Re: [Ksummit-discuss] [TECH(CORE?) TOPIC] Energy conservation bias interfaces

[Amit Kucheria <amit.kucheria@linaro.org>]

On Mon, May 12, 2014 at 6:01 PM, Morten Rasmussen
<morten.rasmussen@arm.com> wrote:
> On Mon, May 12, 2014 at 12:53:11PM +0100, Amit Kucheria wrote:
>> On Tue, May 6, 2014 at 6:24 PM, Rafael J. Wysocki <rjw@rjwysocki.net> wrote:

<snip>

> The middleware power manager as you mention above seems to be a good
> candidate. The kernel wouldn't know which tasks are trusted to behave
> nicely so I think that is a user-space/middleware problem to deal with.
>
>>
>> > It also is not particularly clear what representation of "energy conservation
>> > bias" would be most useful.  Should that be a number or a set of well-defined
>> > discrete levels that can be given names (like "max performance", "high
>> > prerformance", "balanced" etc.)?  If a number, then what units to use and
>> > how many different values to take into account?
>>
>> I have a hard time figuring out how to map these levels to performance
>> / power optimisations I care about. Say I have the following
>> optimisation techniques available today that I can change at runtime.
>>
>> #define XX_TASK_PACKING              0x00000001  /* opposite of the
>> default spread policy */
>> #define XX_DISABLE_OVERDRIVE    0x00000002  /* disables expensive P-states */
>> #define XX_FORCE_DEEP_IDLE        0x00000004  /* go to deep idle
>> states even if activity on system dictates low-latency idling - useful
>> for thermal throttling aka idle injection */
>> #define XX_FORCE_SHALLOW_IDLE 0x00000008  /* keep cpu in low-latency
>> idle states for performance reasons */
>> #define XX_FOO_TECHNIQUE           0x00000010
>>
>> This is a mix of power and performance objectives that apply on a
>> per-cpu and/or per-cluster level. The challenge here is the lack of
>> consistency - some of these conflict with each other but are not
>> necessary opposites of each other. Some of them are good for
>> performance and power. How do I categorize them into 'max
>> performance', 'balanced' or 'power save' ?
>
> You can't. Since platforms are different, different techniques will have
> different impacts on the performance/energy trade-off. As I have said in
> the original thread, we need to distinguish between techniques to change
> behaviour (like the ones you have listed above) and optimization goals.
> Whether a specific technique can bring us closer to our current
> optimization goal (performance/energy trade-off) depends on the
> platform.

Right. So we are saying that state names like "powersave",
"balanced/auto", "performance" will be platform-defined. Is it worth
defining them at all then?

I expect that these techniques can be counted on our fingers, so why
not just expose them directly to the system? The middleware and even
other kernel subsystems can directly toggle their state based on
current conditions.

I'm assuming here that cpufreq and cpuidle mechanism will get merged
into the scheduler core at some point in the near future.

Regards,
Amit