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

[NeilBrown <neilb@suse.com>]

On Mon, 06 Jul 2015 02:22:02 +0200 "Rafael J. Wysocki"
<rjw@rjwysocki.net> wrote:

> Hi All,
> 
> This is a re-occuring theme, but we discussed it last month during LinuxCon
> Japan with Kristen, Grant and other people and pretty much the only conclusion
> we could reach was to propose it as the KS topic, so here it goes.
> 
> As systems get more and more complex and more and more internally integrated
> over time, every new generation of them requires an increased amount of tuning
> to achieve satisfactory balance between energy usage and performance.  You need
> to know what to tune and how to do that, it needs to be done from user space or
> requires special Kconfig options to be set (or even out-of-the-tree patches to
> be applied in extreme cases) and so on.  All that becomes more and more esoteric
> and quite frankly I'm not sure how many users are able to do that on their new
> systems.
> 
> That leads to a question whether or not a global interface (sysfs-based,
> command line etc.) could be added to the kernel that might be used to make a
> certain amount of the tuning happen already at the kernel level.  For example,
> it might change the default runtime PM control setting for all devices from
> "on" to "auto", automatically enable other runtime power management features
> available from various bus types (SATA link power management, USB LPM, others)
> and generally enable power management techiques disabled by default because
> enabling them may lead to performance regressions.
> 
> So do we need such an interface?  If not, why not?  If so, how should it be
> designed, what should it cover etc.?
> 

This sounds like an important topic, but I don't think I quite
understand the question.
We already have ".../power/runtime_enabled" and various other tunables.
What more could you need in a kernel interface?

I can see that much more than an interface is needed - we need a tool
that makes use of that interface.
Maybe a database of different systems together with tuning settings for
different goals.
Then some tools detects the particular hardware it is running on, and
applies the tuning rules.

(a tiny bit like a devicetree database which contains configuration
rules).

Or have I missed the point completely?

Thanks,
NeilBrown