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Commit 60302ff6 authored by Michael S. Tsirkin's avatar Michael S. Tsirkin Committed by David S. Miller
Browse files

virtio: document queue state logic 



commit d631b94e
    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: default avatarMichael S. Tsirkin <mst@redhat.com>
Signed-off-by: default avatarDavid S. Miller <davem@davemloft.net>
parent 91bc4822

drivers/net/virtio_net.c

+9 −5
Original line number Diff line number Diff line
@@ -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);