Re: [PATCH] sched/fair: Optimize dequeue_task_fair()

[Vincent Guittot]

On Mon, 15 Jun 2020 at 16:20, Peng Wang <rocking@xxxxxxxxxxxxxxxxx> wrote:
>
> While looking at enqueue_task_fair and dequeue_task_fair, it occurred
> to me that dequeue_task_fair can also be optimized as Vincent described
> in commit 7d148be69e3a ("sched/fair: Optimize enqueue_task_fair()").

good point

>
> dequeue_throttle label can ensure se not to be NULL, and se is
> always NULL when reaching root level.
>
> Signed-off-by: Peng Wang <rocking@xxxxxxxxxxxxxxxxx>
> ---
>  kernel/sched/fair.c | 6 +++---
>  1 file changed, 3 insertions(+), 3 deletions(-)
>
> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> index cbcb2f7..e6be952 100644
> --- a/kernel/sched/fair.c
> +++ b/kernel/sched/fair.c
> @@ -5614,10 +5614,10 @@ static void dequeue_task_fair(struct rq *rq, struct task_struct *p, int flags)
>
>         }
>
> -dequeue_throttle:
> -       if (!se)
> -               sub_nr_running(rq, 1);
> +       /* At this point se is NULL and we are at root level*/
> +       sub_nr_running(rq, 1);
>
> +dequeue_throttle:
>         /* balance early to pull high priority tasks */
>         if (unlikely(!was_sched_idle && sched_idle_rq(rq)))
>                 rq->next_balance = jiffies;

You can move the label here because sched_idle_rq() uses
rq->nr-running and rq->cfs.idle_h_nr_running so they will not change
if we jump to the label before reaching root level

> --
> 2.9.5
>