Virtualization - Sep 2022 - [PATCH net] virtio-net: add cond_resched() to the command waiting loop

On Mon, Sep 5, 2022 at 3:15 PM Michael S. Tsirkin <mst at redhat.com>
wrote:
>
> On Mon, Sep 05, 2022 at 12:53:41PM +0800, Jason Wang wrote:
> > Adding cond_resched() to the command waiting loop for a better
> > co-operation with the scheduler. This allows to give CPU a breath to
> > run other task(workqueue) instead of busy looping when preemption is
> > not allowed.
> >
> > What's more important. This is a must for some vDPA parent to work
> > since control virtqueue is emulated via a workqueue for those parents.
> >
> > Fixes: bda324fd037a ("vdpasim: control virtqueue support")
>
> That's a weird commit to fix. so it fixes the simulator?
Yes, since the simulator is using a workqueue to handle control virtueue.
>
> > Signed-off-by: Jason Wang <jasowang at redhat.com>
> > ---
> >  drivers/net/virtio_net.c | 4 +++-
> >  1 file changed, 3 insertions(+), 1 deletion(-)
> >
> > diff --git a/drivers/net/virtio_net.c b/drivers/net/virtio_net.c
> > index ece00b84e3a7..169368365d6a 100644
> > --- a/drivers/net/virtio_net.c
> > +++ b/drivers/net/virtio_net.c
> > @@ -2000,8 +2000,10 @@ static bool virtnet_send_command(struct
virtnet_info *vi, u8 class, u8 cmd,
> >        * into the hypervisor, so the request should be handled
immediately.
> >        */
> >       while (!virtqueue_get_buf(vi->cvq, &tmp) &&
> > -            !virtqueue_is_broken(vi->cvq))
> > +            !virtqueue_is_broken(vi->cvq)) {
> > +             cond_resched();
> >               cpu_relax();
> > +     }
>
> with cond_resched do we still need cpu_relax?
I think so, it's possible that cond_sched() just doesn't trigger
scheduling.

Thanks
>
> >       return vi->ctrl->status == VIRTIO_NET_OK;
> >  }
> > --
> > 2.25.1
>

On Mon, 2022-09-05 at 15:49 +0800, Jason Wang wrote:
> On Mon, Sep 5, 2022 at 3:15 PM Michael S. Tsirkin <mst at redhat.com>
wrote:
> > 
> > On Mon, Sep 05, 2022 at 12:53:41PM +0800, Jason Wang wrote:
> > > Adding cond_resched() to the command waiting loop for a better
> > > co-operation with the scheduler. This allows to give CPU a breath
to
> > > run other task(workqueue) instead of busy looping when preemption
is
> > > not allowed.
> > > 
> > > What's more important. This is a must for some vDPA parent to
work
> > > since control virtqueue is emulated via a workqueue for those
parents.
> > > 
> > > Fixes: bda324fd037a ("vdpasim: control virtqueue
support")
> > 
> > That's a weird commit to fix. so it fixes the simulator?
> 
> Yes, since the simulator is using a workqueue to handle control virtueue.
Uhmm... touching a driver for a simulator's sake looks a little weird. 

Additionally, if the bug is vdpasim, I think it's better to try to
solve it there, if possible.

Looking at vdpasim_net_work() and vdpasim_blk_work() it looks like
neither needs a process context, so perhaps you could rework it to run
the work_fn() directly from vdpasim_kick_vq(), at least for the control
virtqueue?

Thanks!

Paolo