linux-kernel - Re: [PATCH] sched: Take thermal pressure into account when determine rt fits capacity

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Message-ID: <9bb00bcb-f984-1cf6-39aa-c11dcf7f07cb@arm.com>
Date:   Mon, 11 Apr 2022 16:07:14 +0200
From:   Dietmar Eggemann <dietmar.eggemann@....com>
To:     Xuewen Yan <xuewen.yan94@...il.com>
Cc:     Xuewen Yan <xuewen.yan@...soc.com>, Lukasz.Luba@....com,
        rafael@...nel.org, viresh.kumar@...aro.org, mingo@...hat.com,
        peterz@...radead.org, vincent.guittot@...aro.org,
        rostedt@...dmis.org, linux-kernel@...r.kernel.org,
        di.shen@...soc.com
Subject: Re: [PATCH] sched: Take thermal pressure into account when determine
 rt fits capacity

On 11/04/2022 10:52, Xuewen Yan wrote:
> HI Dietmar
> 
> On Mon, Apr 11, 2022 at 4:21 PM Dietmar Eggemann
> <dietmar.eggemann@....com> wrote:
>>
>> On 07/04/2022 07:19, Xuewen Yan wrote:
>>> There are cases when the cpu max capacity might be reduced due to thermal.
>>> Take into the thermal pressure into account when judge whether the rt task
>>> fits the cpu. And when schedutil govnor get cpu util, the thermal pressure
>>> also should be considered.
>>>
>>> Signed-off-by: Xuewen Yan <xuewen.yan@...soc.com>
>>> ---
>>>  kernel/sched/cpufreq_schedutil.c | 1 +
>>>  kernel/sched/rt.c                | 1 +
>>>  2 files changed, 2 insertions(+)
>>>
>>> diff --git a/kernel/sched/cpufreq_schedutil.c b/kernel/sched/cpufreq_schedutil.c
>>> index 3dbf351d12d5..285ad51caf0f 100644
>>> --- a/kernel/sched/cpufreq_schedutil.c
>>> +++ b/kernel/sched/cpufreq_schedutil.c
>>> @@ -159,6 +159,7 @@ static void sugov_get_util(struct sugov_cpu *sg_cpu)
>>>       struct rq *rq = cpu_rq(sg_cpu->cpu);
>>>       unsigned long max = arch_scale_cpu_capacity(sg_cpu->cpu);
>>>
>>> +     max -= arch_scale_thermal_pressure(sg_cpu->cpu);
>>
>> max' = arch_scale_cpu_capacity() - arch_scale_thermal_pressure()
>>
>> For the energy part (A) we use max' in compute_energy() to cap sum_util
>> and max_util at max' and to call em_cpu_energy(..., max_util, sum_util,
>> max'). This was done to match (B)'s `policy->max` capping.
>>
>> For the frequency part (B) we have freq_qos_update_request() in:
>>
>> power_actor_set_power()
>>   ...
>>   cdev->ops->set_cur_state()
>>
>>     cpufreq_set_cur_state()
>>       freq_qos_update_request()      <-- !
>>       arch_update_thermal_pressure()
>>
>> restricting `policy->max` which then clamps `target_freq` in:
>>
>>   cpufreq_update_util()
>>     ...
>>     get_next_freq()
>>       cpufreq_driver_resolve_freq()
>>         __resolve_freq()
>>
> 
> Do you mean that the "max" here will not affect the frequency
> conversion, so there is no need to change it?
> But is it better to reflect the influence of thermal here?

I guess your point is that even though max' has no effect on frequency
since QOS caps policy->max anyway, it is still easier to understand the
dependency between schedutil and EAS/EM when it comes to the use of
thermal pressure.

I agree. The way how the "hidden" policy->max capping guarantees that
schedutil and EAS/EM are doing the same even when only the latter uses
max' is not obvious.

I just wanted to mention the historical reason why the code looks like
it does today.

>> [...]
>>
>>> diff --git a/kernel/sched/rt.c b/kernel/sched/rt.c
>>> index a32c46889af8..d9982ebd4821 100644
>>> --- a/kernel/sched/rt.c
>>> +++ b/kernel/sched/rt.c
>>> @@ -466,6 +466,7 @@ static inline bool rt_task_fits_capacity(struct task_struct *p, int cpu)
>>>       max_cap = uclamp_eff_value(p, UCLAMP_MAX);
>>>
>>>       cpu_cap = capacity_orig_of(cpu);
>>> +     cpu_cap -= arch_scale_thermal_pressure(cpu);
>>>
>>>       return cpu_cap >= min(min_cap, max_cap);
>>>  }
>>
>> IMHO, this should follow what we do with rq->cpu_capacity
>> (capacity_of(), the remaining capacity for CFS). E.g. we use
>> capacity_of() in find_energy_efficient_cpu() and select_idle_capacity()
>> to compare capacities. So we would need a function like
>> scale_rt_capacity() for RT (minus the rq->avg_rt.util_avg) but then also
>> one for DL (minus rq->avg_dl.util_avg and rq->avg_rt.util_avg).
> 
> It's a really good idea. And do you already have the corresponding patch?
> If there is, can you tell me the corresponding link?

No, I don't have any code for this. Should be trivial though. But the
issue is that the update would probably have to be decoupled from CFS
load_balance (update_group_capacity()) and the use cases in RT/DL are
only valid for asymmetric CPU capacity systems.