linux-kernel - Re: [PATCH V2] sched: fair: Use the earliest break even

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Date:   Tue, 17 Mar 2020 10:56:11 +0000
From:   Valentin Schneider <valentin.schneider@....com>
To:     Daniel Lezcano <daniel.lezcano@...aro.org>
Cc:     peterz@...radead.org, mingo@...hat.com, juri.lelli@...hat.com,
        vincent.guittot@...aro.org, dietmar.eggemann@....com,
        rostedt@...dmis.org, bsegall@...gle.com,
        linux-kernel@...r.kernel.org, qais.yousef@....com
Subject: Re: [PATCH V2] sched: fair: Use the earliest break even


Hi Daniel,

One more comment on the break even itself, ignoring the rest:

On Wed, Mar 11 2020, Daniel Lezcano wrote:
> diff --git a/kernel/sched/idle.c b/kernel/sched/idle.c
> index b743bf38f08f..3342e7bae072 100644
> --- a/kernel/sched/idle.c
> +++ b/kernel/sched/idle.c
> @@ -19,7 +19,13 @@ extern char __cpuidle_text_start[], __cpuidle_text_end[];
>   */
>  void sched_idle_set_state(struct cpuidle_state *idle_state)
>  {
> -	idle_set_state(this_rq(), idle_state);
> +	struct rq *rq = this_rq();
> +
> +	idle_set_state(rq, idle_state);
> +
> +	if (idle_state)
> +		idle_set_break_even(rq, ktime_get_ns() +
> +				    idle_state->exit_latency_ns);

I'm not sure I follow why we go for entry time + exit latency. If this
is based on the minimum residency, shouldn't this be something depending
on the entry latency? i.e. something like

  break_even = now + entry_latency + idling_time
                     \_________________________/
                            min-residency

or am I missing something?

>  }
>
>  static int __read_mostly cpu_idle_force_poll;