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From: Lukasz Luba <>
To: "Rafael J. Wysocki" <>
Cc: Linux Kernel Mailing List <>,
	Daniel Lezcano <>,
	Linux PM <>,
	Amit Kucheria <>,
	"Zhang, Rui" <>,
	Dietmar Eggemann <>,
	Chris Redpath <>,, Viresh Kumar <>,
	"Rafael J. Wysocki" <>,
	Amit Kachhap <>
Subject: Re: [RFC PATCH v2 0/6] Introduce Active Stats framework with CPU performance statistics
Date: Tue, 6 Jul 2021 16:56:02 +0100	[thread overview]
Message-ID: <> (raw)
In-Reply-To: <>

On 7/6/21 4:28 PM, Rafael J. Wysocki wrote:
> On Tue, Jul 6, 2021 at 3:18 PM Lukasz Luba <> wrote:
>> Hi all,
>> This patch set introduces a new mechanism: Active Stats framework (ASF), which
>> gathers and maintains statistics of CPU performance - time residency at each
>> performance state.
>> The ASF tracks all the frequency transitions as well as CPU
>> idle entry/exit events for all CPUs. Based on that it accounts the active
>> (non-idle) residency time for each CPU at each frequency. This information can
>> be used by some other subsystems (like thermal governor) to enhance their
>> estimations about CPU usage at a given period.
> This seems to mean that what is needed is something like the cpufreq
> stats but only collected during the time when CPUs are not in idle
> states.


>> Does it fix something in mainline?
>> Yes, there is thermal governor Intelligent Power Allocation (IPA), which
>> estimates the CPUs power used in the past. IPA is sampling the CPU utilization
>> and frequency and relies on the info available at the time of sampling
>> and this imposes the estimation errors.
>> The use of ASF solve the issue and enables IPA to make better estimates.
> Obviously the IPA is not used on all platforms where cpufreq and
> cpuidle are used.  What platforms are going to benefit from this
> change?

Arm platforms which still use kernel thermal to control temperature,
such as Chromebooks or mid-, low-end phones.

>> Why it couldn't be done using existing frameworks?
>> The CPUFreq and CPUIdle statistics are not combined, so it is not possible
>> to derive the information on how long exactly the CPU was running with a given
>> frequency.
> But it doesn't mean that the statistics could not be combined.
> For instance, the frequency of the CPU cannot change via cpufreq when
> active_stats_cpu_idle_enter() is running, so instead of using an
> entirely new framework for collecting statistics it might update the
> existing cpufreq stats to register that event.

True, but keep in mind that the cpufreq most likely works for a few
CPUs (policy::related_cpus), while cpuidle in a per-cpu fashion.
I would say that cpuidle should check during enter/exit what is
the currently set frequency for cluster and account its active

> And analogously for the wakeup.
>> This new framework combines that information and provides
>> it in a handy way.
> I'm not convinced about the last piece.

The handy structure is called Active Stats Monitor. It samples
the stats gathered after processing idle. That private
structure maintains statistics which are for a given period
(current snapshot - previous snapshot).

>> IMHO it has to be implemented as a new framework, next to
>> CPUFreq and CPUIdle, due to a clean design and not just hooks from thermal
>> governor into the frequency change and idle code paths.
> As far as the design is concerned, I'm not sure if I agree with it.
>  From my perspective it's all a 1000-line patch that I have to read and
> understand to figure out what the design is.

I can help you with understanding it with some design docs if you want.

>> Tha patch 4/6 introduces a new API for cooling devices, which allows to
>> stop tracking the freq and idle statistics.
>> The patch set contains also a patches 5/6 6/6 which adds the new power model
>> based on ASF into the cpufreq cooling (used by thermal governor IPA).
>> It is added as ifdef option, since Active Stats might be not compiled in.
>> The ASF is a compile time option, but that might be changed and IPA could
>> select it, which would allow to remove some redundant code from
>> cpufreq_cooling.c.
>> Comments and suggestions are very welcome.
> I'm totally not convinced that it is necessary to put the extra 1000
> lines of code into the kernel to address the problem at hand.

I understand your concerns. If you have another idea than this framework
I'm happy to hear it. Maybe better stats in cpuidle, which would be
are of the cpufreq?

  reply	other threads:[~2021-07-06 15:56 UTC|newest]

Thread overview: 13+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2021-07-06 13:18 [RFC PATCH v2 0/6] Introduce Active Stats framework with CPU performance statistics Lukasz Luba
2021-07-06 13:18 ` [RFC PATCH v2 1/6] PM: Introduce Active Stats framework Lukasz Luba
2021-07-06 13:18 ` [RFC PATCH v2 2/6] cpuidle: Add Active Stats calls tracking idle entry/exit Lukasz Luba
2021-07-06 13:18 ` [RFC PATCH v2 3/6] cpufreq: Add Active Stats calls tracking frequency changes Lukasz Luba
2021-07-06 13:18 ` [RFC PATCH v2 4/6] thermal: Add interface to cooling devices to handle governor change Lukasz Luba
2021-07-06 13:18 ` [RFC PATCH v2 5/6] thermal/core/power allocator: Prepare power actors and calm down when not used Lukasz Luba
2021-07-06 13:18 ` [RFC PATCH v2 6/6] thermal: cpufreq_cooling: Improve power estimation based on Active Stats framework Lukasz Luba
2021-07-06 15:28 ` [RFC PATCH v2 0/6] Introduce Active Stats framework with CPU performance statistics Rafael J. Wysocki
2021-07-06 15:56   ` Lukasz Luba [this message]
2021-07-06 16:34     ` Rafael J. Wysocki
2021-07-06 16:51       ` Lukasz Luba
2021-07-14 18:30         ` Rafael J. Wysocki
2021-07-19  8:52           ` Lukasz Luba

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