[Ksummit-discuss] [TECH TOPIC] System-wide interface to specify the level of PM tuning

[Rafael J. Wysocki]

On Sunday, July 12, 2015 12:01:27 PM Daniel Vetter wrote:
> On Fri, Jul 10, 2015 at 7:25 PM, Kevin Hilman <khilman at kernel.org> wrote:
> > "Rafael J. Wysocki" <rjw at rjwysocki.net> writes:
> >
> >> On Monday, July 06, 2015 01:49:45 PM Iyer, Sundar wrote:
> >
> > [...]
> >
> >>> Is a "single setting somewhere" even appropriate? It is actually the intelligence
> >>> needed vs executing the actions?
> >>
> >> For one example, the default for most of the device/.../power/control files in
> >> sysfs is "on" (meaning no runtime PM) while it might be "auto" (use runtime PM
> >> if you can).  Making that change for everybody in one go may lead to various
> >> issues (that may be regarded as regressions then), but if we made it configurable,
> >> people might choose to make that change for themselves if they wanted to.
> >
> > I'd be very supportive of some default knob (or cmdline option) to favor
> > energy efficiency.  For runtime PM, I suspect the resulting performance
> > regressions are mostly (relatively) simple fixes, like enabling
> > autosuspend, etc.
> >
> > Also, having a system-wide way to enable this mode would also enable us
> > to find/report these bugs/regressions in a way that would be easily
> > repeatable.  With the current pile of knobs/tunables, it's often very
> > hard to reproduce problems others may be seeing.
> 
> My approach in drm/i915 is that there's just one default config and
> that's the well-tuned one. Maybe gfx is special, but with todays
> power-envelope constrained chips it's not a question of performance
> _or_ power efficiency, but always _and_: Enabling power saving
> features improves performance.

However, there are places in the kernel where there is a real tradeoff between
power and performance (or power and capacity in general) and there are places
that tend to keep conservative settings for fear of exposing latent bugs to
a wide community of users.

Those might benefit from allowing the users to relax the settings globally
if they want to.

Thanks,
Rafael