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

[Shannon Nelson]

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>
> ---
>   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

>          return vi->ctrl->status == VIRTIO_NET_OK;
>   }
> --
> 2.39.3
>
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