linux-kernel - Re: [PATCH 0/4] powercap/dtpm: Add the DTPM framework

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Date:   Tue, 13 Oct 2020 14:47:44 +0200
From:   Hans de Goede <hdegoede@...hat.com>
To:     Daniel Lezcano <daniel.lezcano@...aro.org>, rafael@...nel.org,
        srinivas.pandruvada@...ux.intel.com
Cc:     lukasz.luba@....com, linux-kernel@...r.kernel.org,
        linux-pm@...r.kernel.org, rui.zhang@...el.com,
        Mark Pearson <mpearson@...ovo.com>
Subject: Re: [PATCH 0/4] powercap/dtpm: Add the DTPM framework

Hi,

On 10/12/20 6:02 PM, Daniel Lezcano wrote:
> On 12/10/2020 13:46, Hans de Goede wrote:
>> Hi Daniel,
>>
>> On 10/12/20 12:30 PM, Daniel Lezcano wrote:

<snip>

>>> Here the proposed interface is already exported in userspace via the
>>> powercap framework which supports today the backend driver for the RAPL
>>> register.
>>
>> You say that some sort of power/ balanced power / balanced /
>> balanced performance / performance setting in is already exported
>> through the powercap interface today (if I understand you correctly)?
> 
> Sorry, I was unclear. I meant 'Here the proposed interface' referring to
> the powercap/dtpm. There is no profile interface in the powercap framework.

Ah, I see.

<snip>

>>> A side note, related to your proposal, not this patch. IMO it suits
>>> better to have /sys/power/profile.
>>>
>>> cat /sys/power/profile
>>>
>>> power
>>> balanced_power *
>>> balanced
>>> balanced_performance
>>> performance
>>>
>>> The (*) being the active profile.
>>
>> Interesting the same thing was brought up in the discussion surrounding
>> RFC which I posted.
>>
>> The downside against this approach is that it assumes that there
>> only is a single system-wide settings. AFAIK that is not always
>> the case, e.g. (AFAIK):
>>
>> 1. The intel pstate driver has something like this
>>     (might this be the rapl setting you mean? )
>>
>> 2. The X1C8 has such a setting for the embedded-controller, controlled
>>     through the ACPI interfaces which thinkpad-acpi used
>>
>> 3. The hp-wmi interface allows selecting a profile which in turn
>>     (through AML code) sets a bunch of variables which influence how
>>     the (dynamic, through mjg59's patches) DPTF code controls various
>>     things
>>
>> At least the pstate setting and the vendor specific settings can
>> co-exist. Also the powercap API has a notion of zones, I can see the
>> same thing here, with a desktop e.g. having separate performance-profile
>> selection for the CPU and a discrete GPU.
>>
>> So limiting the API to a single /sys/power/profile setting seems a
>> bit limited and I have the feeling we will regret making this
>> choice in the future.
>>
>> With that said your proposal would work well for the current
>> thinkpad_acpi / hp-wmi cases, so I'm not 100% against it.
>>
>> This would require adding some internal API to the code which
>> owns the /sys/power root-dir to allow registering a profile
>> provider I guess. But that would also immediately bring the
>> question, what if multiple drivers try to register themselves
>> as /sys/power/profile provider ?
> 
> Did you consider putting the profile on a per device basis ?
> 
> eg.
> 
> /sys/devices/system/cpu/cpu[0-9]/power/profile
> 
> May be make 'energy_performance_preference' obsolete in
> /sys/devices/system/cpu/cpufreq ?
> 
> When one device sets the profile, all children will have the same profile.
> 
> eg.
> 
> A change in /sys/devices/system/cpu/power/profile will impact all the
> underlying cpu[0-9]/power/profile
> 
> Or a change in /sys/devices/power/profile will change all profiles below
> /sys/devices.
> 
> Well that is a high level suggestion, I don't know how that can fit with
> the cyclic sysfs hierarchy.

A problem with I see with making this a per-device power setting is that
only a few devices will actually have this; and then the question becomes
how does userspace discover / find these devices ? Typically for these kinda
discovery problems we use a sysfs class as a solution to group devices
offering the same functionailty (through the same standardized sysfs API)
together. Which circles back to my original RFC proposal for this.

Regards,

Hans