commit d631b94e7a15277858ec5f88d674d93080506999
virtio: change comment in transmit
started clarifying the logic behind queue state management,
but introduced an inaccuracy: TX_BUSY does not cause
a BUG message.
Clean this up some more, explaining the tradeoffs in detail.
Signed-off-by: Michael S. Tsirkin <mst@xxxxxxxxxx>
---
diff --git a/drivers/net/virtio_net.c b/drivers/net/virtio_net.c
index a829930..63c7810 100644
--- a/drivers/net/virtio_net.c
+++ b/drivers/net/virtio_net.c
@@ -939,11 +939,15 @@ static netdev_tx_t start_xmit(struct sk_buff *skb, struct net_device *dev)
skb_orphan(skb);
nf_reset(skb);
- /* It is better to stop queue if running out of space
- * instead of forcing queuing layer to requeue the skb
- * by returning TX_BUSY (and cause a BUG message).
- * Since most packets only take 1 or 2 ring slots
- * this means 16 slots are typically wasted.
+ /* If running out of space, stop queue to avoid getting packets that we
+ * are then unable to transmit.
+ * An alternative would be to force queuing layer to requeue the skb by
+ * returning NETDEV_TX_BUSY. However, NETDEV_TX_BUSY should not be
+ * returned in a normal path of operation: it means that driver is not
+ * maintaining the TX queue stop/start state properly, and causes
+ * the stack to do a non-trivial amount of useless work.
+ * Since most packets only take 1 or 2 ring slots, stopping the queue
+ * early means 16 slots are typically wasted.
*/
if (sq->vq->num_free < 2+MAX_SKB_FRAGS) {
netif_stop_subqueue(dev, qnum);
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