Herbert> However, lockless drivers do not take the xmit_lock so
Herbert> this method is ineffective. Such drivers need to do
Herbert> their own checking inside whatever locks that they do
Herbert> take. For example, tg3 could get around this by checking
Herbert> whether the queue is stopped in its hard_start_xmit
Herbert> function.
Yes, I had to add this to the IPoIB driver, because calling
netif_stop_queue() when the transmit ring was full was sometimes still
allowing hard_start_xmit to be called again:
/*
* Check if our queue is stopped. Since we have the LLTX bit
* set, we can't rely on netif_stop_queue() preventing our
* xmit function from being called with a full queue.
*/
if (unlikely(netif_queue_stopped(dev))) {
spin_unlock_irqrestore(&priv->tx_lock, flags);
return NETDEV_TX_BUSY;
}
this bug started a long thread a while back, but I don't remember if
there was any resolution.
Herbert> I must say though that I'm becoming less and less
Herbert> impressed by the lockless feature based on the number of
Herbert> problems that it has caused. Does anyone have any hard
Herbert> figures as to its effectiveness (excluding any stats
Herbert> relating to the loopback interface which can be easily
Herbert> separated from normal NIC drivers).
I don't have exact figures at hand, but I remember something like a 2 or
3 percent throughput improvement for IPoIB.
- R.
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