[PATCH RFC] virtio_net: use NAPI for xmit (UNTESTED)

[Rusty Russell <rusty@rustcorp.com.au>]

I don't have time to chase this, but it's been sitting in my patch queue
for a while.  Wondered if Michael or Shirley wanted to toy with it

Thanks!
Rusty.


This is closer to the way tg3 and ixgbe do it: use the NAPI framework to
free transmitted packets.  It neatens things a little as well.

Changes since last version:

1) Use the tx lock for the xmit_poll to synchronize against
   start_xmit; it might be overkill, but it's simple.
2) Don't wake queue if the carrier is gone.

(Note: a side effect of this is that we are lazier in freeing old xmit skbs.
 This might be a slight win).

Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>
---
 drivers/net/virtio_net.c |   71 ++++++++++++++++++++++++++++++++---------------
 1 file changed, 49 insertions(+), 22 deletions(-)

diff --git a/drivers/net/virtio_net.c b/drivers/net/virtio_net.c
--- a/drivers/net/virtio_net.c
+++ b/drivers/net/virtio_net.c
@@ -47,6 +47,9 @@ struct virtnet_info
 	struct napi_struct napi;
 	unsigned int status;
 
+	/* We free packets and decide whether to restart xmit here. */
+	struct napi_struct xmit_napi;
+
 	/* Number of input buffers, and max we've ever had. */
 	unsigned int num, max;
 
@@ -60,6 +63,9 @@ struct virtnet_info
 	struct sk_buff_head recv;
 	struct sk_buff_head send;
 
+	/* Capacity left in xmit queue. */
+	unsigned int capacity;
+
 	/* Work struct for refilling if we run low on memory. */
 	struct delayed_work refill;
 
@@ -111,11 +117,8 @@ static void skb_xmit_done(struct virtque
 {
 	struct virtnet_info *vi = svq->vdev->priv;
 
-	/* Suppress further interrupts. */
-	svq->vq_ops->disable_cb(svq);
-
 	/* We were probably waiting for more output buffers. */
-	netif_wake_queue(vi->dev);
+	napi_schedule(&vi->xmit_napi);
 }
 
 static void receive_skb(struct net_device *dev, struct sk_buff *skb,
@@ -455,6 +458,29 @@ static unsigned int free_old_xmit_skbs(s
 	return tot_sgs;
 }
 
+static int virtnet_xmit_poll(struct napi_struct *xmit_napi, int budget)
+{
+	struct virtnet_info *vi =
+		container_of(xmit_napi, struct virtnet_info, xmit_napi);
+
+	/* Don't access vq/capacity at same time as start_xmit. */
+	__netif_tx_lock(netdev_get_tx_queue(vi->dev, 0), smp_processor_id());
+
+	vi->capacity += free_old_xmit_skbs(vi);
+	if (vi->capacity >= 2 + MAX_SKB_FRAGS) {
+		/* Suppress further xmit interrupts. */
+		vi->svq->vq_ops->disable_cb(vi->svq);
+		napi_complete(xmit_napi);
+
+		/* Don't wake it if link is down. */
+		if (likely(netif_carrier_ok(vi->vdev)))
+			netif_wake_queue(vi->dev);
+	}
+
+	__netif_tx_unlock(netdev_get_tx_queue(vi->dev, 0));
+	return 1;
+}
+
 static int xmit_skb(struct virtnet_info *vi, struct sk_buff *skb)
 {
 	struct scatterlist sg[2+MAX_SKB_FRAGS];
@@ -509,10 +535,6 @@ static netdev_tx_t start_xmit(struct sk_
 	struct virtnet_info *vi = netdev_priv(dev);
 	int capacity;
 
-again:
-	/* Free up any pending old buffers before queueing new ones. */
-	free_old_xmit_skbs(vi);
-
 	/* Try to transmit */
 	capacity = xmit_skb(vi, skb);
 
@@ -520,14 +542,13 @@ again:
 	if (unlikely(capacity < 0)) {
 		netif_stop_queue(dev);
 		dev_warn(&dev->dev, "Unexpected full queue\n");
-		if (unlikely(!vi->svq->vq_ops->enable_cb(vi->svq))) {
-			vi->svq->vq_ops->disable_cb(vi->svq);
-			netif_start_queue(dev);
-			goto again;
-		}
+		/* If we missed an interrupt, we let virtnet_xmit_poll deal. */
+		if (unlikely(!vi->svq->vq_ops->enable_cb(vi->svq)))
+			napi_schedule(&vi->xmit_napi);
 		return NETDEV_TX_BUSY;
 	}
 	vi->svq->vq_ops->kick(vi->svq);
+	vi->capacity = capacity;
 
 	/*
 	 * Put new one in send queue.  You'd expect we'd need this before
@@ -545,14 +566,13 @@ again:
 	/* Apparently nice girls don't return TX_BUSY; stop the queue
 	 * before it gets out of hand.  Naturally, this wastes entries. */
 	if (capacity < 2+MAX_SKB_FRAGS) {
-		netif_stop_queue(dev);
-		if (unlikely(!vi->svq->vq_ops->enable_cb(vi->svq))) {
-			/* More just got used, free them then recheck. */
-			capacity += free_old_xmit_skbs(vi);
-			if (capacity >= 2+MAX_SKB_FRAGS) {
-				netif_start_queue(dev);
-				vi->svq->vq_ops->disable_cb(vi->svq);
-			}
+		/* Free old skbs; might make more capacity. */
+		vi->capacity = capacity + free_old_xmit_skbs(vi);
+		if (unlikely(vi->capacity < 2+MAX_SKB_FRAGS)) {
+			netif_stop_queue(dev);
+			/* Missed xmit irq? virtnet_xmit_poll will deal. */
+			if (unlikely(!vi->svq->vq_ops->enable_cb(vi->svq)))
+				napi_schedule(&vi->xmit_napi);
 		}
 	}
 
@@ -590,6 +610,7 @@ static int virtnet_open(struct net_devic
 	struct virtnet_info *vi = netdev_priv(dev);
 
 	napi_enable(&vi->napi);
+	napi_enable(&vi->xmit_napi);
 
 	/* If all buffers were filled by other side before we napi_enabled, we
 	 * won't get another interrupt, so process any outstanding packets
@@ -652,6 +673,7 @@ static int virtnet_close(struct net_devi
 	struct virtnet_info *vi = netdev_priv(dev);
 
 	napi_disable(&vi->napi);
+	napi_disable(&vi->xmit_napi);
 
 	return 0;
 }
@@ -818,9 +840,13 @@ static void virtnet_update_status(struct
 
 	if (vi->status & VIRTIO_NET_S_LINK_UP) {
 		netif_carrier_on(vi->dev);
-		netif_wake_queue(vi->dev);
+		/* Make sure virtnet_xmit_poll sees carrier enabled. */
+		wmb();
+		napi_schedule(&vi->xmit_napi);
 	} else {
 		netif_carrier_off(vi->dev);
+		/* Make sure virtnet_xmit_poll sees carrier disabled. */
+		wmb();
 		netif_stop_queue(vi->dev);
 	}
 }
@@ -883,6 +909,7 @@ static int virtnet_probe(struct virtio_d
 	/* Set up our device-specific information */
 	vi = netdev_priv(dev);
 	netif_napi_add(dev, &vi->napi, virtnet_poll, napi_weight);
+	netif_napi_add(dev, &vi->xmit_napi, virtnet_xmit_poll, 64);
 	vi->dev = dev;
 	vi->vdev = vdev;
 	vdev->priv = vi;