[PATCH 2/2] virtio_net: remove send completion interrupts and avoid TX queue overrun through packet drop

[Shirley Ma]

Signed-off-by: Shirley Ma <x...@us.ibm.com>
---
 drivers/net/virtio_net.c |   39 ++++++++++++++++++++-------------------
 1 files changed, 20 insertions(+), 19 deletions(-)

diff --git a/drivers/net/virtio_net.c b/drivers/net/virtio_net.c
index 82dba5a..c603daa 100644
--- a/drivers/net/virtio_net.c
+++ b/drivers/net/virtio_net.c
@@ -23,6 +23,7 @@
 #include <linux/module.h>
 #include <linux/virtio.h>
 #include <linux/virtio_net.h>
+#include <linux/virtio_ring.h>
 #include <linux/scatterlist.h>
 #include <linux/if_vlan.h>
 #include <linux/slab.h>
@@ -509,19 +510,18 @@ again:
        return received;
 }
 
-static unsigned int free_old_xmit_skbs(struct virtnet_info *vi)
+static void free_old_xmit_skbs(struct virtnet_info *vi)
 {
        struct sk_buff *skb;
-       unsigned int len, tot_sgs = 0;
+       unsigned int len;
 
        while ((skb = virtqueue_get_buf(vi->svq, &len)) != NULL) {
                pr_debug("Sent skb %p\n", skb);
                vi->dev->stats.tx_bytes += skb->len;
                vi->dev->stats.tx_packets++;
-               tot_sgs += skb_vnet_hdr(skb)->num_sg;
                dev_kfree_skb_any(skb);
        }
-       return tot_sgs;
+       return;
 }
 
 static int xmit_skb(struct virtnet_info *vi, struct sk_buff *skb)
@@ -574,11 +574,26 @@ static int xmit_skb(struct virtnet_info *vi, struct 
sk_buff *skb)
 static netdev_tx_t start_xmit(struct sk_buff *skb, struct net_device *dev)
 {
        struct virtnet_info *vi = netdev_priv(dev);
+       bool drop;
+       bool indirect = virtio_has_feature(vi->vdev,
+                                          VIRTIO_RING_F_INDIRECT_DESC);
        int capacity;
 
        /* Free up any pending old buffers before queueing new ones. */
        free_old_xmit_skbs(vi);
-
+       capacity = virtqueue_get_capacity(vi->svq);
+
+       /* Drop packet instead of stop queue for better performance */
+       drop = (capacity == 0 && indirect) ||
+              ((capacity < MAX_SKB_FRAGS + 2) && !indirect);
+       if (unlikely(drop)) {
+               dev->stats.tx_fifo_errors++;
+               dev_warn(&dev->dev, "Unexpected TX queue failure: %d\n",
+                        capacity);
+               dev->stats.tx_dropped++;
+               kfree_skb(skb);
+               return NETDEV_TX_OK;
+       }
        /* Try to transmit */
        capacity = xmit_skb(vi, skb);
 
@@ -605,20 +620,6 @@ static netdev_tx_t start_xmit(struct sk_buff *skb, struct 
net_device *dev)
        skb_orphan(skb);
        nf_reset(skb);
 
-       /* 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(!virtqueue_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);
-                               virtqueue_disable_cb(vi->svq);
-                       }
-               }
-       }
-
        return NETDEV_TX_OK;
 }
 


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