virtio-net: clean tx descriptors from rx napi

Amortize the cost of virtual interrupts by doing both rx and tx work
on reception of a receive interrupt if tx napi is enabled. With
VIRTIO_F_EVENT_IDX, this suppresses most explicit tx completion
interrupts for bidirectional workloads.
Signed-off-by: Willem de Bruijn <willemb@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

drivers/net/virtio_net.c

@@ -1075,12 +1075,33 @@ static void free_old_xmit_skbs(struct send_queue *sq)
 	u64_stats_update_end(&stats->tx_syncp);
 }
 
+static void virtnet_poll_cleantx(struct receive_queue *rq)
+{
+	struct virtnet_info *vi = rq->vq->vdev->priv;
+	unsigned int index = vq2rxq(rq->vq);
+	struct send_queue *sq = &vi->sq[index];
+	struct netdev_queue *txq = netdev_get_tx_queue(vi->dev, index);
+
+	if (!sq->napi.weight)
+		return;
+
+	if (__netif_tx_trylock(txq)) {
+		free_old_xmit_skbs(sq);
+		__netif_tx_unlock(txq);
+	}
+
+	if (sq->vq->num_free >= 2 + MAX_SKB_FRAGS)
+		netif_tx_wake_queue(txq);
+}
+
 static int virtnet_poll(struct napi_struct *napi, int budget)
 {
 	struct receive_queue *rq =
 		container_of(napi, struct receive_queue, napi);
 	unsigned int received;
 
+	virtnet_poll_cleantx(rq);
+
 	received = virtnet_receive(rq, budget);
 
 	/* Out of packets? */