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

Waiman Long
2018-12-03T16:11:04Z

On 12/03/2018 12:31 AM, Davidlohr Bueso wrote:
> 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);

Acked-by: Waiman Long <longman@redhat.com>