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[NET_SCHED]: explict hold dev tx lock 

For N cpus, with full throttle traffic on all N CPUs, funneling traffic
to the same ethernet device, the devices queue lock is contended by all
N CPUs constantly. The TX lock is only contended by a max of 2 CPUS.
In the current mode of operation, after all the work of entering the
dequeue region, we may endup aborting the path if we are unable to get
the tx lock and go back to contend for the queue lock. As N goes up,
this gets worse.

The changes in this patch result in a small increase in performance
with a 4CPU (2xdual-core) with no irq binding. Both e1000 and tg3
showed similar behavior;

Signed-off-by: Jamal Hadi Salim <hadi@cyberus.ca>
Signed-off-by: David S. Miller <davem@davemloft.net>
This commit is contained in:
Jamal Hadi Salim 2007-09-25 19:27:13 -07:00 committed by David S. Miller
parent 854d8363f3
commit 8236632fb3
1 changed files with 2 additions and 17 deletions
Show Stats Download Patch File Download Diff File Expand all files Collapse all files

19
net/sched/sch_generic.c
View File

@ -134,34 +134,19 @@ static inline int qdisc_restart(struct net_device *dev)
{
struct Qdisc *q = dev->qdisc;
struct sk_buff *skb;
unsigned lockless;
int ret;
/* Dequeue packet */
if (unlikely((skb = dev_dequeue_skb(dev, q)) == NULL))
return 0;
/*
* When the driver has LLTX set, it does its own locking in
* start_xmit. These checks are worth it because even uncongested
* locks can be quite expensive. The driver can do a trylock, as
* is being done here; in case of lock contention it should return
* NETDEV_TX_LOCKED and the packet will be requeued.
*/
lockless = (dev->features & NETIF_F_LLTX);
if (!lockless && !netif_tx_trylock(dev)) {
/* Another CPU grabbed the driver tx lock */
return handle_dev_cpu_collision(skb, dev, q);
}
/* And release queue */
spin_unlock(&dev->queue_lock);
HARD_TX_LOCK(dev, smp_processor_id());
ret = dev_hard_start_xmit(skb, dev);
if (!lockless)
netif_tx_unlock(dev);
HARD_TX_UNLOCK(dev);
spin_lock(&dev->queue_lock);
q = dev->qdisc;