Re: [RFC PATCH] sched/fair: update the vruntime to be max vruntime when yield

[Xuewen Yan]

Hi all

Looking forward to your comments.

Thanks！

On Wed, Feb 22, 2023 at 4:21 PM Xuewen Yan <xuewen.yan@xxxxxxxxxx> wrote:
>
> When task call the sched_yield, cfs would set the cfs's skip buddy.
> If there is no other task call the sched_yield syscall, the task would
> always be skiped when there are tasks in rq. As a result, the task's
> vruntime would not be updated for long time, and the cfs's min_vruntime
> is almost not updated.
> When this scenario happens, when the yield task had wait for a long time,
> and other tasks run a long time, once there is other task call the sched_yield,
> the cfs's skip_buddy is covered, at this time, the first task can run normally,
> but the task's vruntime is small, as a result, the task would always run,
> because other task's vruntime is big. This would lead to other tasks can not
> run for a long time.
> In order to mitigate this, update the yield_task's vruntime to be cfs's max vruntime.
> This way, the cfs's min vruntime can be updated as the process running.
>
> Signed-off-by: Xuewen Yan <xuewen.yan@xxxxxxxxxx>
> ---
>  kernel/sched/fair.c | 16 +++++++++++++---
>  1 file changed, 13 insertions(+), 3 deletions(-)
>
> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> index ff4dbbae3b10..a9ff1921fc07 100644
> --- a/kernel/sched/fair.c
> +++ b/kernel/sched/fair.c
> @@ -670,7 +670,6 @@ static struct sched_entity *__pick_next_entity(struct sched_entity *se)
>         return __node_2_se(next);
>  }
>
> -#ifdef CONFIG_SCHED_DEBUG
>  struct sched_entity *__pick_last_entity(struct cfs_rq *cfs_rq)
>  {
>         struct rb_node *last = rb_last(&cfs_rq->tasks_timeline.rb_root);
> @@ -681,6 +680,7 @@ struct sched_entity *__pick_last_entity(struct cfs_rq *cfs_rq)
>         return __node_2_se(last);
>  }
>
> +#ifdef CONFIG_SCHED_DEBUG
>  /**************************************************************
>   * Scheduling class statistics methods:
>   */
> @@ -7751,8 +7751,18 @@ static void set_next_buddy(struct sched_entity *se)
>
>  static void set_skip_buddy(struct sched_entity *se)
>  {
> -       for_each_sched_entity(se)
> -               cfs_rq_of(se)->skip = se;
> +       for_each_sched_entity(se) {
> +               struct sched_entity *last;
> +               struct cfs_rq *cfs_rq = cfs_rq_of(se);
> +
> +               last = __pick_last_entity(cfs_rq);
> +               if (last) {
> +                       se->vruntime = last->vruntime;
> +                       update_min_vruntime(cfs_rq);
> +               }
> +
> +               cfs_rq->skip = se;
> +       }
>  }
>
>  /*
> --
> 2.25.1
>