linux-kernel - Re: [PATCH v3 1/5] cpufreq: Add a cpufreq pressure feedback for the scheduler

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Date: Mon, 8 Jan 2024 17:35:08 +0100
From: Dietmar Eggemann <dietmar.eggemann@....com>
To: Vincent Guittot <vincent.guittot@...aro.org>, linux@...linux.org.uk,
 catalin.marinas@....com, will@...nel.org, sudeep.holla@....com,
 rafael@...nel.org, viresh.kumar@...aro.org, agross@...nel.org,
 andersson@...nel.org, konrad.dybcio@...aro.org, mingo@...hat.com,
 peterz@...radead.org, juri.lelli@...hat.com, rostedt@...dmis.org,
 bsegall@...gle.com, mgorman@...e.de, bristot@...hat.com,
 vschneid@...hat.com, lukasz.luba@....com, rui.zhang@...el.com,
 mhiramat@...nel.org, daniel.lezcano@...aro.org, amit.kachhap@...il.com,
 corbet@....net, gregkh@...uxfoundation.org,
 linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org,
 linux-pm@...r.kernel.org, linux-arm-msm@...r.kernel.org,
 linux-trace-kernel@...r.kernel.org, linux-doc@...r.kernel.org
Cc: qyousef@...alina.io
Subject: Re: [PATCH v3 1/5] cpufreq: Add a cpufreq pressure feedback for the
 scheduler

On 08/01/2024 14:48, 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.

Is this then related to the 'medium pace system pressure' you mentioned
in your OSPM '23 talk?

> 
> Signed-off-by: Vincent Guittot <vincent.guittot@...aro.org>
> ---
>  drivers/cpufreq/cpufreq.c | 36 ++++++++++++++++++++++++++++++++++++
>  include/linux/cpufreq.h   | 10 ++++++++++
>  2 files changed, 46 insertions(+)
> 
> diff --git a/drivers/cpufreq/cpufreq.c b/drivers/cpufreq/cpufreq.c
> index 44db4f59c4cc..fa2e2ea26f7f 100644
> --- a/drivers/cpufreq/cpufreq.c
> +++ b/drivers/cpufreq/cpufreq.c
> @@ -2563,6 +2563,40 @@ int cpufreq_get_policy(struct cpufreq_policy *policy, unsigned int cpu)
>  }
>  EXPORT_SYMBOL(cpufreq_get_policy);
>  
> +DEFINE_PER_CPU(unsigned long, cpufreq_pressure);
> +
> +/**
> + * cpufreq_update_pressure() - Update cpufreq pressure for CPUs
> + * @policy: cpufreq policy of the CPUs.
> + *
> + * Update the value of cpufreq pressure for all @cpus in the policy.
> + */
> +static void cpufreq_update_pressure(struct cpufreq_policy *policy)
> +{
> +	unsigned long max_capacity, capped_freq, pressure;
> +	u32 max_freq;
> +	int cpu;
> +
> +	/*
> +	 * Handle properly the boost frequencies, which should simply clean
> +	 * the thermal pressure value.
               ^^^^^^^
IMHO, this is a copy & paste error from topology_update_thermal_pressure()?

> +	 */
> +	if (max_freq <= capped_freq) {

max_freq seems to be uninitialized.

> +		pressure = 0;

Is this x86 (turbo boost) specific? IMHO at arm we follow this max freq
(including boost) relates to 1024 in capacity? Or haven't we made this
discussion yet?

> +	} else {
> +		cpu = cpumask_first(policy->related_cpus);
> +		max_capacity = arch_scale_cpu_capacity(cpu);
> +		capped_freq = policy->max;
> +		max_freq = arch_scale_freq_ref(cpu);
> +
> +		pressure = max_capacity -
> +			   mult_frac(max_capacity, capped_freq, max_freq);
> +	}
> +
> +	for_each_cpu(cpu, policy->related_cpus)
> +		WRITE_ONCE(per_cpu(cpufreq_pressure, cpu), pressure);
> +}
> +

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