linux-kernel - Re: [PATCH] sched/fair: Fix cfs_rq_is

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Date:   Fri, 18 Aug 2023 14:25:03 +0200
From:   Vincent Guittot <vincent.guittot@...aro.org>
To:     chengming.zhou@...ux.dev
Cc:     mingo@...hat.com, peterz@...radead.org, ycliang@...estech.com,
        juri.lelli@...hat.com, dietmar.eggemann@....com,
        rostedt@...dmis.org, bsegall@...gle.com, mgorman@...e.de,
        bristot@...hat.com, vschneid@...hat.com,
        zhouchengming@...edance.com, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] sched/fair: Fix cfs_rq_is_decayed() on !SMP

On Fri, 18 Aug 2023 at 13:37, <chengming.zhou@...ux.dev> wrote:
>
> From: Chengming Zhou <zhouchengming@...edance.com>
>
> We don't need to maintain per-queue leaf_cfs_rq_list on !SMP, since
> it's used for cfs_rq load tracking & balance on SMP.
>
> But sched debug interface use it to print per-cfs_rq stats, which
> maybe better to change to use walk_tg_tree_from() instead.
>
> This patch just fix the !SMP version cfs_rq_is_decayed(), so the
> per-queue leaf_cfs_rq_list is also maintained correctly on !SMP,
> to fix the warning in assert_list_leaf_cfs_rq().
>
> Fixes: 0a00a354644e ("sched/fair: Delete useless condition in tg_unthrottle_up()")
> Reported-by: Leo Liang <ycliang@...estech.com>
> Signed-off-by: Chengming Zhou <zhouchengming@...edance.com>
> ---
>  kernel/sched/fair.c | 2 ++
>  1 file changed, 2 insertions(+)
>
> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> index a80a73909dc2..00ef7e86a95b 100644
> --- a/kernel/sched/fair.c
> +++ b/kernel/sched/fair.c
> @@ -4654,6 +4654,8 @@ static inline void update_misfit_status(struct task_struct *p, struct rq *rq)
>
>  static inline bool cfs_rq_is_decayed(struct cfs_rq *cfs_rq)
>  {
> +       if (cfs_rq->load.weight)
> +               return false;
>         return true;

Why not :

return !(cfs_rq->nr_running);

The above seems easier to understand although I agree that both do the
same thing at the end

>  }
>
> --
> 2.41.0
>