[PATCH v1 net-next] bonding: Optimise is_netpoll_tx_blocked().

[Kuniyuki Iwashima <kuniyu@google.com>]

bond_start_xmit() spends some cycles in is_netpoll_tx_blocked():

  if (unlikely(is_netpoll_tx_blocked(dev)))
      return NETDEV_TX_BUSY;

because of the "pushf;pop reg" sequence (aka irqs_disabled()).

Let's swap the conditions in is_netpoll_tx_blocked() and
convert netpoll_block_tx to a static key.

Before:

   1.23 │       mov    %gs:0x28,%rax
   1.24 │       mov    %rax,0x18(%rsp)
  29.45 │       pushfq
   0.50 │       pop    %rax
   0.47 │       test   $0x200,%eax
        │     ↓ je     1b4
   0.49 │ 32:   lea    0x980(%rsi),%rbx

After:

   0.72 │       mov    %gs:0x28,%rax
   0.81 │       mov    %rax,0x18(%rsp)
   0.82 │       nop
   2.77 │ 2a:   lea    0x980(%rsi),%rbx

Suggested-by: Eric Dumazet <edumazet@google.com>
Signed-off-by: Kuniyuki Iwashima <kuniyu@google.com>
---
 drivers/net/bonding/bond_main.c |  4 ++--
 include/net/bonding.h           | 10 +++++-----
 2 files changed, 7 insertions(+), 7 deletions(-)

diff --git a/drivers/net/bonding/bond_main.c b/drivers/net/bonding/bond_main.c
index 55a960da42b5..2f2dddcf9d30 100644
--- a/drivers/net/bonding/bond_main.c
+++ b/drivers/net/bonding/bond_main.c
@@ -206,7 +206,7 @@ MODULE_PARM_DESC(lp_interval, "The number of seconds between instances where "
 /*----------------------------- Global variables ----------------------------*/
 
 #ifdef CONFIG_NET_POLL_CONTROLLER
-atomic_t netpoll_block_tx = ATOMIC_INIT(0);
+DEFINE_STATIC_KEY_FALSE(netpoll_block_tx);
 #endif
 
 unsigned int bond_net_id __read_mostly;
@@ -6589,7 +6589,7 @@ static void __exit bonding_exit(void)
 
 #ifdef CONFIG_NET_POLL_CONTROLLER
 	/* Make sure we don't have an imbalance on our netpoll blocking */
-	WARN_ON(atomic_read(&netpoll_block_tx));
+	WARN_ON(static_branch_unlikely(&netpoll_block_tx));
 #endif
 }
 
diff --git a/include/net/bonding.h b/include/net/bonding.h
index 4ad5521e7731..7f8fda2fdfcb 100644
--- a/include/net/bonding.h
+++ b/include/net/bonding.h
@@ -91,22 +91,22 @@
 			    NETIF_F_GSO_ESP)
 
 #ifdef CONFIG_NET_POLL_CONTROLLER
-extern atomic_t netpoll_block_tx;
+DECLARE_STATIC_KEY_FALSE(netpoll_block_tx);
 
 static inline void block_netpoll_tx(void)
 {
-	atomic_inc(&netpoll_block_tx);
+	static_branch_inc(&netpoll_block_tx);
 }
 
 static inline void unblock_netpoll_tx(void)
 {
-	atomic_dec(&netpoll_block_tx);
+	static_branch_dec(&netpoll_block_tx);
 }
 
 static inline int is_netpoll_tx_blocked(struct net_device *dev)
 {
-	if (unlikely(netpoll_tx_running(dev)))
-		return atomic_read(&netpoll_block_tx);
+	if (static_branch_unlikely(&netpoll_block_tx))
+		return netpoll_tx_running(dev);
 	return 0;
 }
 #else
-- 
2.53.0.414.gf7e9f6c205-goog