RFC: setting performance_cx_lowest=C2 in -HEAD to avoid lock contention on many-CPU boxes

[Adrian Chadd]

Oh the other thing, which I just mentioned to kip in IRC - all of the
intel laptops I've tested (and that's a long list) don't enter CPU C7
if the power is plugged in.

Ie:

* power in, ACPI C2 -> CPU C6
* power in, ACPI C3 - CPU C6
* battery - ACPI C2 -> CPU C6
* battery -> ACPI C3 -> CPU C7

So having performance_cx_lowest=C3 is effectively a no-op on the
devices that it'd matter on, so it's okay to just flip it to C2.



-adrian