Re: [PATCH net V3] virtio-net: correctly enable callback during start_xmit

Laurent Vivier
2023-01-17T16:35:21Z

On 1/17/23 04:47, Jason Wang wrote:
> Commit a7766ef18b33("virtio_net: disable cb aggressively") enables
> virtqueue callback via the following statement:
>
> do {
> if (use_napi)
> virtqueue_disable_cb(sq->vq);
>
> free_old_xmit_skbs(sq, false);
>
> } while (use_napi && kick &&
> unlikely(!virtqueue_enable_cb_delayed(sq->vq)));
>
> When NAPI is used and kick is false, the callback won't be enabled
> here. And when the virtqueue is about to be full, the tx will be
> disabled, but we still don't enable tx interrupt which will cause a TX
> hang. This could be observed when using pktgen with burst enabled.
>
> TO be consistent with the logic that tries to disable cb only for
> NAPI, fixing this by trying to enable delayed callback only when NAPI
> is enabled when the queue is about to be full.
>
> Fixes: a7766ef18b33 ("virtio_net: disable cb aggressively")
> Signed-off-by: Jason Wang <jasowang@redhat.com>
> ---
> The patch is needed for -stable.
> Changes since V2:
> - try to enabled delayed callback and schedule NAPI instead of try to
> poll as when TX NAPI is disabled
> Changes since V1:
> - enable tx interrupt after we disable TX
> ---
> drivers/net/virtio_net.c | 6 ++++--
> 1 file changed, 4 insertions(+), 2 deletions(-)
>
> diff --git a/drivers/net/virtio_net.c b/drivers/net/virtio_net.c
> index 7723b2a49d8e..18b3de854aeb 100644
> --- a/drivers/net/virtio_net.c
> +++ b/drivers/net/virtio_net.c
> @@ -1877,8 +1877,10 @@ static netdev_tx_t start_xmit(struct sk_buff *skb, struct net_device *dev)
> */
> if (sq->vq->num_free < 2+MAX_SKB_FRAGS) {
> netif_stop_subqueue(dev, qnum);
> - if (!use_napi &&
> - unlikely(!virtqueue_enable_cb_delayed(sq->vq))) {
> + if (use_napi) {
> + if (unlikely(!virtqueue_enable_cb_delayed(sq->vq)))
> + virtqueue_napi_schedule(&sq->napi, sq->vq);
> + } else if (unlikely(!virtqueue_enable_cb_delayed(sq->vq))) {
> /* More just got used, free them then recheck. */
> free_old_xmit_skbs(sq, false);
> if (sq->vq->num_free >= 2+MAX_SKB_FRAGS) {

This fix works fine with my test case (netdev stream + passt)

Tested-by: Laurent Vivier <lvivier@redhat.com>

Thanks,
Laurent