Re: [RFC PATCH v6 6/9] thermal: cpu_cooling: implement the power cooling device API

[Eduardo Valentin <edubezval@gmail.com>]

:x
> Hi Eduardo,
> 
> Eduardo Valentin <edubezval@gmail.com> writes:
> 
> > Hello Javi,
> >
> > On Fri, Dec 05, 2014 at 07:04:17PM +0000, Javi Merino wrote:
> >> Add a basic power model to the cpu cooling device to implement the
> >> power cooling device API.  The power model uses the current frequency,
> >> current load and OPPs for the power calculations.  The cpus must have
> >> registered their OPPs using the OPP library.
> >> 
> >> Cc: Zhang Rui <rui.zhang@intel.com>
> >> Cc: Eduardo Valentin <edubezval@gmail.com>
> >> Signed-off-by: Punit Agrawal <punit.agrawal@arm.com>
> >> Signed-off-by: Javi Merino <javi.merino@arm.com>
> >
> > <big cut>
> >
> >> +
> >> +/**
> >> + * get_load() - get load for a cpu since last updated
> >> + * @cpufreq_device:	&struct cpufreq_cooling_device for this cpu
> >> + * @cpu:	cpu number
> >> + *
> >> + * Return: The average load of cpu @cpu in percentage since this
> >> + * function was last called.
> >> + */
> >> +static u32 get_load(struct cpufreq_cooling_device *cpufreq_device, int cpu)
> >> +{
> >> +	u32 load;
> >> +	u64 now, now_idle, delta_time, delta_idle;
> >> +
> >> +	now_idle = get_cpu_idle_time(cpu, &now, 0);
> >> +	delta_idle = now_idle - cpufreq_device->time_in_idle[cpu];
> >> +	delta_time = now - cpufreq_device->time_in_idle_timestamp[cpu];
> >> +
> >> +	if (delta_time <= delta_idle)
> >> +		load = 0;
> >> +	else
> >> +		load = div64_u64(100 * (delta_time - delta_idle), delta_time);
> >> +
> >> +	cpufreq_device->time_in_idle[cpu] = now_idle;
> >> +	cpufreq_device->time_in_idle_timestamp[cpu] = now;
> >> +
> >> +	return load;
> >> +}
> >
> > <cut>
> >
> >>  
> >> +/**
> >> + * cpufreq_get_actual_power() - get the current power
> >> + * @cdev:	&thermal_cooling_device pointer
> >> + *
> >> + * Return the current power consumption of the cpus in milliwatts.
> >> + */
> >> +static u32 cpufreq_get_actual_power(struct thermal_cooling_device *cdev)
> >> +{
> >> +	unsigned long freq;
> >> +	int cpu;
> >> +	u32 static_power, dynamic_power, total_load = 0;
> >> +	struct cpufreq_cooling_device *cpufreq_device = cdev->devdata;
> >> +
> >> +	freq = cpufreq_quick_get(cpumask_any(&cpufreq_device->allowed_cpus));
> >> +
> >> +	for_each_cpu(cpu, &cpufreq_device->allowed_cpus) {
> >> +		u32 load;
> >> +
> >> +		if (cpu_online(cpu))
> >> +			load = get_load(cpufreq_device, cpu);
> >> +		else
> >> +			load = 0;
> >> +
> >> +		total_load += load;
> >> +	}
> >> +
> >> +	cpufreq_device->last_load = total_load;
> >> +
> >> +	static_power = get_static_power(cpufreq_device, freq);
> >> +	dynamic_power = get_dynamic_power(cpufreq_device, freq);
> >> +
> >> +	return static_power + dynamic_power;
> >> +}
> >
> > With respect to load computation vs. frequency usage vs. power
> > estimation, while getting actual power for a given interval T. What if
> > in interval T, we have used, say, 3 different cpu frequencies, and the
> > load on the first was 50%, on the second 80%, and on the last frequency,
> > the load was 60%, what should be the right load value for computing the
> > actual power? 
> >
> > I mean, we are using the total idle time for a given interval, but 1 -
> > idle not always seams to reflect actual load on different opps, if opps
> > change over time within T time interval window.
> 
> The value returned by cpufreq_get_actual_power is used as a proxy for
> the estimate of the requested power of the actor for the next window
> duration. Even though the frequency might have changed in the previous
> period, the current frequency reflects the latest information about the
> required performance. As it is an estimate, and to avoid making the
> power calculations more complicated, we used utilisation (1 - idle time)
> to calculate the request. The estimate for the T+1 period becomes more
> accurate as the load stabilises.
> 
> In our testing on different workloads using 100ms as the polling period
> for thermal control, we didn't see any problems arising from the use of
> this definition of utilisation.
> 
> Having said that, there are a number of ways to improve the accuracy of
> the power calculations. As part of investigating the effects of
> improving model accuracy and it's effect on thermal control and
> performance, we plan to look at fine-grained frequency and load tracking
> once the initial set of patches are merged.

In this case, I believe we must mark the code at least with a TODO or
REVISIT mark. Can we add the above comments within a REVISIT: mark in
this part of the code?

> 
> Cheers,
> Punit
> 
> >
> > BR,
> >
> >
> > BR,
> >
> > Eduardo Valentin