Virtualization - Dec 2022 - [PATCH net V2] virtio-net: correctly enable callback during start_xmit

Commit a7766ef18b33("virtio_net: disable cb aggressively") enables
virtqueue callback via the following statement:

        do {
           ......
	} 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.

Fixing this by trying to enable tx interrupt after we disable TX when
we're not using napi or kick is false.

Fixes: a7766ef18b33 ("virtio_net: disable cb aggressively")
Signed-off-by: Jason Wang <jasowang at redhat.com>
---
The patch is needed for -stable.
Changes since V1:
- enable tx interrupt after we disable tx
---
 drivers/net/virtio_net.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/drivers/net/virtio_net.c b/drivers/net/virtio_net.c
index 86e52454b5b5..dcf3a536d78a 100644
--- a/drivers/net/virtio_net.c
+++ b/drivers/net/virtio_net.c
@@ -1873,7 +1873,7 @@ 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 &&
+		if ((!use_napi || !kick) &&
 		    unlikely(!virtqueue_enable_cb_delayed(sq->vq))) {
 			/* More just got used, free them then recheck. */
 			free_old_xmit_skbs(sq, false);
-- 
2.25.1

On Thu, Dec 15, 2022 at 11:27:19AM +0800, Jason Wang
wrote:
> Commit a7766ef18b33("virtio_net: disable cb aggressively")
enables
> virtqueue callback via the following statement:
> 
>         do {
>            ......
> 	} 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.
> 
> Fixing this by trying to enable tx interrupt after we disable TX when
> we're not using napi or kick is false.
> 
> Fixes: a7766ef18b33 ("virtio_net: disable cb aggressively")
> Signed-off-by: Jason Wang <jasowang at redhat.com>
> ---
> The patch is needed for -stable.
> Changes since V1:
> - enable tx interrupt after we disable tx
> ---
>  drivers/net/virtio_net.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/drivers/net/virtio_net.c b/drivers/net/virtio_net.c
> index 86e52454b5b5..dcf3a536d78a 100644
> --- a/drivers/net/virtio_net.c
> +++ b/drivers/net/virtio_net.c
> @@ -1873,7 +1873,7 @@ 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 &&
> +		if ((!use_napi || !kick) &&
>  		    unlikely(!virtqueue_enable_cb_delayed(sq->vq))) {
>  			/* More just got used, free them then recheck. */
>  			free_old_xmit_skbs(sq, false);
This will work but the following lines are:

                       if (sq->vq->num_free >= 2+MAX_SKB_FRAGS) {
                                netif_start_subqueue(dev, qnum);
                                virtqueue_disable_cb(sq->vq);
                        }


and I thought we are supposed to keep callbacks enabled with napi?
One of the ideas of napi is to free on napi callback, not here
immediately.

I think it is easier to just do a separate branch here. Along the
lines of:

		if (use_napi) {
			if (unlikely(!virtqueue_enable_cb_delayed(sq->vq)))
				virtqueue_napi_schedule(napi, vq);
		} else {
			... old code ...
		}

also reduces chances of regressions on !napi (which is not well tested)
and keeps callbacks off while we free skbs.

No?

> -- 
> 2.25.1