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

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Date:   Tue, 17 Mar 2020 14:59:34 +0100
From:   Daniel Lezcano <daniel.lezcano@...aro.org>
To:     Valentin Schneider <valentin.schneider@....com>
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

On 17/03/2020 11:56, Valentin Schneider wrote:
> 
> 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?
Oh, no it is a stupid mistake from me. Thanks for pointing this out!

It should be:

	break_even = now + idle_state->target_residency_ns;

 - Documentation/devicetree/bindings/arm/idle-states.yaml

* min-residency: Minimum period, including preparation and entry, for a
given idle state to be worthwhile energywise.

(target_residency_ns == min-residency).

> 
>>  }
>>
>>  static int __read_mostly cpu_idle_force_poll;


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