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

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Date:   Wed, 27 Apr 2022 14:58:07 +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



On 4/26/22 17:29, Artem Bityutskiy wrote:
> On Tue, 2022-04-26 at 16:01 +0100, Lukasz Luba wrote:
>>> 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.
> 
> Thanks. That is preferable in general: we do not do things with interrupts
> disabled unless there is a very good reason to.
> 
>>
>> 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?
> 
> I did not thoroughly read your patches so can't comment on the details.
> 
> Just pointing that in general idle path is to be considered the critical path,
> especially the part before interrupts are re-enabled. Not only on x86,
> but on all platforms using cpuidle. This does not mean we can't read more
> statistics there, but it does mean that we should be very careful about added
> overhead, keep it under control, etc.

I totally agree with you. I didn't know that the interrupts were not
enabled at that moment. I'll address it.

Regards,
Lukasz