linux-kernel - Re: [PATCH v2 2/3] sched/uclamp: Ignore (util == 0) optimization in feec() when p_util

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Date:   Wed, 8 Feb 2023 12:52:33 +0100
From:   Dietmar Eggemann <dietmar.eggemann@....com>
To:     Vincent Guittot <vincent.guittot@...aro.org>,
        Qais Yousef <qyousef@...alina.io>
Cc:     Ingo Molnar <mingo@...nel.org>,
        Peter Zijlstra <peterz@...radead.org>,
        linux-kernel@...r.kernel.org, Lukasz Luba <lukasz.luba@....com>,
        Wei Wang <wvw@...gle.com>, Xuewen Yan <xuewen.yan94@...il.com>,
        Hank <han.lin@...iatek.com>,
        Jonathan JMChen <Jonathan.JMChen@...iatek.com>
Subject: Re: [PATCH v2 2/3] sched/uclamp: Ignore (util == 0) optimization in
 feec() when p_util_max = 0

On 07/02/2023 11:04, Vincent Guittot wrote:
> On Sun, 5 Feb 2023 at 23:43, Qais Yousef <qyousef@...alina.io> wrote:
>>
>> find_energy_efficient_cpu() bails out early if effective util of the
>> task is 0. When uclamp is being used, this could lead to wrong decisions
>> when uclamp_max is set to 0. Cater for that.

IMHO this needs a little bit more explanation. Someone could argue that
'util > 0, uclamp_min=0, uclamp_max=0' is a valid setup for a task which
should let it appear as a task with 0 util (capped to 0).

>> Fixes: d81304bc6193 ("sched/uclamp: Cater for uclamp in find_energy_efficient_cpu()'s early exit condition")
>> Signed-off-by: Qais Yousef <qyousef@...alina.io>
>> ---
>>  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 7a21ee74139f..a8c3d92ff3f6 100644
>> --- a/kernel/sched/fair.c
>> +++ b/kernel/sched/fair.c
>> @@ -7374,7 +7374,7 @@ static int find_energy_efficient_cpu(struct task_struct *p, int prev_cpu)
>>         target = prev_cpu;
>>
>>         sync_entity_load_avg(&p->se);
>> -       if (!uclamp_task_util(p, p_util_min, p_util_max))
>> +       if (!uclamp_task_util(p, p_util_min, p_util_max) && p_util_max != 0)
> 
> The below should do the same without testing twice p_util_max:
> uclamp_task_util(p, p_util_min, ULONG_MAX)

Since uclamp_task_util() is only used here and we don't want to test for
capping to 0 anymore, why not just get rid of this function and use:

  !(task_util_est(p) || p_util_min)

[...]