net: rps: add the shortcut for one rps_cpus

When there is only one rps_cpus, skb_get_rxhash() can be eliminated.
Signed-off-by: Changli Gao <xiaosuo@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/core/dev.c

@@ -2343,7 +2343,7 @@ static int get_rps_cpu(struct net_device *dev, struct sk_buff *skb,
 		       struct rps_dev_flow **rflowp)
 {
 	struct netdev_rx_queue *rxqueue;
-	struct rps_map *map;
+	struct rps_map *map = NULL;
 	struct rps_dev_flow_table *flow_table;
 	struct rps_sock_flow_table *sock_flow_table;
 	int cpu = -1;
@@ -2361,8 +2361,17 @@ static int get_rps_cpu(struct net_device *dev, struct sk_buff *skb,
 	} else
 		rxqueue = dev->_rx;
 
-	if (!rxqueue->rps_map && !rxqueue->rps_flow_table)
+	if (rxqueue->rps_map) {
+		map = rcu_dereference(rxqueue->rps_map);
+		if (map && map->len == 1) {
+			tcpu = map->cpus[0];
+			if (cpu_online(tcpu))
+				cpu = tcpu;
+			goto done;
+		}
+	} else if (!rxqueue->rps_flow_table) {
 		goto done;
+	}
 
 	skb_reset_network_header(skb);
 	if (!skb_get_rxhash(skb))
@@ -2407,7 +2416,6 @@ static int get_rps_cpu(struct net_device *dev, struct sk_buff *skb,
 		}
 	}
 
-	map = rcu_dereference(rxqueue->rps_map);
 	if (map) {
 		tcpu = map->cpus[((u64) skb->rxhash * map->len) >> 32];