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

[Stefan Hajnoczi <stefanha@redhat.com>]

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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://git.kernel.org/pub/scm/linux/kernel/git/mst/vhost.git/log/?h=vhost
> 
> and the vhost-scsi bug fix patchset:
> 
> https://lore.kernel.org/linux-scsi/20201112170008.GB1555653@stefanha-x1.localdomain/T/#t
> 
> 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.

The purpose of the ioctl isn't clear to me because the kernel could
automatically create 1 thread per vq without a new ioctl. On the other
hand, if userspace is supposed to control worker threads then a
different interface would be more powerful:

  struct vhost_vq_worker_info {
      /*
       * The pid of an existing vhost worker that this vq will be
       * assigned to. When pid is 0 the virtqueue is assigned to the
       * default vhost worker. When pid is -1 a new worker thread is
       * created for this virtqueue. When pid is -2 the virtqueue's
       * worker thread is unchanged.
       *
       * If a vhost worker no longer has any virtqueues assigned to it
       * then it will terminate.
       *
       * The pid of the vhost worker is stored to this field when the
       * ioctl completes successfully. Use pid -2 to query the current
       * vhost worker pid.
       */
      __kernel_pid_t pid;  /* in/out */

      /* The virtqueue index*/
      unsigned int vq_idx; /* in */
  };

  ioctl(vhost_fd, VHOST_SET_VQ_WORKER, &info);

Stefan

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