netdev - Re: [PATCH v5] rps: Receive Packet Steering

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Date:	Fri, 15 Jan 2010 10:22:03 +0800
From:	Changli Gao <xiaosuo@...il.com>
To:	Tom Herbert <therbert@...gle.com>
Cc:	davem@...emloft.net, netdev@...r.kernel.org,
	Netfilter Developer Mailing List 
	<netfilter-devel@...r.kernel.org>
Subject: Re: [PATCH v5] rps: Receive Packet Steering

On Fri, Jan 15, 2010 at 5:56 AM, Tom Herbert <therbert@...gle.com> 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@...il.com)