Re: [RFC] Documentation: Add documentation for new performance_profile sysfs class
From: Hans de Goede
Date: Wed Oct 14 2020 - 10:16:57 EST
Hi,
On 10/14/20 3:55 PM, Rafael J. Wysocki wrote:
On Tue, Oct 13, 2020 at 3:09 PM Hans de Goede <hdegoede@xxxxxxxxxx> wrote:
Hi,
On 10/12/20 6:42 PM, Rafael J. Wysocki wrote:
On Wed, Oct 7, 2020 at 8:41 PM Limonciello, Mario
<Mario.Limonciello@xxxxxxxx> wrote:
On Wed, 2020-10-07 at 15:58 +0000, Limonciello, Mario wrote:
On Mon, 2020-10-05 at 12:58 +0000, Limonciello, Mario wrote:
On modern systems CPU/GPU/... performance is often dynamically
configurable
in the form of e.g. variable clock-speeds and TPD. The
performance
is often
automatically adjusted to the load by some automatic-mechanism
(which may
very well live outside the kernel).
These auto performance-adjustment mechanisms often can be
configured with
one of several performance-profiles, with either a bias towards
low-power
consumption (and cool and quiet) or towards performance (and
higher
power
consumption and thermals).
Introduce a new performance_profile class/sysfs API which
offers a
generic
API for selecting the performance-profile of these automatic-
mechanisms.
If introducing an API for this - let me ask the question, why
even let each
driver offer a class interface and userspace need to change
"each" driver's
performance setting?
I would think that you could just offer something kernel-wide
like
/sys/power/performance-profile
Userspace can read and write to a single file. All drivers can
get notified
on this sysfs file changing.
The systems that react in firmware (such as the two that prompted
this discussion) can change at that time. It leaves the
possibility for a
more open kernel implementation that can do the same thing though
too by
directly modifying device registers instead of ACPI devices.
The problem, as I've mentioned in previous discussions we had about
this, is that, as you've seen in replies to this mail, this would
suddenly be making the kernel apply policy.
There's going to be pushback as soon as policy is enacted in the
kernel, and you take away the different knobs for individual
components
(or you can control them centrally as well as individually). As
much as
I hate the quantity of knobs[1], I don't think that trying to
reduce
the number of knobs in the kernel is a good use of our time, and
easier
to enact, coordinated with design targets, in user-space.
Unless you can think of a way to implement this kernel wide setting
without adding one more exponent on the number of possibilities for
the
testing matrix, I'll +1 Hans' original API.
Actually I offered two proposals in my reply. So are you NAKing
both?
No, this is only about the first portion of the email, which I quoted.
And I'm not NAK'ing it, but I don't see how it can work without being
antithetical to what kernel "users" expect, or what the folks consuming
those interfaces (presumably us both) would expect to be able to test
and maintain.
(Just so others are aware, Bastien and I had a previous discussion on this topic
that he alluded to here: https://gitlab.freedesktop.org/hadess/power-profiles-daemon/-/issues/1)
In general I agree that we shouldn't be offering 100's of knobs to change
things and protect users from themselves where possible.
Whether the decisions are made in the kernel or in userspace you still have a matrix once
you're letting someone change 2 different kernel devices that offer policy. I'd argue it's
actually worse if you let userspace change it though.
Let's go back to the my GPU and platform example and lets say both offer the new knob here
for both. Userspace software such as your PPD picks performance. Both the platform device
and GPU device get changed, hopefully no conflicts.
Then user decides no, I don't want my GPU in performance mode, I only want my platform.
So they change the knob for the GPU manually, and now you have a new config in your matrix.
However if you left it to a single kernel knob, both GPU and platform get moved together and
you don't have these extra configs in your matrix anymore.
The other point I mentioned, that platform might also do something to GPU via a sideband and
you race, you can solve it with kernel too by modifying the ordering the kernel handles it.
Userspace however, you give two knobs and now you have to worry about them getting it right
and supporting them doing them in the wrong order.
The other one suggested to use the same firmware attributes class
being
introduced by the new Dell driver (
https://patchwork.kernel.org/patch/11818343/)
since this is actually a knob to a specific firmware setting.
This seemed to me like an implementation detail (eg. the same metadata
is being exported, but in a different way), and I don't feel strongly
about it either way.
OK thanks.
IMV there are two choices here: One is between exposing the low-level
interfaces verbatim to user space and wrapping them up into a certain
"translation" layer allowing user space to use a unified interface (I
think that is what everybody wants) and the other boils down to how
the unified interface between the kernel and user space will look
like.
Personally, I think that something line /sys/power/profile allowing
drivers (and other kernel entities) to register callbacks might work
(as stated in my last reply to Hans).
Note to others reading along I pointed to this thread in this thread:
https://lore.kernel.org/linux-pm/20201006122024.14539-1-daniel.lezcano@xxxxxxxxxx/T/#t
and Rafael's "last reply" above refers to his reply in that thread.
For the sake of people reading along I'm reproducing my reply
there below.
For completeness, my response in the other thread is here:
https://lore.kernel.org/linux-pm/CAJZ5v0jpYpu3Tk7qq_MCVs0wUr-Dw0rY5EZELrVbQta0NZaoVA@xxxxxxxxxxxxxx/T/#t
Rafael, it seems more appropriate to continue this discussion
in this thread, so lets discuss this further here ?
And because I sent it before reading this message, let me reproduce it
below (with some additions).
And I just did the same thing (replied to your reply in the other thread),
I guess I was too quick.
So I too will reproduce my reply here.
And I still believe it is best to then stick to this thread from now on,
because this reproducing thing is not really productive...
