Subject: [RFC,PATCH] loopback: calls netif_receive_skb() instead of netif_rx()

From: Eric Dumazet <[email-masked]>
Date: Thu, 21 Feb 2008 19:51:52 +0100

Hi David

This is an RFC, based on net-2.6 for convenience only.

Thank you

[RFC,PATCH] loopback: calls netif_receive_skb() instead of netif_rx()

Loopback transmit function loopback_xmit() actually calls netif_rx() to 
queue
a skb to the softnet queue, and arms a softirq so that this skb can be 
handled later.

This has a cost on SMP, because we need to hold a reference on the 
device, and free this
reference when softirq dequeues packet.

Following patch directly calls netif_receive_skb() and avoids lot of 
atomic operations.
(atomic_inc(&dev->refcnt), set_and_set_bit(NAPI_STATE_SCHED, &n->state), ...
  atomic_dec(&dev->refcnt)...), cache line ping-pongs on device refcnt, 
but also softirq overhead.

This gives a nice boost on tbench for example (5 % on my machine)

Signed-off-by: Eric Dumazet <[email-masked]>

diff --git a/drivers/net/loopback.c b/drivers/net/loopback.c
index f2a6e71..9bed7ed 100644
--- a/drivers/net/loopback.c
+++ b/drivers/net/loopback.c
@@ -158,7 +158,7 @@ static int loopback_xmit(struct sk_buff *skb, struct net_device *dev)
 	lb_stats->bytes += skb->len;
 	lb_stats->packets++;
 
-	netif_rx(skb);
+	netif_receive_skb(skb);
 
 	return 0;
 }