linux-kernel - Re: [RFC PATCH 2/4] cpuidle: Add Active Stats calls tracking idle entry/exit

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Message-ID: <2f7d855c-5232-ddbe-8403-db3596dcebc5@arm.com>
Date:   Tue, 22 Jun 2021 14:58:56 +0100
From:   Lukasz Luba <lukasz.luba@....com>
To:     "Rafael J. Wysocki" <rafael@...nel.org>
Cc:     Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Daniel Lezcano <daniel.lezcano@...aro.org>,
        Linux PM <linux-pm@...r.kernel.org>,
        Amit Kucheria <amitk@...nel.org>,
        "Zhang, Rui" <rui.zhang@...el.com>,
        Dietmar Eggemann <dietmar.eggemann@....com>,
        Chris Redpath <Chris.Redpath@....com>, Beata.Michalska@....com,
        Viresh Kumar <viresh.kumar@...aro.org>,
        "Rafael J. Wysocki" <rjw@...ysocki.net>,
        Amit Kachhap <amit.kachhap@...il.com>
Subject: Re: [RFC PATCH 2/4] cpuidle: Add Active Stats calls tracking idle
 entry/exit



On 6/22/21 1:33 PM, Rafael J. Wysocki wrote:
> On Tue, Jun 22, 2021 at 9:59 AM Lukasz Luba <lukasz.luba@....com> wrote:
>>
>> The Active Stats framework tracks and accounts the activity of the CPU
>> for each performance level. It accounts the real residency,
> 
> No, it doesn't.  It just measures the time between the entry and exit
> and that's not the real residency (because it doesn't take the exit
> latency into account, for example).

It's 'just' a 'model' and as other models has limitations, but it's
better than existing one, which IPA has to use:
cpu_util + currect_freq_at_sampling_time

> 
>> when the CPU was not idle, at a given performance level. This patch adds needed calls
>> which provide the CPU idle entry/exit events to the Active Stats
>> framework.
> 
> And it adds overhead to overhead-sensitive code.
> 
> AFAICS, some users of that code will not really get the benefit, so
> adding the overhead to it is questionable.
> 
> First, why is the existing instrumentation in the idle loop insufficient?

The instrumentation (tracing) cannot be used at run time AFAIK. I need
this idle + freq information combined in a running platform, not for
post-processing (like we have in LISA). The thermal governor IPA would
use them for used power estimation.

> 
> Second, why do you need to add locking to this code?

The idle entry/exit updates the CPU's accounting data structure.
There is a reader of those data structures: thermal governor,
run from different CPU, which is the reason why I put locking for them.