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

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Date: Thu, 20 Jul 2023 16:58:18 -0400
From: "Michael S. Tsirkin" <mst@...hat.com>
To: Shannon Nelson <shannon.nelson@....com>
Cc: Jason Wang <jasowang@...hat.com>, xuanzhuo@...ux.alibaba.com,
	netdev@...r.kernel.org, linux-kernel@...r.kernel.org,
	virtualization@...ts.linux-foundation.org, edumazet@...gle.com,
	maxime.coquelin@...hat.com, kuba@...nel.org, pabeni@...hat.com,
	davem@...emloft.net
Subject: Re: [PATCH net-next v4 2/2] virtio-net: add cond_resched() to the
 command waiting loop

On Thu, Jul 20, 2023 at 08:31:13AM -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@...hat.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 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();
> > +       }
> 
> The cover letter suggests that this addresses the infinite poll for buggy
> devices, but I don't see how that is resolved here.  This should make it a
> little nicer to the system, but it still is going to poll forever on a
> device that has gone catatonic.  Is there a reason that I'm missing that we
> don't have a polling limit here?
> 
> sln

we don't know what the limit would be. but given it's a workqueue
now, why does it still have to poll as opposed to blocking?

> > 
> >          return vi->ctrl->status == VIRTIO_NET_OK;
> >   }
> > --
> > 2.39.3
> > 
> > _______________________________________________
> > Virtualization mailing list
> > Virtualization@...ts.linux-foundation.org
> > https://lists.linuxfoundation.org/mailman/listinfo/virtualization

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