linux-kernel - Re: [RFC PATCH 03/10] cpufreq: Architecture specific callback for frequency changes

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Message-ID: <CAKfTPtAXGQRPGNBkAinzeR65xF0-WRr6wtTqf2-XtGzXGM5LNA@mail.gmail.com>
Date:	Wed, 17 Dec 2014 08:56:14 +0100
From:	Vincent Guittot <vincent.guittot@...aro.org>
To:	Morten Rasmussen <morten.rasmussen@....com>
Cc:	Peter Zijlstra <peterz@...radead.org>,
	"mingo@...hat.com" <mingo@...hat.com>,
	Dietmar Eggemann <dietmar.eggemann@....com>,
	Paul Turner <pjt@...gle.com>,
	Benjamin Segall <bsegall@...gle.com>,
	Michael Turquette <mturquette@...aro.org>,
	linux-kernel <linux-kernel@...r.kernel.org>,
	"linux-pm@...r.kernel.org" <linux-pm@...r.kernel.org>,
	"Rafael J. Wysocki" <rjw@...ysocki.net>,
	Viresh Kumar <viresh.kumar@...aro.org>
Subject: Re: [RFC PATCH 03/10] cpufreq: Architecture specific callback for
 frequency changes

On 2 December 2014 at 15:06, Morten Rasmussen <morten.rasmussen@....com> wrote:
> From: Morten Rasmussen <Morten.Rasmussen@....com>
>
> Architectures that don't have any other means for tracking cpu frequency
> changes need a callback from cpufreq to implement a scaling factor to
> enable scale-invariant per-entity load-tracking in the scheduler.
>
> To compute the scale invariance correction factor the architecture would
> need to know both the max frequency and the current frequency. This
> patch defines weak functions for setting both from cpufreq.
>
> Related architecture specific functions use weak function definitions.
> The same approach is followed here.
>
> These callbacks can be used to implement frequency scaling of cpu
> capacity later.
>
> Cc: Rafael J. Wysocki <rjw@...ysocki.net>
> Cc: Viresh Kumar <viresh.kumar@...aro.org>
> Signed-off-by: Morten Rasmussen <morten.rasmussen@....com>
> ---
>  drivers/cpufreq/cpufreq.c | 10 +++++++++-
>  1 file changed, 9 insertions(+), 1 deletion(-)
>
> diff --git a/drivers/cpufreq/cpufreq.c b/drivers/cpufreq/cpufreq.c
> index 644b54e..1b17608 100644
> --- a/drivers/cpufreq/cpufreq.c
> +++ b/drivers/cpufreq/cpufreq.c
> @@ -278,6 +278,10 @@ static inline void adjust_jiffies(unsigned long val, struct cpufreq_freqs *ci)
>  }
>  #endif
>
> +void __weak arch_scale_set_curr_freq(int cpu, unsigned long freq) {}
> +
> +void __weak arch_scale_set_max_freq(int cpu, unsigned long freq) {}
> +
>  static void __cpufreq_notify_transition(struct cpufreq_policy *policy,
>                 struct cpufreq_freqs *freqs, unsigned int state)
>  {
> @@ -315,6 +319,7 @@ static void __cpufreq_notify_transition(struct cpufreq_policy *policy,
>                 pr_debug("FREQ: %lu - CPU: %lu\n",
>                          (unsigned long)freqs->new, (unsigned long)freqs->cpu);
>                 trace_cpu_frequency(freqs->new, freqs->cpu);
> +               arch_scale_set_curr_freq(freqs->cpu, freqs->new);

You must ensure that arch_scale_set_curr_freq will be called at least
once per CPU in order to ensure that you have a defined current
frequency.

If cpufreq don't have to change the frequency of a CPU, you will never
set the current frequency and the notification handler will never be
called. A typical example is a CPU booting at max frequency and we use
the performance governor; The CPU frequency is already the right one
so cpufreq will not change it and you will never call
arch_scale_set_curr_freq. The current frequency will be at least
undefined or set to 0 for ARM platform (according to patch 04) and the
arch_scale_freq_capacity will return 0

Regards,
Vincent

>                 srcu_notifier_call_chain(&cpufreq_transition_notifier_list,
>                                 CPUFREQ_POSTCHANGE, freqs);
>                 if (likely(policy) && likely(policy->cpu == freqs->cpu))
> @@ -2164,7 +2169,7 @@ static int cpufreq_set_policy(struct cpufreq_policy *policy,
>                                 struct cpufreq_policy *new_policy)
>  {
>         struct cpufreq_governor *old_gov;
> -       int ret;
> +       int ret, cpu;
>
>         pr_debug("setting new policy for CPU %u: %u - %u kHz\n",
>                  new_policy->cpu, new_policy->min, new_policy->max);
> @@ -2202,6 +2207,9 @@ static int cpufreq_set_policy(struct cpufreq_policy *policy,
>         policy->min = new_policy->min;
>         policy->max = new_policy->max;
>
> +       for_each_cpu(cpu, policy->cpus)
> +               arch_scale_set_max_freq(cpu, policy->max);
> +
>         pr_debug("new min and max freqs are %u - %u kHz\n",
>                  policy->min, policy->max);
>
> --
> 1.9.1
>
>
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