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

[Peng Wang]

On 6/15/20 11:09 PM, Vincent Guittot wrote:
> 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

Yes, then we can also skip the early balance check.

> > --
> > 2.9.5