Re: [PATCH net-next] net: Optimize IPv6 path in ip_neigh_for_gw()

[David Ahern <dsahern@kernel.org>]

On 10/4/24 10:27 AM, Breno Leitao wrote:
> Branch annotation traces from approximately 200 IPv6-enabled hosts
> revealed that the 'likely' branch in ip_neigh_for_gw() was consistently
> mispredicted. Given the increasing prevalence of IPv6 in modern networks,
> this commit adjusts the function to favor the IPv6 path.
> 
> Swap the order of the conditional statements and move the 'likely'
> annotation to the IPv6 case. This change aims to improve performance in
> IPv6-dominant environments by reducing branch mispredictions.
> 
> This optimization aligns with the trend of IPv6 becoming the default IP
> version in many deployments, and should benefit modern network
> configurations.
> 
> Signed-off-by: Breno Leitao <leitao@debian.org>
> ---
>  include/net/route.h | 6 +++---
>  1 file changed, 3 insertions(+), 3 deletions(-)
> 
> diff --git a/include/net/route.h b/include/net/route.h
> index 1789f1e6640b..b90b7b1effb8 100644
> --- a/include/net/route.h
> +++ b/include/net/route.h
> @@ -389,11 +389,11 @@ static inline struct neighbour *ip_neigh_for_gw(struct rtable *rt,
>  	struct net_device *dev = rt->dst.dev;
>  	struct neighbour *neigh;
>  
> -	if (likely(rt->rt_gw_family == AF_INET)) {
> -		neigh = ip_neigh_gw4(dev, rt->rt_gw4);
> -	} else if (rt->rt_gw_family == AF_INET6) {
> +	if (likely(rt->rt_gw_family == AF_INET6)) {
>  		neigh = ip_neigh_gw6(dev, &rt->rt_gw6);
>  		*is_v6gw = true;
> +	} else if (rt->rt_gw_family == AF_INET) {
> +		neigh = ip_neigh_gw4(dev, rt->rt_gw4);
>  	} else {
>  		neigh = ip_neigh_gw4(dev, ip_hdr(skb)->daddr);
>  	}

This is an IPv4 function allowing support for IPv6 addresses as a
nexthop. It is appropriate for IPv4 family checks to be first.