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

[Paolo Abeni <pabeni@redhat.com>]

On 10/4/24 19:37, Breno Leitao wrote:
> On Fri, Oct 04, 2024 at 11:01:29AM -0600, David Ahern wrote:
>> 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.
> 
> Right. In which case is this called on IPv6 only systems?
> 
> On my IPv6-only 200 systems, the annotated branch predictor is showing
> it is mispredicted 100% of the time.

perf probe -a ip_neigh_for_gw; perf record -e probe:ip_neigh_for_gw -ag; 
perf script

should give you an hint.

Cheers,

Paolo