Re: [PATCH] sched/fair: favor non-idle group in tick preemption

[Josh Don <joshdon@google.com>]

Hi Chuyi,

On Thu, Oct 27, 2022 at 1:16 AM Chuyi Zhou <zhouchuyi@bytedance.com> wrote:
>
> The non-idle se dominates competition vs the idle se when they
> are belong to the same group. We ensure that idle groups would not
> preempt non-idle group in wakeup preemption(see check_preempt_wakeup()).
> However, this can happen in tick preemption, since check_preempt_tick()
> dose not check current/se is idle or not. This patch adds this check.
>
> Signed-off-by: Chuyi Zhou <zhouchuyi@bytedance.com>
> ---
>  kernel/sched/fair.c | 12 +++++++++++-
>  1 file changed, 11 insertions(+), 1 deletion(-)
>
> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> index e4a0b8bd941c..f3324b8753b3 100644
> --- a/kernel/sched/fair.c
> +++ b/kernel/sched/fair.c
> @@ -4750,6 +4750,7 @@ static void
>  check_preempt_tick(struct cfs_rq *cfs_rq, struct sched_entity *curr)
>  {
>         unsigned long ideal_runtime, delta_exec;
> +       int cse_is_idle, pse_is_idle;
>         struct sched_entity *se;
>         s64 delta;
>
> @@ -4779,8 +4780,17 @@ check_preempt_tick(struct cfs_rq *cfs_rq, struct sched_entity *curr)
>         if (delta < 0)
>                 return;
>
> -       if (delta > ideal_runtime)
> +       if (delta > ideal_runtime) {
> +               /*
> +                * Favor non-idle group even in tick preemption
> +                */
> +               cse_is_idle = se_is_idle(curr);
> +               pse_is_idle = se_is_idle(se);
> +               if (unlikely(!cse_is_idle && pse_is_idle))
> +                       return;

This would make it so that we never have tick based preemption of a
non-idle entity by an idle entity. That's a recipe for starvation of
the idle entity, if the non-idle entity is cpu bound.

Beyond that though, I'm not quite sure the issue being solved here.
The large difference in weight between the idle and non-idle entity
means that the non-idle entity will not be preempted for quite a while
due to its ideal_runtime being quite high. The idle entity will
quickly be preempted on the next tick it takes due to the smaller
value of sysctl_sched_idle_min_granularity.

The wakeup check is useful for latency sensitive non-idle tasks.
However, in steady state competition between idle and non-idle, we
must allow some amount of round-robin.

> +
>                 resched_curr(rq_of(cfs_rq));
> +       }
>  }
>
>  static void
> --
> 2.20.1
>

Best,
Josh