From: David Miller <davem@davemloft.net>
Date: Fri, 01 May 2009 09:17:47 -0700 (PDT)

From: Eric Dumazet <redacted>
Date: Fri, 01 May 2009 11:29:54 +0200

quoted

-	} else if (skb->sk && skb->sk->sk_hash) {
+		/*
+		 * Try to avoid an expensive divide, for symmetric setups :
+		 *   number of tx queues of output device ==
+		 *   number of rx queues of incoming device
+		 */
+		if (hash >= dev->real_num_tx_queues)
+			hash %= dev->real_num_tx_queues;
+		return hash;
+	}

Subtraction in a while() loop is almost certainly a lot
faster.

To move forward on this, I've commited the following to
net-next-2.6, thanks!

net: Avoid modulus in skb_tx_hash() for forwarding case.

Based almost entirely upon a patch by Eric Dumazet.

The common case is to have num-tx-queues <= num_rx_queues
and even if num_tx_queues is larger it will not be significantly
larger.

Therefore, a subtraction loop is always going to be faster than
modulus.

Signed-off-by: David S. Miller <davem@davemloft.net>
---
 net/core/dev.c |    8 ++++++--
 1 files changed, 6 insertions(+), 2 deletions(-)

diff --git a/net/core/dev.c b/net/core/dev.c
index 8144295..3c8073f 100644
--- a/net/core/dev.c
+++ b/net/core/dev.c

@@ -1735,8 +1735,12 @@ u16 skb_tx_hash(const struct net_device *dev, const struct sk_buff *skb)
 {
 	u32 hash;
 
-	if (skb_rx_queue_recorded(skb))
-		return skb_get_rx_queue(skb) % dev->real_num_tx_queues;
+	if (skb_rx_queue_recorded(skb)) {
+		hash = skb_get_rx_queue(skb);
+		while (unlikely (hash >= dev->real_num_tx_queues))
+			hash -= dev->real_num_tx_queues;
+		return hash;
+	}
 
 	if (skb->sk && skb->sk->sk_hash)
 		hash = skb->sk->sk_hash;

-- 
1.6.2.4