My reply to Rafael from the other thread:
First of all thank you for your input, with your expertise in this
area your input is very much appreciated, after all we only get
one chance to get the userspace API for this right.
Your proposal to have a single sysfs file for userspace to talk
to and then use an in kernel subscription mechanism for drivers
to get notified of writes to this file is interesting.
But I see 2 issues with it:
1. How will userspace know which profiles are actually available ?
An obvious solution is to pick a set of standard names and let
subscribers map those as close to their own settings as possible,
the most often mentioned set of profile names in this case seems to be:
low_power
balanced_power
balanced
balanced_performance
performance
Which works fine for the thinkpad_acpi case, but not so much for
the hp-wmi case. In the HP case what happens is that a WMI call
is made which sets a bunch of ACPI variables which influence
the DPTF code (this assumes we have some sort of DPTF support
such as mjg59's reverse engineered support) but the profile-names
under Windows are: "Performance", "HP recommended", "Cool" and
"Quiet". If you read the discussion from the
"[RFC] Documentation: Add documentation for new performance_profile sysfs class"
thread you will see this was brought up as an issue there.
Two different things seem to be conflated here. One is how to pass a
possible performance-vs-power preference coming from user space down
to device drivers or generally pieces of kernel code that can adjust
the behavior and/or hardware settings depending on what that
preference is and the other is how to expose OEM-provided DPTF system
profile interfaces to user space.
I was hoping / thinking that we could use a single API for both of
these. But I guess that it makes sense to see them as 2 separate
things, esp. since DPTF profiles seem to be somewhat free-form
where as a way to pass a performance-pref to a device could use
a fixes set of values.
So lets say that we indeed want to treat these 2 separately,
then I guess that the issue at hand / my reason to start a
discussion surrounding this is allowing userspace to selecting
the DPTF system profile.
The thinkpad_acpi case at hand is not using DPTF, but that is
because Lenovo decided to implement dynamic DPTF like behavior
inside their embedded controller (for when running Linux) since
DPTF is atm not really supported all that well under Linux and
Lenovo was getting a lot of complaints about sub-optimal
performance because of this.
So the thinkpad_acpi solution is in essence a replacement
for DPTF and it should thus use the same userspace API as
other mechanisms to select DPTF system profiles.
And if we limit this new userspace API solely to setting DPTF
system profiles, then their will indeed be only 1 provider for
this for the entire system.
The former assumes that there is a common set of values that can be
understood and acted on in a consistent way by all of the interested
entities within the kernel and the latter is about passing information
from user space down to a side-band power control mechanism working in
its own way behind the kernel's back (and possibly poking at multiple
hardware components in the platform in its own way).
Ack.
IMO there is no way to provide a common interface covering these two
cases at the same time.
I see your point, esp. the free form vs common set of values
argument seems to be exactly what we have been going in circles
about during the discussion about this so far.
The problem here is that both "cool" and "quiet" could be
interpreted as low-power. But it seems that they actually mean
what they say, cool focuses on keeping temps low, which can
also be done by making the fan-profile more aggressive. And quiet
is mostly about keeping fan speeds down, at the cost of possible
higher temperatures.
<edit in this version of the reply:>
I wonder if the HP profiles are actually just fan speed profiles ?
Elia do you know ?
</edit>
I don't think so.
AFAICS, in both the Thinkpad and HP cases the profile covers the
entire platform, which in particular means that they cannot co-exist.
Ok.
IOW we don't really have a 1 dimensional axis.
Well, AFAICS, DPTF system profile interfaces coming from different
OEMs will be different, but they are about side-band power control and
there can be only one thing like that in a platform at the same time.
Ack.
My class proposal fixes this by having a notion of both
standardized names (because anything else would suck) combined
with a way for drivers to advertise which standardized names
the support. So in my proposal I simply add quiet and cool
to the list of standard profile names, and then the HP-wmi
driver can list those as supported, while not listing
low_power as a supported profile. This way we export the
hardware interface to userspace as is (as much as possible)
while still offering a standardized interface for userspace
to consume. Granted if userspace now actually want to set
a low_power profile, we have just punted the problem to userspace
but I really do not see a better solution.
First, a common place to register a DPTF system profile seems to be
needed and, as I said above, I wouldn't expect more than one such
thing to be present in the system at any given time, so it may be
registered along with the list of supported profiles and user space
will have to understand what they mean.
Mostly Ack, I would still like to have an enum for DPTF system
profiles in the kernel and have a single piece of code map that
enum to profile names. This enum can then be extended as
necessary, but I want to avoid having one driver use
"Performance" and the other "performance" or one using
"performance-balanced" and the other "balanced-performance", etc.
With the goal being that new drivers use existing values from
the enum as much as possible, but we extend it where necessary.
Second, irrespective of the above, it may be useful to have a
consistent way to pass performance-vs-power preference information
from user space to different parts of the kernel so as to allow them
to adjust their operation and this could be done with a system-wide
power profile attribute IMO.
I agree, which is why I tried to tackle both things in one go,
but as you said doing both in 1 API is probably not the best idea.
So I believe we should park this second issue for now and revisit it
when we find a need for it.
Do you have any specific userspace API in mind for the
DPTF system profile selection?
And to get one thing out of the way, in the other thread we had some
discussion about using a single attribute where a cat would result in:
low-power [balanced] performance
Where the [] indicate the active profile, vs having 2 sysfs attributes
one ro with space-separated available (foo_available) values and one
wr with the actual/current value. FWIW userspace folks have indicated
they prefer the solution with 2 separate sysfs-attributes and that is
also what e.g. cpufreq is currently using for governor selection.
I don't really have a strong opinion either way.
Regards,
Hans