linux-kernel - Re: [PATCH v2 02/10] cpufreq: provide data for frequency-invariant load-tracking support

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Date:   Fri, 7 Jul 2017 18:18:15 +0200
From:   "Rafael J. Wysocki" <rafael@...nel.org>
To:     Dietmar Eggemann <dietmar.eggemann@....com>
Cc:     Viresh Kumar <viresh.kumar@...aro.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Linux PM <linux-pm@...r.kernel.org>,
        Russell King - ARM Linux <linux@....linux.org.uk>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Russell King <rmk+kernel@...linux.org.uk>,
        Catalin Marinas <catalin.marinas@....com>,
        Will Deacon <will.deacon@....com>,
        Juri Lelli <juri.lelli@....com>,
        Vincent Guittot <vincent.guittot@...aro.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Morten Rasmussen <morten.rasmussen@....com>,
        "Rafael J . Wysocki" <rjw@...ysocki.net>
Subject: Re: [PATCH v2 02/10] cpufreq: provide data for frequency-invariant
 load-tracking support

On Fri, Jul 7, 2017 at 6:01 PM, Dietmar Eggemann
<dietmar.eggemann@....com> wrote:
> On 06/07/17 11:40, Viresh Kumar wrote:
>> On 06-07-17, 10:49, Dietmar Eggemann wrote:
>
> [...]
>
>>> In case arch_set_freq_scale() is not defined (and because of the
>>> pr_debug() drivers/cpufreq/cpufreq.c is not compiled with -DDEBUG)
>>
>> The line within () needs to be improved to convey a clear message.
>
> Probably not needed anymore. See below.
>
> [...]
>
>>> diff --git a/drivers/cpufreq/cpufreq.c b/drivers/cpufreq/cpufreq.c
>>> index 9bf97a366029..a04c5886a5ce 100644
>>> --- a/drivers/cpufreq/cpufreq.c
>>> +++ b/drivers/cpufreq/cpufreq.c
>>> @@ -347,6 +347,28 @@ static void __cpufreq_notify_transition(struct cpufreq_policy *policy,
>>>      }
>>>  }
>>>
>>> +/*********************************************************************
>>> + *           FREQUENCY INVARIANT CPU CAPACITY SUPPORT                *
>>> + *********************************************************************/
>>> +
>>> +#ifndef arch_set_freq_scale
>>> +static void arch_set_freq_scale(struct cpumask *cpus, unsigned long cur_freq,
>>> +                            unsigned long max_freq)
>>> +{}
>>> +#endif
>>> +
>>> +static void cpufreq_set_freq_scale(struct cpufreq_policy *policy,
>>> +                               struct cpufreq_freqs *freqs)
>>> +{
>>> +    unsigned long cur_freq = freqs ? freqs->new : policy->cur;
>>> +    unsigned long max_freq = policy->cpuinfo.max_freq;
>>> +
>>> +    pr_debug("cpus %*pbl cur/cur max freq %lu/%lu kHz\n",
>>> +             cpumask_pr_args(policy->related_cpus), cur_freq, max_freq);
>>> +
>>> +    arch_set_freq_scale(policy->related_cpus, cur_freq, max_freq);
>>
>> I am not sure why all these are required to be sent here and will come back to
>> it later on after going through other patches.
>
> See below.
>
>>> +}
>>> +
>>>  /**
>>>   * cpufreq_notify_transition - call notifier chain and adjust_jiffies
>>>   * on frequency transition.
>>> @@ -405,6 +427,8 @@ void cpufreq_freq_transition_begin(struct cpufreq_policy *policy,
>>>
>>>      spin_unlock(&policy->transition_lock);
>>>
>>> +    cpufreq_set_freq_scale(policy, freqs);
>>> +
>>
>> Why do this before even changing the frequency ? We may fail while changing it.
>>
>> IMHO, you should call this routine whenever we update policy->cur and that
>> happens regularly in __cpufreq_notify_transition() and few other places..
>
> See below.
>
>>>      cpufreq_notify_transition(policy, freqs, CPUFREQ_PRECHANGE);
>>>  }
>>>  EXPORT_SYMBOL_GPL(cpufreq_freq_transition_begin);
>>> @@ -2203,6 +2227,8 @@ static int cpufreq_set_policy(struct cpufreq_policy *policy,
>>>      blocking_notifier_call_chain(&cpufreq_policy_notifier_list,
>>>                      CPUFREQ_NOTIFY, new_policy);
>>>
>>> +    cpufreq_set_freq_scale(new_policy, NULL);
>>
>> Why added it here ? To get it initialized ? If yes, then we should do that in
>> cpufreq_online() where we first initialize policy->cur.
>
> I agree. This can go away. Initialization is not really needed here. We initialize
> the scale values to SCHED_CAPACITY_SCALE at boot-time.
>
>> Apart from this, you also need to update this in the schedutil governor (if you
>> haven't done that in this series later) as that also updates policy->cur in the
>> fast path.
>
> So what about I call arch_set_freq_scale() in __cpufreq_notify_transition() in the
> CPUFREQ_POSTCHANGE case for slow-switching and in cpufreq_driver_fast_switch() for
> fast-switching?

Why don't you do this in drivers instead of in the core?

Ultimately, the driver knows what frequency it has requested, so why
can't it call arch_set_freq_scale()?

Thanks,
Rafael