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virtio: document queue state logic

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commit d631b94e7a
    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>
This commit is contained in:
Michael S. Tsirkin 2015-04-02 13:05:47 +02:00 committed by David S. Miller
parent 91bc4822c3
commit 60302ff631
1 changed files with 9 additions and 5 deletions
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14
drivers/net/virtio_net.c
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@ -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);