linux-kernel - Re: [RFC PATCH v3 2/5] cpuidle: Add Cpufreq Active Stats calls tracking idle entry/exit

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Date:   Tue, 26 Apr 2022 16:01:46 +0100
From:   Lukasz Luba <lukasz.luba@....com>
To:     Artem Bityutskiy <dedekind1@...il.com>
Cc:     dietmar.eggemann@....com, viresh.kumar@...aro.org,
        rafael@...nel.org, daniel.lezcano@...aro.org, amitk@...nel.org,
        rui.zhang@...el.com, amit.kachhap@...il.com,
        linux-pm@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [RFC PATCH v3 2/5] cpuidle: Add Cpufreq Active Stats calls
 tracking idle entry/exit

Hi Artem,

Thanks for comments!

On 4/26/22 13:05, Artem Bityutskiy wrote:
> Hi Lukasz,
> 
> On Wed, 2022-04-06 at 23:08 +0100, Lukasz Luba wrote:
>> @@ -231,6 +232,8 @@ int cpuidle_enter_state(struct cpuidle_device *dev, struct
>> cpuidle_driver *drv,
>>          trace_cpu_idle(index, dev->cpu);
>>          time_start = ns_to_ktime(local_clock());
>>   
>> +       cpufreq_active_stats_cpu_idle_enter(time_start);
>> +
>>          stop_critical_timings();
>>          if (!(target_state->flags & CPUIDLE_FLAG_RCU_IDLE))
>>                  rcu_idle_enter();
>> @@ -243,6 +246,8 @@ int cpuidle_enter_state(struct cpuidle_device *dev, struct
>> cpuidle_driver *drv,
>>          time_end = ns_to_ktime(local_clock());
>>          trace_cpu_idle(PWR_EVENT_EXIT, dev->cpu);
>>   
>> +       cpufreq_active_stats_cpu_idle_exit(time_end);
>> +
> 
> At this point the interrupts are still disabled, and they get enabled later. So
> the more code you add here and the longer it executes, the longer you delay the
> interrupts. Therefore, you are effectively increasing IRQ latency from idle by
> adding more code here.

Good point, I've added it just below the trace_cpu_idle().

> 
> How much? I do not know, depends on how much code you need to execute. But the
> amount of code in functions like this tends to increase over time.
> 
> So the risk is that we'll keep making 'cpufreq_active_stats_cpu_idle_exit()',
> and (may be unintentionally) increase idle interrupt latency.
> 
> This is not ideal.

I agree,  I will try to find a better place to put this call.

> 
> We use the 'wult' tool (https://github.com/intel/wult) to measure C-states
> latency and interrupt latency on Intel platforms, and for fast C-states like
> Intel C1, we can see that even the current code between C-state exit and
> interrupt re-enabled adds measurable overhead.

Thanks for the hint and the link. I'll check that tool. I don't know if
that would work with my platforms. I might create some tests for this
latency measurements.

> 
> I am worried about adding more stuff here.
> 
> Please, consider getting the stats after interrupts are re-enabled. You may lose
> some "precision" because of that, but it is probably overall better that adding
> to idle interrupt latency.

Definitely. I don't need such precision, so later when interrupts are
re-enabled is OK for me.

> 
>>          /* The cpu is no longer idle or about to enter idle. */
>>          sched_idle_set_state(NULL);
> 
> 

This new call might be empty for your x86 kernels, since probably
you set the CONFIG_CPU_FREQ_STAT.I can add additional config
so platforms might still have CONFIG_CPU_FREQ_STAT but avoid this
new feature and additional overhead in idle exit when e.g.
CONFIG_CPU_FREQ_ACTIVE_STAT is not set.

The x86 platforms won't use IPA governor, so it's reasonable to
do this way.

Does this sounds good?

Regards,
Lukasz