On Fri, Jan 15, 2010 at 5:56 AM, Tom Herbert [off-list ref] wrote:

+
+       if (skb->rxhash)
+               goto got_hash; /* Skip hash computation on packet header */
+
+       switch (skb->protocol) {
+       case __constant_htons(ETH_P_IP):
+               if (!pskb_may_pull(skb, sizeof(*ip)))
+                       goto done;
+
+               ip = (struct iphdr *) skb->data;
+               ip_proto = ip->protocol;
+               addr1 = ip->saddr;
+               addr2 = ip->daddr;
+               ihl = ip->ihl;
+               break;
+       case __constant_htons(ETH_P_IPV6):
+               if (!pskb_may_pull(skb, sizeof(*ip6)))
+                       goto done;
+
+               ip6 = (struct ipv6hdr *) skb->data;
+               ip_proto = ip6->nexthdr;

This code can't work, when there are extra headers. ipv6_skip_exthdr()
can be used to get the l4 header.

+               addr1 = ip6->saddr.s6_addr32[3];
+               addr2 = ip6->daddr.s6_addr32[3];
+               ihl = (40 >> 2);
+               break;
+       default:
+               goto done;
+       }
+       ports = 0;
+       switch (ip_proto) {
+       case IPPROTO_TCP:
+       case IPPROTO_UDP:
+       case IPPROTO_DCCP:
+       case IPPROTO_ESP:
+       case IPPROTO_AH:
+       case IPPROTO_SCTP:
+       case IPPROTO_UDPLITE:
+               if (pskb_may_pull(skb, (ihl * 4) + 4))
+                       ports = *((u32 *) (skb->data + (ihl * 4)));
+               break;
+
+       default:
+               break;
+       }
+
+       skb->rxhash = jhash_3words(addr1, addr2, ports, hashrnd);

For connection based packet processing, such as netfilter,
distributing the packets in two directions into one CPU will reduce
cache miss, when NAT isn't used. I think the code bellow will help:
if (addr1 > addr2)
  swap(addr1, addr2);

+       if (!skb->rxhash)
+               skb->rxhash = 1;

Why not put the above code into a new function, and add more protocols
support, such as 802.1Q.  Though rxhash is based on 4-tuple, I think
netfilter will benefit from it.

-- 
Regards，
Changli Gao(xiaosuo@gmail.com)