Some improvements to the cpuidle menu governor

Hi,

two patches to the menu governor that, together, boost the quality of
the decisions it makes significantly (results below).
The first patch is basically a bugfix, while the second patch adds some
smarts to the algorithm.

Both patches only impact the idle-duration predictor (for those
unfamiliar with the menu governor, the menu governor first tries to
predict the idle duration, and then uses a set of heuristics to pick
a suitable idle state based on this prediction. The better the
prediction the better the decision ends up being)

To show the quality of the predictor, I added some instrumentation
where I had perf report out the result of the current, after patch 1
and after patch 2 prediction value as well as the actual idle duration
as measured at the end of the idle period. The test run I give below had
69854 samples (other runs had pretty much the same result).

The quality indicator I use is the percentage of samples for which the
prediction is within a certain factor of the actual duration.

Factor         before patches       patch 1          patch 1+2
1.5x            29.1%               30.4%            34.1%
2x              33.6%               49.3%            52.9%
4x              43.6%               73.6%            75.2%
10x             56.9%               86.1%            86.5%
100x            78.4%               97.0%            97.2%



-- 
Arjan van de Ven 	Intel Open Source Technology Centre
For development, discussion and tips for power savings, 
visit http://www.lesswatts.org
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