Messages in this thread Patch in this message | | | Date | Sun, 2 Dec 2018 21:31:30 -0800 | From | Davidlohr Bueso <> | Subject | [PATCH -tip] kernel/sched,wake_q: Branch predict wake_q_add() cmpxchg |
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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
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