linux-kernel - Re: [PATCH] sched/cpufreq: Align trace event behavior of fast switching

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Date:   Mon, 26 Aug 2019 11:51:17 +0200
From:   Dietmar Eggemann <dietmar.eggemann@....com>
To:     Peter Zijlstra <peterz@...radead.org>,
        "Rafael J. Wysocki" <rjw@...ysocki.net>
Cc:     Douglas RAILLARD <douglas.raillard@....com>,
        linux-kernel@...r.kernel.org, linux-pm@...r.kernel.org,
        viresh.kumar@...aro.org, mingo@...hat.com
Subject: Re: [PATCH] sched/cpufreq: Align trace event behavior of fast
 switching

On 26/08/2019 11:40, Peter Zijlstra wrote:
> On Mon, Aug 26, 2019 at 11:10:52AM +0200, Rafael J. Wysocki wrote:
>> On Wednesday, August 7, 2019 5:33:40 PM CEST Douglas RAILLARD wrote:
>>> Fast switching path only emits an event for the CPU of interest, whereas the
>>> regular path emits an event for all the CPUs that had their frequency changed,
>>> i.e. all the CPUs sharing the same policy.
>>>
>>> With the current behavior, looking at cpu_frequency event for a given CPU that
>>> is using the fast switching path will not give the correct frequency signal.
>>>
>>> Signed-off-by: Douglas RAILLARD <douglas.raillard@....com>
>>> ---
>>>  kernel/sched/cpufreq_schedutil.c | 7 ++++++-
>>>  1 file changed, 6 insertions(+), 1 deletion(-)
>>>
>>> diff --git a/kernel/sched/cpufreq_schedutil.c b/kernel/sched/cpufreq_schedutil.c
>>> index 1f82ab108bab..975ccc3de807 100644
>>> --- a/kernel/sched/cpufreq_schedutil.c
>>> +++ b/kernel/sched/cpufreq_schedutil.c
>>> @@ -153,6 +153,7 @@ static void sugov_fast_switch(struct sugov_policy *sg_policy, u64 time,
>>>  			      unsigned int next_freq)
>>>  {
>>>  	struct cpufreq_policy *policy = sg_policy->policy;
>>> +	int cpu;
>>>  
>>>  	if (!sugov_update_next_freq(sg_policy, time, next_freq))
>>>  		return;
>>> @@ -162,7 +163,11 @@ static void sugov_fast_switch(struct sugov_policy *sg_policy, u64 time,
>>>  		return;
>>>  
>>>  	policy->cur = next_freq;
>>> -	trace_cpu_frequency(next_freq, smp_processor_id());
>>> +
>>> +	if (trace_cpu_frequency_enabled()) {
>>> +		for_each_cpu(cpu, policy->cpus)
>>> +			trace_cpu_frequency(next_freq, cpu);
>>> +	}
>>>  }
>>>  
>>>  static void sugov_deferred_update(struct sugov_policy *sg_policy, u64 time,
>>>
>>
>> Peter, any comments here?
> 
> I was thinking this would be a static map and dealing with it would be
> something trivially done in post (or manually while reading), but sure,
> whatever:
> 
> Acked-by: Peter Zijlstra (Intel) <peterz@...radead.org>

I think our EAS tooling expects the behavior of the non-fast-switching
driver (cpufreq.c cpufreq_notify_transition() CPUFREQ_POSTCHANGE). Pixel
3 is the first device with a fast-switching driver we test on.

Not sure about the extra  'if trace_cpu_frequency_enabled()' but I guess
it doesn't hurt.