linux-kernel - Re: [Patch v5 5/6] thermal/cpu-cooling: Update thermal pressure in case of a maximum frequency capping

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Date:   Thu, 7 Nov 2019 14:00:22 +0100
From:   Dietmar Eggemann <dietmar.eggemann@....com>
To:     Thara Gopinath <thara.gopinath@...aro.org>, mingo@...hat.com,
        peterz@...radead.org, ionela.voinescu@....com,
        vincent.guittot@...aro.org, rui.zhang@...el.com,
        edubezval@...il.com, qperret@...gle.com
Cc:     linux-kernel@...r.kernel.org, amit.kachhap@...il.com,
        javi.merino@...nel.org, daniel.lezcano@...aro.org
Subject: Re: [Patch v5 5/6] thermal/cpu-cooling: Update thermal pressure in
 case of a maximum frequency capping



On 06/11/2019 13:50, Dietmar Eggemann wrote:
> On 05/11/2019 19:49, Thara Gopinath wrote:
> 
> [...]
> 
>> diff --git a/drivers/thermal/cpu_cooling.c b/drivers/thermal/cpu_cooling.c
>> index 391f397..c65b7c4 100644
>> --- a/drivers/thermal/cpu_cooling.c
>> +++ b/drivers/thermal/cpu_cooling.c
>> @@ -218,6 +218,27 @@ static u32 cpu_power_to_freq(struct cpufreq_cooling_device *cpufreq_cdev,
>>  }
>>  
>>  /**
>> + * update_sched_max_capacity - update scheduler about change in cpu
>> + *				max frequency.
>> + * @cpus: list of cpus whose max capacity needs udating in scheduler.
>> + * @cur_max_freq: current maximum frequency.
>> + * @max_freq: highest possible frequency.
>> + */
>> +static void update_sched_max_capacity(struct cpumask *cpus,
>> +				      unsigned int cur_max_freq,
>> +				      unsigned int max_freq)
>> +{
>> +	int cpu;
>> +	unsigned long capacity;
>> +
>> +	for_each_cpu(cpu, cpus) {
>> +		capacity = cur_max_freq * arch_scale_cpu_capacity(cpu);
>> +		capacity /= max_freq;
>> +		update_thermal_pressure(cpu, capacity);
>> +	}
>> +}
>> +
>> +/**
> 
> Have you seen
> https://lore.kernel.org/r/2b19d7da-412c-932f-7251-110eadbef3e3@arm.com ?
> 
> Also the naming 'update_thermal_pressure()' is not really suitable for
> an external task scheduler interface. If I see this called in the
> cooling device code, I wouldn't guess that this is a task scheduler
> interface.
> 
> [...]

The current interface to bring in frequency and uarch (CPU) invariance
information into the task scheduler is arch_scale_[freq|cpu]_capacity.

So why not follow this approach for the thermal->sched interface?

+#ifndef arch_scale_thermal_capacity
+static __always_inline
+unsigned long arch_scale_thermal_capacity(int cpu)
+{
+       return 0;
+}
+#endif

Leave per_cpu(thermal_pressure, cpu) within cpu cooling and let cpu
cooling (or even platform) #define arch_scale_thermal_capacity.