soreuseport: TCP/IPv6 implementation

Motivation for soreuseport would be something like a web server
binding to port 80 running with multiple threads, where each thread
might have it's own listener socket.  This could be done as an
alternative to other models: 1) have one listener thread which
dispatches completed connections to workers. 2) accept on a single
listener socket from multiple threads.  In case #1 the listener thread
can easily become the bottleneck with high connection turn-over rate.
In case #2, the proportion of connections accepted per thread tends
to be uneven under high connection load (assuming simple event loop:
while (1) { accept(); process() }, wakeup does not promote fairness
among the sockets.  We have seen the  disproportion to be as high
as 3:1 ratio between thread accepting most connections and the one
accepting the fewest.  With so_reusport the distribution is
uniform.

Signed-off-by: Tom Herbert <therbert@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

include/net/inet6_hashtables.h

@@ -71,6 +71,8 @@ extern struct sock *__inet6_lookup_established(struct net *net,
 
 extern struct sock *inet6_lookup_listener(struct net *net,
 					  struct inet_hashinfo *hashinfo,
+					  const struct in6_addr *saddr,
+					  const __be16 sport,
 					  const struct in6_addr *daddr,
 					  const unsigned short hnum,
 					  const int dif);
@@ -88,7 +90,8 @@ static inline struct sock *__inet6_lookup(struct net *net,
 	if (sk)
 		return sk;
 
-	return inet6_lookup_listener(net, hashinfo, daddr, hnum, dif);
+	return inet6_lookup_listener(net, hashinfo, saddr, sport,
+				     daddr, hnum, dif);
 }
 
 static inline struct sock *__inet6_lookup_skb(struct inet_hashinfo *hashinfo,

include/net/netfilter/nf_tproxy_core.h

@@ -152,6 +152,7 @@ nf_tproxy_get_sock_v6(struct net *net, const u8 protocol,
 			break;
 		case NFT_LOOKUP_LISTENER:
 			sk = inet6_lookup_listener(net, &tcp_hashinfo,
+						   saddr, sport,
 						   daddr, ntohs(dport),
 						   in->ifindex);