Re: [RFC PATCH 3/4] cpufreq: Add Active Stats calls tracking frequency changes

["Rafael J. Wysocki" <rafael@kernel.org>]

On Tue, Jun 22, 2021 at 9:59 AM Lukasz Luba <lukasz.luba@arm.com> wrote:
>
> The Active Stats framework tracks and accounts the activity of the CPU
> for each performance level. It accounts the real residency, when the CPU
> was not idle, at a given performance level. This patch adds needed calls
> which provide the CPU frequency transition events to the Active Stats
> framework.
>
> Signed-off-by: Lukasz Luba <lukasz.luba@arm.com>
> ---
>  drivers/cpufreq/cpufreq.c | 5 +++++
>  1 file changed, 5 insertions(+)
>
> diff --git a/drivers/cpufreq/cpufreq.c b/drivers/cpufreq/cpufreq.c
> index 802abc925b2a..d79cb9310572 100644
> --- a/drivers/cpufreq/cpufreq.c
> +++ b/drivers/cpufreq/cpufreq.c
> @@ -14,6 +14,7 @@
>
>  #define pr_fmt(fmt) KBUILD_MODNAME ": " fmt
>
> +#include <linux/active_stats.h>
>  #include <linux/cpu.h>
>  #include <linux/cpufreq.h>
>  #include <linux/cpu_cooling.h>
> @@ -387,6 +388,8 @@ static void cpufreq_notify_transition(struct cpufreq_policy *policy,
>
>                 cpufreq_stats_record_transition(policy, freqs->new);
>                 policy->cur = freqs->new;
> +
> +               active_stats_cpu_freq_change(policy->cpu, freqs->new);
>         }
>  }
>
> @@ -2085,6 +2088,8 @@ unsigned int cpufreq_driver_fast_switch(struct cpufreq_policy *policy,
>                             policy->cpuinfo.max_freq);
>         cpufreq_stats_record_transition(policy, freq);
>
> +       active_stats_cpu_freq_fast_change(policy->cpu, freq);
> +

This is quite a bit of overhead and so why is it needed in addition to
the code below?

And pretty much the same goes for the idle loop change.  There is
quite a bit of instrumentation in that code already and it avoids
adding new locking for a reason.  Why is it a good idea to add more
locking to that code?

>         if (trace_cpu_frequency_enabled()) {
>                 for_each_cpu(cpu, policy->cpus)
>                         trace_cpu_frequency(freq, cpu);
> --