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

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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();
+       }

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?


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

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