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

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

On Fri, Nov 20, 2020 at 08:45:49AM +0000, Stefan Hajnoczi wrote:
> On Thu, Nov 19, 2020 at 5:08 PM Stefan Hajnoczi <stefanha@gmail.com> wrote:
> >
> > On Thu, Nov 19, 2020 at 4:43 PM Mike Christie
> > <michael.christie@oracle.com> wrote:
> > >
> > > On 11/19/20 10:24 AM, Stefan Hajnoczi wrote:
> > > > On Thu, Nov 19, 2020 at 4:13 PM Mike Christie
> > > > <michael.christie@oracle.com> wrote:
> > > >>
> > > >> On 11/19/20 8:46 AM, Michael S. Tsirkin wrote:
> > > >>> On Wed, Nov 18, 2020 at 11:31:17AM +0000, Stefan Hajnoczi wrote:
> > > > struct vhost_run_worker_info {
> > > >      struct timespec *timeout;
> > > >      sigset_t *sigmask;
> > > >
> > > >      /* List of virtqueues to process */
> > > >      unsigned nvqs;
> > > >      unsigned vqs[];
> > > > };
> > > >
> > > > /* This blocks until the timeout is reached, a signal is received, or
> > > > the vhost device is destroyed */
> > > > int ret = ioctl(vhost_fd, VHOST_RUN_WORKER, &info);
> > > >
> > > > As you can see, userspace isn't involved with dealing with the
> > > > requests. It just acts as a thread donor to the vhost driver.
> > > >
> > > > We would want the VHOST_RUN_WORKER calls to be infrequent to avoid the
> > > > penalty of switching into the kernel, copying in the arguments, etc.
> > >
> > > I didn't get this part. Why have the timeout? When the timeout expires,
> > > does userspace just call right back down to the kernel or does it do
> > > some sort of processing/operation?
> > >
> > > You could have your worker function run from that ioctl wait for a
> > > signal or a wake up call from the vhost_work/poll functions.
> >
> > An optional timeout argument is common in blocking interfaces like
> > poll(2), recvmmsg(2), etc.
> >
> > Although something can send a signal to the thread instead,
> > implementing that in an application is more awkward than passing a
> > struct timespec.
> >
> > Compared to other blocking calls we don't expect
> > ioctl(VHOST_RUN_WORKER) to return soon, so maybe the timeout will
> > rarely be used and can be dropped from the interface.
> >
> > BTW the code I posted wasn't a carefully thought out proposal :). The
> > details still need to be considered and I'm going to be offline for
> > the next week so maybe someone else can think it through in the
> > meantime.
> 
> One final thought before I'm offline for a week. If
> ioctl(VHOST_RUN_WORKER) is specific to a single vhost device instance
> then it's hard to support poll-mode (busy waiting) workers because
> each device instance consumes a whole CPU. If we stick to an interface
> where the kernel manages the worker threads then it's easier to share
> workers between devices for polling.


Yes that is the reason vhost did its own reason in the first place.


I am vaguely thinking about poll(2) or a similar interface,
which can wait for an event on multiple FDs.


> I have CCed Stefano Garzarella, who is looking at similar designs for
> vDPA software device implementations.
> 
> Stefan