[kernel, android] cpufreq: new 'interactive' governor

cpufreq: interactive: New 'interactive' governor

New interactive governor.

This governor is designed for latency sensitive workloads, UI interaction for
example.

Advantages:
+ significantly more responsive to ramp cpu up when required (UI interaction)
+ more consistent ramping, existing governors do their cpu load sampling in a
workqueue context, the 'interactive' governor does this in a timer context, 
which
gives more consistent cpu load sampling.
+ higher priority for cpu frequency increase, rt_workqueue is used for scaling
up, giving the remaining tasks the cpu performance benefit, unlike existing
governors which schedule rampup work to occur after your performance starved
tasks have completed.

Existing governors sample cpu load at a particular rate, typically
every X ms. Which can lead to under powering UI threads when the user has
interacted with an idle system until the next sample period happns.

The 'interactive' governor has a different approach. Instead of sampling the cpu

at a specified rate, the governor will scale the cpu frequency up when coming
out of idle. When the cpu comes out of idle, a timer is configured to fire
within 1-2 ticks. If the cpu is 100% busy from exiting idle to when the timer
fires then we assume the cpu is underpowered and ramp to MAX speed.

If the cpu was not 100% busy, then the governor evaluates the cpu load over the
last 'min_sample_rate' (default 50000 uS) to determine the cpu speed to ramp 
down
to.

There is only one tuneable for this governor:
/sys/devices/system/cpu/cpufreq/interactive/min_sample_rate:
  The minimum ammount of time to spend at the current frequency before
  ramping down. This is to ensure that the governor has seen enough
  historic cpu load data to determine the appropriate workload.
  Default is 5000 uS.

Signed-off-by: Mike Chan <mike@android.com>
Change-Id: I686d2f57b0ed9cbb73217403b7438be5719588d2