linux-kernel - Re: [PATCH v2 1/5] arch_topology: Introduce thermal pressure update function

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Date:   Tue, 26 Oct 2021 18:51:03 +0200
From:   Dietmar Eggemann <dietmar.eggemann@....com>
To:     Lukasz Luba <lukasz.luba@....com>, linux-kernel@...r.kernel.org,
        linux-pm@...r.kernel.org
Cc:     linux-arm-kernel@...ts.infradead.org,
        linux-arm-msm@...r.kernel.org, sudeep.holla@....com,
        will@...nel.org, catalin.marinas@....com, linux@...linux.org.uk,
        gregkh@...uxfoundation.org, rafael@...nel.org,
        viresh.kumar@...aro.org, amitk@...nel.org,
        daniel.lezcano@...aro.org, amit.kachhap@...il.com,
        thara.gopinath@...aro.org, bjorn.andersson@...aro.org,
        agross@...nel.org
Subject: Re: [PATCH v2 1/5] arch_topology: Introduce thermal pressure update
 function

On 15/10/2021 16:45, Lukasz Luba wrote:
> The thermal pressure is a mechanism which is used for providing
> information about reduced CPU performance to the scheduler. Usually code
> has to convert the value from frequency units into capacity units,
> which are understandable by the scheduler. Create a common conversion code
> which can be just used via a handy API.
> 
> Signed-off-by: Lukasz Luba <lukasz.luba@....com>
> ---
>  arch/arm/include/asm/topology.h   |  1 +
>  arch/arm64/include/asm/topology.h |  1 +
>  drivers/base/arch_topology.c      | 36 ++++++++++++++++++++++++++++++-
>  include/linux/arch_topology.h     |  3 +++
>  include/linux/sched/topology.h    |  7 ++++++
>  5 files changed, 47 insertions(+), 1 deletion(-)
> 
> diff --git a/arch/arm/include/asm/topology.h b/arch/arm/include/asm/topology.h
> index 470299ee2fba..aee6c456c085 100644
> --- a/arch/arm/include/asm/topology.h
> +++ b/arch/arm/include/asm/topology.h
> @@ -24,6 +24,7 @@
>  /* Replace task scheduler's default thermal pressure API */
>  #define arch_scale_thermal_pressure topology_get_thermal_pressure
>  #define arch_set_thermal_pressure   topology_set_thermal_pressure
> +#define arch_thermal_pressure_update	topology_thermal_pressure_update
>  
>  #else
>  
> diff --git a/arch/arm64/include/asm/topology.h b/arch/arm64/include/asm/topology.h
> index ec2db3419c41..c997015402bc 100644
> --- a/arch/arm64/include/asm/topology.h
> +++ b/arch/arm64/include/asm/topology.h
> @@ -33,6 +33,7 @@ void update_freq_counters_refs(void);
>  /* Replace task scheduler's default thermal pressure API */
>  #define arch_scale_thermal_pressure topology_get_thermal_pressure
>  #define arch_set_thermal_pressure   topology_set_thermal_pressure
> +#define arch_thermal_pressure_update	topology_thermal_pressure_update

s/thermal_pressure_update/update_thermal_pressure ?

The scheme seems to be {arch|topology}_*foo*_thermal_pressure

But ...

>  
>  #include <asm-generic/topology.h>
>  
> diff --git a/drivers/base/arch_topology.c b/drivers/base/arch_topology.c
> index 43407665918f..1fa28b5afdb2 100644
> --- a/drivers/base/arch_topology.c
> +++ b/drivers/base/arch_topology.c
> @@ -25,6 +25,7 @@
>  static DEFINE_PER_CPU(struct scale_freq_data __rcu *, sft_data);
>  static struct cpumask scale_freq_counters_mask;
>  static bool scale_freq_invariant;
> +static DEFINE_PER_CPU(u32, freq_factor) = 1;
>  
>  static bool supports_scale_freq_counters(const struct cpumask *cpus)
>  {
> @@ -168,6 +169,40 @@ void topology_set_thermal_pressure(const struct cpumask *cpus,
>  }
>  EXPORT_SYMBOL_GPL(topology_set_thermal_pressure);
>  
> +/**
> + * topology_thermal_pressure_update() - Update thermal pressure for CPUs
> + * @cpus	: The related CPUs for which capacity has been reduced
> + * @capped_freq	: The maximum allowed frequency that CPUs can run at
> + *
> + * Update the value of thermal 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 must be less or equal to the max possible frequency and
> + * reflects the currently allowed max CPUs frequency due to thermal capping.
> + * The @capped_freq must be provided in kHz.
> + */
> +void topology_thermal_pressure_update(const struct cpumask *cpus,
> +				      unsigned long capped_freq)
> +{

... why not just s/unsigned long th_pressure/unsigned long capped_freq
in existing topology_set_thermal_pressure() and move code the
frequency/capacity conversion in there? The patch set will become
considerably smaller.

 void topology_set_thermal_pressure(const struct cpumask *cpus,
-                              unsigned long th_pressure)
+                              unsigned long capped_freq)
 {
+       unsigned long max_capacity, capacity;
        int cpu;

-       for_each_cpu(cpu, cpus)
-               WRITE_ONCE(per_cpu(thermal_pressure, cpu), th_pressure);
+       if (!cpus)
+               return;
+
+       cpu = cpumask_first(cpus);
+       max_capacity = arch_scale_cpu_capacity(cpu);
+
+       /* Convert to MHz scale which is used in 'freq_factor' */
+       capped_freq /= 1000;
+
+       capacity = mult_frac(capped_freq, max_capacity,
+                            per_cpu(freq_factor, cpu));
+
+       for_each_cpu(cpu, cpus) {
+               WRITE_ONCE(per_cpu(thermal_pressure, cpu),
+                          max_capacity - capacity);
+       }
 }
 EXPORT_SYMBOL_GPL(topology_set_thermal_pressure);

And a user like [drivers/thermal/cpufreq_cooling.c] can call
arch_set_thermal_pressure(cpus, frequency).

[...]