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

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Date:	Thu, 21 Jan 2010 15:04:13 +0800
From:	Changli Gao <xiaosuo@...il.com>
To:	David Miller <davem@...emloft.net>
Cc:	eric.dumazet@...il.com, therbert@...gle.com,
	netdev@...r.kernel.org, netfilter-devel@...r.kernel.org
Subject: Re: [PATCH v5] rps: Receive Packet Steering

On Fri, Jan 15, 2010 at 5:26 PM, David Miller <davem@...emloft.net> wrote:
> From: Changli Gao <xiaosuo@...il.com>
> Date: Fri, 15 Jan 2010 17:20:43 +0800
>
> Calling a function is expensive.
>
> What was now a leaf function deep in the call chain, will no longer
> be, so GCC will need to push all live registers onto the stack,
> then reload them back into registers when ipv6_skip_exthdr() returns.
>
> And that function is expensive, it's a lot of code that %99 of the
> time serves no purpose at all.
>
> This will be executed for every single packet we process, and Linux
> can process millions of packets per second, so every cycle and every
> memory reference matters.
>

We can write a new inline function like this:

static inline int ipv6_get_ports(const struct sk_buff *skb, u16 *port1,
                u16 *port2)
{
        u8 nexthdr;
        int hdrlen;

        nexthdr = ipv6_hdr(skb)->nexthdr;
        hdrlen = sizeof(struct ipv6hdr);
        while (1) {
                switch (nexthdr) {
                case IPPROTO_TCP:
                case IPPROTO_UDP:
                case IPPROTO_DCCP:
                case IPPROTO_ESP:
                case IPPROTO_AH:
                case IPPROTO_SCTP:
                case IPPROTO_UDPLITE:
                        skb_copy_bits(skb, hdrlen, port1, 2);
                        skb_copy_bits(skb, hdrlen + 2, port2, 2);
                        return 0;
                case NEXTHDR_HOP:
                case NEXTHDR_ROUTING:
                case NEXTHDR_FRAGMENT:
                case NEXTHDR_AUTH:
                case NEXTHDR_DEST:
                        // some code like ipv6_skip_exthdr()
                        ....
                        break;
                case NEXTHDR_NONE:
                        return -1;
                default:
                        return -1;
                }
        }
}

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