linux-kernel - Re: [PATCH] sched/fair: reduce preemption with IDLE tasks runable

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Date:   Mon, 3 Aug 2020 10:16:37 +0200
From:   Dietmar Eggemann <dietmar.eggemann@....com>
To:     Jiang Biao <benbjiang@...il.com>, mingo@...hat.com,
        peterz@...radead.org, juri.lelli@...hat.com,
        vincent.guittot@...aro.org
Cc:     rostedt@...dmis.org, bsegall@...gle.com, mgorman@...e.de,
        linux-kernel@...r.kernel.org, Jiang Biao <benbjiang@...cent.com>
Subject: Re: [PATCH] sched/fair: reduce preemption with IDLE tasks runable

On 01/08/2020 04:32, Jiang Biao wrote:
> From: Jiang Biao <benbjiang@...cent.com>
> 
> No need to preempt when there are only one runable CFS task with
> other IDLE tasks on runqueue. The only one CFS task would always
> be picked in that case.
> 
> Signed-off-by: Jiang Biao <benbjiang@...cent.com>
> ---
>  kernel/sched/fair.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> index 04fa8dbcfa4d..8fb80636b010 100644
> --- a/kernel/sched/fair.c
> +++ b/kernel/sched/fair.c
> @@ -4527,7 +4527,7 @@ entity_tick(struct cfs_rq *cfs_rq, struct sched_entity *curr, int queued)
>  		return;
>  #endif
>  
> -	if (cfs_rq->nr_running > 1)
> +	if (cfs_rq->nr_running > cfs_rq.idle_h_nr_running + 1)

cfs_rq is a pointer.

>  		check_preempt_tick(cfs_rq, curr);
>  }

You can't compare cfs_rq->nr_running with cfs_rq->idle_h_nr_running!

There is a difference between cfs_rq->h_nr_running and
cfs_rq->nr_running. The '_h_' stands for hierarchical.

The former gives you hierarchical task accounting whereas the latter is
the number of sched entities (representing tasks or taskgroups) enqueued
in cfs_rq.

In entity_tick(), cfs_rq->nr_running has to be used for the condition to
call check_preempt_tick(). We want to check if curr has to be preempted
by __pick_first_entity(cfs_rq) on this cfs_rq.

entity_tick() is called for each sched entity (and so for each
cfs_rq_of(se)) of the task group hierarchy (e.g. task p running in
taskgroup /A/B : se(p) -> se(A/B) -> se(A)).