O > > > > But the spin is still there, just more complex.. > > > > In qdisc_restart() processing of NETDEV_TX_LOCKED causes: > > > > spin_lock(dev->xmit_lock) > > > > > > > > q->requeue() > > > > netif_schedule(dev); > > > > > > > > SOFTIRQ: > > > > net_tx_action() > > > > qdisc_run() --> qdisc_restart() > > > > > > > > So instead of spinning in tight loop, you end up with a longer code > > > > path. > > > > > > Stephen, sorry for insisting a bit but I'm failing to see how B is > > > different from C in that respect. With method B, in qdisc_restart(), > > > if netif_tx_trylock() fails to acquire the lock then we also > > > requeue(), etc. Same long code path in case of contention. > > > > > > > Method C LLTX causes repeated softirq's which will be slower since the loop > > requires more instructions than a simple spin loop (Method B). > > What I'm saying above is that Method B also causes repeated tx > softirqs in case of contention on netif_tx_lock. The code path is : > netif_tx_trylock() fails -> requeue() -> netif_schedule() -> > raise_softirq(NET_TX_SOFTIRQ). Am I missing anything? >
Never mind, you right. I think the netif_tx_trylock() there is a bad idea because of the long route as shown. Trylock's are usually bad SMP design anyway. Not sure if it would be possible to change the locking in qdisc_restart() without causing collateral damage in network drivers. -- Stephen Hemminger <[EMAIL PROTECTED]> - To unsubscribe from this list: send the line "unsubscribe netdev" in the body of a message to [EMAIL PROTECTED] More majordomo info at http://vger.kernel.org/majordomo-info.html
