Re: [PATCH net-next v4 2/2] virtio-net: add cond_resched() to the command waiting loop

[Maxime Coquelin]

On 7/21/23 16:45, Michael S. Tsirkin wrote:
> On Fri, Jul 21, 2023 at 04:37:00PM +0200, Maxime Coquelin wrote:
> > On 7/20/23 23:02, Michael S. Tsirkin wrote:
> > > On Thu, Jul 20, 2023 at 01:26:20PM -0700, Shannon Nelson wrote:
> > > > On 7/20/23 1:38 AM, 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 on a device whose CVQ might be slow.
> > > > >
> > > > > Signed-off-by: Jason Wang <jasowang@xxxxxxxxxx>
> > > >
> > > > This still leaves hung processes, but at least it doesn't pin the CPU any
> > > > more.  Thanks.
> > > > Reviewed-by: Shannon Nelson <shannon.nelson@xxxxxxx>
> > >
> > > I'd like to see a full solution
> > > 1- block until interrupt
> >
> > Would it make sense to also have a timeout?
> > And when timeout expires, set FAILED bit in device status?
>
> virtio spec does not set any limits on the timing of vq
> processing.

Indeed, but I thought the driver could decide it is too long for it.

The issue is we keep waiting with rtnl locked, it can quickly make the
system unusable.

> > > 2- still handle surprise removal correctly by waking in that case
> > >
> > >
> > >
> > > > > ---
> > > > >     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 9f3b1d6ac33d..e7533f29b219 100644
> > > > > --- a/drivers/net/virtio_net.c
> > > > > +++ b/drivers/net/virtio_net.c
> > > > > @@ -2314,8 +2314,10 @@ 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();
> > > > > +       }
> > > > >
> > > > >            return vi->ctrl->status == VIRTIO_NET_OK;
> > > > >     }
> > > > > --
> > > > > 2.39.3
> > > > >
> > > > > _______________________________________________
> > > > > Virtualization mailing list
> > > > > Virtualization@xxxxxxxxxxxxxxxxxxxxxxxxxx
> > > > > https://lists.linuxfoundation.org/mailman/listinfo/virtualization