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Message-ID: <20241004-straight-prompt-auk-ada09a@leitao>
Date: Fri, 4 Oct 2024 10:37:52 -0700
From: Breno Leitao <leitao@...ian.org>
To: David Ahern <dsahern@...nel.org>
Cc: "David S. Miller" <davem@...emloft.net>,
Eric Dumazet <edumazet@...gle.com>,
Jakub Kicinski <kuba@...nel.org>, Paolo Abeni <pabeni@...hat.com>,
rmikey@...a.com, kernel-team@...a.com, horms@...nel.org,
"open list:NETWORKING [IPv4/IPv6]" <netdev@...r.kernel.org>,
open list <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH net-next] net: Optimize IPv6 path in ip_neigh_for_gw()
Hello David,
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@...ian.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.
Thanks for the review
--breno
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