linux-kernel - Re: [PATCH V2 2/2] sched/fair: Use actual_cpu_capacity everywhere in util_fits

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Message-ID: <66f2ef35-cbc0-4205-96f1-c857efb6f9cd@arm.com>
Date: Fri, 5 Jul 2024 08:25:59 +0200
From: Dietmar Eggemann <dietmar.eggemann@....com>
To: Qais Yousef <qyousef@...alina.io>,
 Vincent Guittot <vincent.guittot@...aro.org>
Cc: Xuewen Yan <xuewen.yan@...soc.com>, mingo@...hat.com,
 peterz@...radead.org, juri.lelli@...hat.com, rostedt@...dmis.org,
 bsegall@...gle.com, mgorman@...e.de, bristot@...hat.com,
 vschneid@...hat.com, christian.loehle@....com, vincent.donnefort@....com,
 ke.wang@...soc.com, di.shen@...soc.com, xuewen.yan94@...il.com,
 linux-kernel@...r.kernel.org
Subject: Re: [PATCH V2 2/2] sched/fair: Use actual_cpu_capacity everywhere in
 util_fits_cpu()

On 05/07/2024 01:56, Qais Yousef wrote:
> On 07/03/24 16:54, Vincent Guittot wrote:
>> On Wed, 3 Jul 2024 at 13:54, Qais Yousef <qyousef@...alina.io> wrote:
>>>
>>> On 07/02/24 15:25, Vincent Guittot wrote:
>>>
>>>>>>        *
>>>>>>        * Only exception is for HW or cpufreq pressure since it has a direct impact
>>>>>>        * on available OPP of the system.
>>>>>> @@ -5011,7 +5011,7 @@ static inline int util_fits_cpu(unsigned long util,
>>>>>>        * For uclamp_max, we can tolerate a drop in performance level as the
>>>>>>        * goal is to cap the task. So it's okay if it's getting less.
>>>>>>        */
>>>>>> -     capacity_orig = arch_scale_cpu_capacity(cpu);
>>>>>> +     capacity_actual = get_actual_cpu_capacity(cpu);
>>>>>>
>>>>>>       /*
>>>>>>        * We want to force a task to fit a cpu as implied by uclamp_max.
>>>>>> @@ -5039,7 +5039,7 @@ static inline int util_fits_cpu(unsigned long util,
>>>>>>        *     uclamp_max request.
>>>>>>        *
>>>>>>        *   which is what we're enforcing here. A task always fits if
>>>>>> -      *   uclamp_max <= capacity_orig. But when uclamp_max > capacity_orig,
>>>>>> +      *   uclamp_max <= capacity_actual. But when uclamp_max > capacity_actual,
>>>>>>        *   the normal upmigration rules should withhold still.
>>>>>>        *
>>>>>>        *   Only exception is when we are on max capacity, then we need to be
>>>>>> @@ -5050,8 +5050,8 @@ static inline int util_fits_cpu(unsigned long util,
>>>>>>        *     2. The system is being saturated when we're operating near
>>>>>>        *        max capacity, it doesn't make sense to block overutilized.
>>>>>>        */
>>>>>> -     uclamp_max_fits = (capacity_orig == SCHED_CAPACITY_SCALE) && (uclamp_max == SCHED_CAPACITY_SCALE);
>>>>>> -     uclamp_max_fits = !uclamp_max_fits && (uclamp_max <= capacity_orig);
>>>>>> +     uclamp_max_fits = (capacity_actual == SCHED_CAPACITY_SCALE) && (uclamp_max == SCHED_CAPACITY_SCALE);
>>>>>
>>>>> We should use capacity_orig here. We are checking if the CPU is the max
>>>>> capacity CPU.
>>>>
>>>> I was also wondering what would be the best choice there. If we
>>>> consider that we have only one performance domain with all max
>>>> capacity cpus then I agree that we should keep capacity_orig as we
>>>> can't find a better cpu that would fit. But is it always true that all
>>>> max cpu are tied to the same perf domain ?

This should be the case. Perf domains follow Frequency Domains (FD). So
even if we have the most powerful CPUs in 2 different FDs, only the CPUs
in the FD with the higher max OPP get SCHED_CAPACITY_SCALE assigned,
i.e. only they become big CPUs.

>>> Hmm I could be not thinking straight today. But the purpose of this check is to
>>> ensure overutilized can trigger for the common case where a task will always
>>> fit the max capacity cpu (whether it's on a single pd or multiple ones). For
>>> that corner case fits_capacity() should be the only fitness check otherwise
>>> overutilized will never trigger by default.
>>
>> Ok, so I messed up several use cases but in fact both are similar ...
>>
>> if capacity_actual != SCHED_CAPACITY_SCALE and uclamp_max ==
>> SCHED_CAPACITY_SCALE
>> then uclamp_max_fits = (capacity_actual == SCHED_CAPACITY_SCALE) &&
>> (uclamp_max == SCHED_CAPACITY_SCALE) is false
>> and uclamp_max_fits = !uclamp_max_fits && (uclamp_max <=
>> capacity_actual); is also false because (uclamp_max <=
>> capacity_actual) is always false
>>
>> if capacity_actual == SCHED_CAPACITY_SCALE and uclamp_max ==
>> SCHED_CAPACITY_SCALE
>> then we are the same as with capacity_orig
> 
> Right. The condition is becoming less readable, but you're right it doesn't
> change functionality.

+1. 'capacity_actual < SCHED_CAPACITY_SCALE' on big CPUs just make them
non-big CPUs.

> Xuewen, could you put something in the commit message please to remind us in
> the future that we thought about this and it is fine?
> 
> Thanks!
> 
> --
> Qais Yousef