Subject: Re: [PATCH] IPIP tunnel performance improvement

From: zhao ya <[email-masked]>
Date: Sat, 27 Feb 2016 12:52:02 +0800

BTW,before the version 3.5 kernel, the source code contains the logic.
2.6.32, for example, in arp_bind_neighbour function, there are the following logic:

__be32 nexthop = ((struct rtable *) DST) - > rt_gateway;
if (dev - > flags & (IFF_LOOPBACK | IFF_POINTOPOINT))
nexthop = 0;
n = __neigh_lookup_errno (
...

zhao ya said, at 2/27/2016 12:40 PM:
> From: Zhao Ya <[email-masked]>
> Date: Sat, 27 Feb 2016 10:06:44 +0800
> Subject: [PATCH] IPIP tunnel performance improvement
> 
> bypass the logic of each packet's own neighbour creation when using 
> pointopint or loopback device.
> 
> Recently, in our tests, met a performance problem.
> In a large number of packets with different target IP address through 
> ipip tunnel, PPS will decrease sharply.
> 
> The output of perf top are as follows, __write_lock_failed is of the first:
>   - 5.89% [kernel]		[k] __write_lock_failed
>    -__write_lock_failed 					a
>    -_raw_write_lock_bh						a
>    -__neigh_create						a
>    -ip_finish_output						a
>    -ip_output							a
>    -ip_local_out						a
> 
> The neighbour subsystem will create a neighbour object for each target 
> when using pointopint device. When massive amounts of packets with diff-
> erent target IP address to be xmit through a pointopint device, these 
> packets will suffer the bottleneck at write_lock_bh(&tbl->lock) after 
> creating the neighbour object and then inserting it into a hash-table 
> at the same time. 
> 
> This patch correct it. Only one or little amounts of neighbour objects 
> will be created when massive amounts of packets with different target IP 
> address through ipip tunnel. 
> 
> As the result, performance will be improved.
> 
> 
> Signed-off-by: Zhao Ya <[email-masked]>
> Signed-off-by: Zhaoya <[email-masked]>
> ---
>  net/ipv4/ip_output.c | 2 ++
>  1 file changed, 2 insertions(+)
> 
> diff --git a/net/ipv4/ip_output.c b/net/ipv4/ip_output.c
> index 64878ef..d7c0594 100644
> --- a/net/ipv4/ip_output.c
> +++ b/net/ipv4/ip_output.c
> @@ -202,6 +202,8 @@ static int ip_finish_output2(struct net *net, struct sock *sk, struct sk_buff *s
>  
>  	rcu_read_lock_bh();
>  	nexthop = (__force u32) rt_nexthop(rt, ip_hdr(skb)->daddr);
> +	if (dev->flags & (IFF_LOOPBACK | IFF_POINTOPOINT))
> +		nexthop = 0;
>  	neigh = __ipv4_neigh_lookup_noref(dev, nexthop);
>  	if (unlikely(!neigh))
>  		neigh = __neigh_create(&arp_tbl, &nexthop, dev, false);
> 
>