diff options
| author | Michael S. Tsirkin <mst@redhat.com> | 2015-04-02 13:05:47 +0200 |
|---|---|---|
| committer | David S. Miller <davem@davemloft.net> | 2015-04-06 16:44:24 -0400 |
| commit | 60302ff631f0f3eac0ec592e128b776f0676b397 (patch) | |
| tree | 2dabf60760cf7e12d0d40bce8e332f375de22ba5 | |
| parent | 91bc4822c3d61b9bb7ef66d3b77948a4f9177954 (diff) | |
virtio: document queue state logic
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@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
| -rw-r--r-- | drivers/net/virtio_net.c | 14 |
1 files changed, 9 insertions, 5 deletions
diff --git a/drivers/net/virtio_net.c b/drivers/net/virtio_net.c index a829930dac15..63c7810e1545 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); |