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

From: Lukasz Luba <lukasz.luba@arm.com>
To: "Rafael J. Wysocki" <rafael@kernel.org>
Cc: Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
	Daniel Lezcano <daniel.lezcano@linaro.org>,
	Linux PM <linux-pm@vger.kernel.org>,
	Amit Kucheria <amitk@kernel.org>,
	"Zhang, Rui" <rui.zhang@intel.com>,
	Dietmar Eggemann <dietmar.eggemann@arm.com>,
	Chris Redpath <Chris.Redpath@arm.com>,
	Beata.Michalska@arm.com, Viresh Kumar <viresh.kumar@linaro.org>,
	"Rafael J. Wysocki" <rjw@rjwysocki.net>,
	Amit Kachhap <amit.kachhap@gmail.com>
Subject: Re: [RFC PATCH 3/4] cpufreq: Add Active Stats calls tracking frequency changes
Date: Tue, 22 Jun 2021 14:42:35 +0100	[thread overview]
Message-ID: <4e5476a6-fa9f-a9ef-ff26-8fa1b4bb90c0@arm.com> (raw)
In-Reply-To: <CAJZ5v0iVwpn0_wCZOh43DOeR2mudWYJyseMdtMsZGR-sjQ1X9Q@mail.gmail.com>

On 6/22/21 1:28 PM, Rafael J. Wysocki wrote:
> 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?

The code below is tracing, which is good for post-processing. We use in
our tool LISA, when we analyze the EAS decision, based on captured
trace data.

This new code is present at run time, so subsystems like our thermal
governor IPA can use it and get better estimation about CPU used power
for any arbitrary period, e.g. 50ms, 100ms, 300ms, ...

> 
> 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?

This active_stats_cpu_freq_fast_change() doesn't use the locking, it
relies on schedutil lock in [1].

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

[1] 
https://elixir.bootlin.com/linux/latest/source/kernel/sched/cpufreq_schedutil.c#L447