ipv6: Use hash-threshold instead of modulo-N

Now that each nexthop stores its region boundary in the multipath hash
function's output space, we can use hash-threshold instead of modulo-N
in multipath selection.

This reduces the number of checks we need to perform during lookup, as
dead and linkdown nexthops are assigned a negative region boundary. In
addition, in contrast to modulo-N, only flows near region boundaries are
affected when a nexthop is added or removed.
Signed-off-by: Ido Schimmel <idosch@mellanox.com>
Acked-by: David Ahern <dsahern@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/ipv6/route.c

@@ -455,7 +455,6 @@ static struct rt6_info *rt6_multipath_select(struct rt6_info *match,
 					     int strict)
 {
 	struct rt6_info *sibling, *next_sibling;
-	int route_choosen;
 
 	/* We might have already computed the hash for ICMPv6 errors. In such
 	 * case it will always be non-zero. Otherwise now is the time to do it.
@@ -463,28 +462,19 @@ static struct rt6_info *rt6_multipath_select(struct rt6_info *match,
 	if (!fl6->mp_hash)
 		fl6->mp_hash = rt6_multipath_hash(fl6, NULL);
 
-	route_choosen = fl6->mp_hash % (match->rt6i_nsiblings + 1);
-	/* Don't change the route, if route_choosen == 0
-	 * (siblings does not include ourself)
-	 */
-	if (route_choosen)
-		list_for_each_entry_safe(sibling, next_sibling,
-				&match->rt6i_siblings, rt6i_siblings) {
-			route_choosen--;
-			if (route_choosen == 0) {
-				struct inet6_dev *idev = sibling->rt6i_idev;
-
-				if (sibling->rt6i_nh_flags & RTNH_F_DEAD)
-					break;
-				if (sibling->rt6i_nh_flags & RTNH_F_LINKDOWN &&
-				    idev->cnf.ignore_routes_with_linkdown)
-					break;
-				if (rt6_score_route(sibling, oif, strict) < 0)
-					break;
-				match = sibling;
-				break;
-			}
-		}
+	if (fl6->mp_hash <= atomic_read(&match->rt6i_nh_upper_bound))
+		return match;
+
+	list_for_each_entry_safe(sibling, next_sibling, &match->rt6i_siblings,
+				 rt6i_siblings) {
+		if (fl6->mp_hash > atomic_read(&sibling->rt6i_nh_upper_bound))
+			continue;
+		if (rt6_score_route(sibling, oif, strict) < 0)
+			break;
+		match = sibling;
+		break;
+	}
+
 	return match;
 }