[PATCH -tip] kernel/sched,wake_q: Branch predict wake_q_add() cmpxchg

Davidlohr Bueso
2018-12-03T05:31:45Z

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@suse.de>
---
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 091e089063be..f7747cf6e427 100644
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -408,7 +408,7 @@ void wake_q_add(struct wake_q_head *head, struct task_struct *task)
* This cmpxchg() executes a full barrier, which pairs with the full
* barrier executed by the wakeup in wake_up_q().
*/
- if (cmpxchg(&node->next, NULL, WAKE_Q_TAIL))
+ if (unlikely(cmpxchg(&node->next, NULL, WAKE_Q_TAIL)))
return;

get_task_struct(task);
--
2.16.4