Re: [PATCH 1/4] cpufreq: Add a cpufreq pressure feedback for the scheduler

[Tim Chen]

On Tue, 2023-12-12 at 15:27 +0100, Vincent Guittot wrote:
> Provide to the scheduler a feedback about the temporary max available
> capacity. Unlike arch_update_thermal_pressure, this doesn't need to be
> filtered as the pressure will happen for dozens ms or more.
> 
> Signed-off-by: Vincent Guittot <vincent.guittot@xxxxxxxxxx>
> ---
>  drivers/cpufreq/cpufreq.c | 48 +++++++++++++++++++++++++++++++++++++++
>  include/linux/cpufreq.h   | 10 ++++++++
>  2 files changed, 58 insertions(+)
> 
> diff --git a/drivers/cpufreq/cpufreq.c b/drivers/cpufreq/cpufreq.c
> index 44db4f59c4cc..7d5f71be8d29 100644
> --- a/drivers/cpufreq/cpufreq.c
> +++ b/drivers/cpufreq/cpufreq.c
> @@ -2563,6 +2563,50 @@ int cpufreq_get_policy(struct cpufreq_policy *policy, unsigned int cpu)
>  }
>  EXPORT_SYMBOL(cpufreq_get_policy);
>  
> +DEFINE_PER_CPU(unsigned long, cpufreq_pressure);
> +EXPORT_PER_CPU_SYMBOL_GPL(cpufreq_pressure);
> +
> +/**
> + * cpufreq_update_pressure() - Update cpufreq pressure for CPUs
> + * @cpus        : The related CPUs for which max capacity has been reduced
> + * @capped_freq : The maximum allowed frequency that CPUs can run at
> + *
> + * Update the value of cpufreq pressure for all @cpus in the mask. The
> + * cpumask should include all (online+offline) affected CPUs, to avoid
> + * operating on stale data when hot-plug is used for some CPUs. The
> + * @capped_freq reflects the currently allowed max CPUs frequency due to
> + * freq_qos capping. It might be also a boost frequency value, which is bigger
> + * than the internal 'capacity_freq_ref' max frequency. In such case the
> + * pressure value should simply be removed, since this is an indication that
> + * there is no capping. The @capped_freq must be provided in kHz.
> + */
> +static void cpufreq_update_pressure(const struct cpumask *cpus,
> +				      unsigned long capped_freq)
> +{
> +	unsigned long max_capacity, capacity, pressure;
> +	u32 max_freq;
> +	int cpu;
> +
> +	cpu = cpumask_first(cpus);
> +	max_capacity = arch_scale_cpu_capacity(cpu);
> +	max_freq = arch_scale_freq_ref(cpu);
> +
> +	/*
> +	 * Handle properly the boost frequencies, which should simply clean
> +	 * the thermal pressure value.
> +	 */
> +	if (max_freq <= capped_freq)
> +		capacity = max_capacity;
> +	else
> +		capacity = mult_frac(max_capacity, capped_freq, max_freq);
> +
> +	pressure = max_capacity - capacity;
> +
> +
> +	for_each_cpu(cpu, cpus)
> +		WRITE_ONCE(per_cpu(cpufreq_pressure, cpu), pressure);

Seems like the pressure value computed from the first CPU applies to all CPU.
Will this be valid for non-homogeneous CPUs that could have different
max_freq and max_capacity?

Tim