net: use jump_label to shortcut RPS if not setup

Most machines dont use RPS/RFS, and pay a fair amount of instructions in
netif_receive_skb() / netif_rx() / get_rps_cpu() just to discover
RPS/RFS is not setup.

Add a jump_label named rps_needed.

If no device rps_map or global rps_sock_flow_table is setup,
netif_receive_skb() / netif_rx() do a single instruction instead of many
ones, including conditional jumps.

jmp +0    (if CONFIG_JUMP_LABEL=y)

Signed-off-by: Eric Dumazet <eric.dumazet@gmail.com>
CC: Tom Herbert <therbert@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

1 files changed, 9 insertions, 12 deletions

diff --git a/net/core/dev.c b/net/core/dev.c
index 26c49d55e79d..f78959996148 100644
--- a/net/core/dev.c
+++ b/net/core/dev.c
@@ -2711,6 +2711,8 @@ EXPORT_SYMBOL(__skb_get_rxhash);
 struct rps_sock_flow_table __rcu *rps_sock_flow_table __read_mostly;
 EXPORT_SYMBOL(rps_sock_flow_table);
 
+struct jump_label_key rps_needed __read_mostly;
+
 static struct rps_dev_flow *
 set_rps_cpu(struct net_device *dev, struct sk_buff *skb,
             struct rps_dev_flow *rflow, u16 next_cpu)
@@ -2994,7 +2996,7 @@ int netif_rx(struct sk_buff *skb)
 
         trace_netif_rx(skb);
 #ifdef CONFIG_RPS
-        {
+        if (static_branch(&rps_needed)) {
                 struct rps_dev_flow voidflow, *rflow = &voidflow;
                 int cpu;
 
@@ -3009,14 +3011,13 @@ int netif_rx(struct sk_buff *skb)
 
                 rcu_read_unlock();
                 preempt_enable();
-        }
-#else
+        } else
+#endif
         {
                 unsigned int qtail;
                 ret = enqueue_to_backlog(skb, get_cpu(), &qtail);
                 put_cpu();
         }
-#endif
         return ret;
 }
 EXPORT_SYMBOL(netif_rx);
@@ -3359,7 +3360,7 @@ int netif_receive_skb(struct sk_buff *skb)
                 return NET_RX_SUCCESS;
 
 #ifdef CONFIG_RPS
-        {
+        if (static_branch(&rps_needed)) {
                 struct rps_dev_flow voidflow, *rflow = &voidflow;
                 int cpu, ret;
 
@@ -3370,16 +3371,12 @@ int netif_receive_skb(struct sk_buff *skb)
                 if (cpu >= 0) {
                         ret = enqueue_to_backlog(skb, cpu, &rflow->last_qtail);
                         rcu_read_unlock();
-                } else {
-                        rcu_read_unlock();
-                        ret = __netif_receive_skb(skb);
+                        return ret;
                 }
-
-                return ret;
+                rcu_read_unlock();
         }
-#else
-        return __netif_receive_skb(skb);
 #endif
+        return __netif_receive_skb(skb);
 }
 EXPORT_SYMBOL(netif_receive_skb);