Re: [PATCH] sched/fair: eliminate redundant code in sched_slice()

[Qais Yousef]

On 08/16/19 22:12, Peng Liu wrote:
> Since sched_slice() is used in high frequency,
> small change should also make sense.
> 
> Signed-off-by: Peng Liu <iwtba...@gmail.com>
> ---
>  kernel/sched/fair.c | 11 ++++-------
>  1 file changed, 4 insertions(+), 7 deletions(-)
> 
> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> index 1054d2cf6aaa..6ae2a507aac0 100644
> --- a/kernel/sched/fair.c
> +++ b/kernel/sched/fair.c
> @@ -694,19 +694,16 @@ static u64 sched_slice(struct cfs_rq *cfs_rq, struct 
> sched_entity *se)
>       u64 slice = __sched_period(cfs_rq->nr_running + !se->on_rq);
>  
>       for_each_sched_entity(se) {
> -             struct load_weight *load;
>               struct load_weight lw;
>  
>               cfs_rq = cfs_rq_of(se);
> -             load = &cfs_rq->load;
> +             lw = cfs_rq->load;
>  
> -             if (unlikely(!se->on_rq)) {
> +             if (unlikely(!se->on_rq))
>                       lw = cfs_rq->load;
>  
> -                     update_load_add(&lw, se->load.weight);
> -                     load = &lw;
> -             }
> -             slice = __calc_delta(slice, se->load.weight, load);
> +             update_load_add(&lw, se->load.weight);
> +             slice = __calc_delta(slice, se->load.weight, &lw);

Unless I misread the diff, you changed the behavior.

update_load_add() is only called if (unlikely(!se->on_rq)), but with your
change it is called unconditionally. And lw is set twice.

I think what you intended is this?


diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index bc9cfeaac8bd..6bbe17d5943f 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -694,19 +694,15 @@ static u64 sched_slice(struct cfs_rq *cfs_rq, struct 
sched_entity *se)
        u64 slice = __sched_period(cfs_rq->nr_running + !se->on_rq);

        for_each_sched_entity(se) {
-               struct load_weight *load;
                struct load_weight lw;

                cfs_rq = cfs_rq_of(se);
-               load = &cfs_rq->load;
-
-               if (unlikely(!se->on_rq)) {
-                       lw = cfs_rq->load;
+               lw = cfs_rq->load;

+               if (unlikely(!se->on_rq))
                        update_load_add(&lw, se->load.weight);
-                       load = &lw;
-               }
-               slice = __calc_delta(slice, se->load.weight, load);
+
+               slice = __calc_delta(slice, se->load.weight, &lw);
        }
        return slice;
 }


Cheers

--
Qais Yousef