues. It
>>>> implments Jason's suggestion to create the initial worker like
>>>> normal, then create the extra workers for IO vqs with the
>>>> VHOST_SET_VRING_ENABLE ioctl command added in this patchset.
>>> How does userspace find out the tids and set their CPU affinity?
>>>
>>> What is the meaning of the new VHOST_SET_VRING_ENABLE ioctl? It doesn't
>>> really "enable" or "disable" the vq, requests are processed regardless.
>>
>>
>> Actually I think it should do the real "enable/disable" that tries to follow 
>> the virtio spec.
>>
> 
> What does real mean here? For the vdpa enable call for example, would it be 
> like
> ifcvf_vdpa_set_vq_ready where it sets the ready bit or more like 
> mlx5_vdpa_set_vq_ready
> where it can do some more work in the disable case?
> 
> For net and something like ifcvf_vdpa_set_vq_ready's design would we have
> vhost_ring_ioctl() set some vhost_virtqueue enable bit. We then have some 
> helper
> vhost_vq_is_enabled() and some code to detect if userspace supports the new 
> ioctl.
> And then in vhost_net_set_backend do we call vhost_vq_is_enabled()? What is 
> done
> for disable then? It doesn't seem to buy a lot of new functionality. Is it 
> just
> so we follow the spec?
> 
> Or do you want it work more like mlx5_vdpa_set_vq_ready? For this in 
> vhost_ring_ioctl
> when we get the new ioctl we would call into the drivers and have it start 
> queues
> and stop queues? For enable, what we you do for net for this case? For 
> disable,
> would you do something like vhost_net_stop_vq (we don't free up anything 
> allocated
> in vhost_vring_ioctl calls, but we can stop what we setup in the net driver)?
> Is this useful for the current net mq design or is this for something like 
> where
> you would do one vhost net device with multiple vqs?
> 
> My issue/convern is that in general these calls seems useful, but we don't 
> really
> need them for scsi because vhost scsi is already stuck creating vqs like how 
> it does
> due to existing users. If we do the ifcvf_vdpa_set_vq_ready type of design 
> where
> we just set some bit, then the new ioctl does not give us a lot. It's just an 
> extra
> check and extra code.
> 
> And for the mlx5_vdpa_set_vq_ready type of design, it doesn't seem like it's 
> going
> to happen a lot where the admin is going to want to remove vqs from a running 
> device.
> And for both addition/removal for scsi we would need code in virtio scsi to 
> handle
> hot plug removal/addition of a queue and then redoing the multiqueue mappings 
> which
> would be difficult to add with no one requesting it.

Actually I want to half take this last chunk back. When I said in general these 
calls
seem useful, I meant for the mlx5_vdpa_set_vq_ready type design. For example, 
if a
user was going to remove/add vCPUs then this functionality where we are 
completely
adding/removing virtqueues would be useful. We would need a lot more than just
the new ioctl though, because we would want to completely create/setup a new
virtqueue

I do not have any of our users asking for this. You guys work on this more so
you know better.

Another option is to kick it down the road again since I'm not sure my patches
here have a lot to do with this. We could also just do the kernel only approach
(no new ioctl) and then add some new design when we have users asking for it.