[tip:locking/core] sched/wake_q: Add branch prediction hint to wake_q_add() cmpxchg

[tip-bot for Davidlohr Bueso]

Commit-ID:  87ff19cb2f1aa55a5d8b691e6690cc059a59d2ec
Gitweb:     https://git.kernel.org/tip/87ff19cb2f1aa55a5d8b691e6690cc059a59d2ec
Author:     Davidlohr Bueso <dave@xxxxxxxxxxxx>
AuthorDate: Sun, 2 Dec 2018 21:31:30 -0800
Committer:  Ingo Molnar <mingo@xxxxxxxxxx>
CommitDate: Mon, 21 Jan 2019 11:18:50 +0100

sched/wake_q: Add branch prediction hint to wake_q_add() cmpxchg

The cmpxchg() will fail when the task is already in the process
of waking up, and as such is an extremely rare occurrence.
Micro-optimize the call and put an unlikely() around it.

To no surprise, when using CONFIG_PROFILE_ANNOTATED_BRANCHES
under a number of workloads the incorrect rate was a mere 1-2%.

Signed-off-by: Davidlohr Bueso <dbueso@xxxxxxx>
Signed-off-by: Peter Zijlstra (Intel) <peterz@xxxxxxxxxxxxx>
Acked-by: Waiman Long <longman@xxxxxxxxxx>
Cc: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
Cc: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx>
Cc: Paul E. McKenney <paulmck@xxxxxxxxxxxxxxxxxx>
Cc: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
Cc: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
Cc: Will Deacon <will.deacon@xxxxxxx>
Cc: Yongji Xie <elohimes@xxxxxxxxx>
Cc: andrea.parri@xxxxxxxxxxxxxxxxxxxx
Cc: lilin24@xxxxxxxxx
Cc: liuqi16@xxxxxxxxx
Cc: nixun@xxxxxxxxx
Cc: xieyongji@xxxxxxxxx
Cc: yuanlinsi01@xxxxxxxxx
Cc: zhangyu31@xxxxxxxxx
Link: https://lkml.kernel.org/r/20181203053130.gwkw6kg72azt2npb@linux-r8p5
Signed-off-by: Ingo Molnar <mingo@xxxxxxxxxx>
---
 kernel/sched/core.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/kernel/sched/core.c b/kernel/sched/core.c
index d8d76a65cfdd..b05eef7d7a1f 100644
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -421,7 +421,7 @@ void wake_q_add(struct wake_q_head *head, struct task_struct *task)
 	 * state, even in the failed case, an explicit smp_mb() must be used.
 	 */
 	smp_mb__before_atomic();
-	if (cmpxchg_relaxed(&node->next, NULL, WAKE_Q_TAIL))
+	if (unlikely(cmpxchg_relaxed(&node->next, NULL, WAKE_Q_TAIL)))
 		return;
 
 	get_task_struct(task);