Re: [RFC] Documentation: Add documentation for new performance_profile sysfs class

["Rafael J. Wysocki" <rafael@kernel.org>]

On Wed, Oct 7, 2020 at 8:41 PM Limonciello, Mario
<Mario.Limonciello@dell.com> 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).

Cheers!