sched: Make scheduler aware of cpu frequency state

Capacity of a cpu (how much performance it can deliver) is partly
determined by its frequency (P) state, both current frequency as well
as max frequency it can reach.  Knowing frequency state of cpus will
help scheduler optimize various functions such as tracking every
task's cpu demand and placing tasks on various cpus.

This patch has scheduler registering for cpufreq notifications to
become aware of cpu's frequency state. Subsequent patches will make
use of derived information for various purposes, such as task's scaled
load (cpu demand) accounting and task placement.

Change-Id: I376dffa1e7f3f47d0496cd7e6ef8b5642ab79016
Signed-off-by: Srivatsa Vaddagiri <vatsa@codeaurora.org>
Signed-off-by: franciscofranco <franciscofranco.1990@gmail.com>
sched: Calculate max_possible_freq based on current policy->max
sched: reinitialize rq->next_balance when a CPU is hot-added

Reinitialize rq->next_balance when a CPU is hot-added.  Otherwise,
scheduler domain rebalancing may be skipped if rq->next_balance was
set to a future time when the CPU was last active, and the
newly-re-added CPU is in idle_balance().  As a result, the
newly-re-added CPU will remain idle with no tasks scheduled until the
softlockup watchdog runs - potentially 4 seconds later.  This can
waste energy and reduce performance.

This behavior can be observed in some SoC kernels, which use CPU
hotplug to dynamically remove and add CPUs in response to load.  In
one case that triggered this behavior,

0. the system started with all cores enabled, running multi-threaded
   CPU-bound code;

1. the system entered some single-threaded code;

2. a CPU went idle and was hot-removed;

3. the system started executing a multi-threaded CPU-bound task;

4. the CPU from event 2 was re-added, to respond to the load.

The time interval between events 2 and 4 was approximately 300
milliseconds.

Of course, ideally CPU hotplug would not be used in this manner,
but this patch does appear to fix a real bug.

Nvidia folks: this patch is submitted as at least a partial fix for
bug 1243368 ("[sched] Load-balancing not happening correctly after
cores brought online")

Change-Id: Iabac21e110402bb581b7db40c42babc951d378d0
Signed-off-by: Paul Walmsley <pwalmsley@nvidia.com>
Cc: Peter Boonstoppel <pboonstoppel@nvidia.com>
Reviewed-on: http://git-master/r/208927
Reviewed-by: Automatic_Commit_Validation_User
Reviewed-by: Peter Zu <pzu@nvidia.com>
Reviewed-by: Peter Boonstoppel <pboonstoppel@nvidia.com>
GVS: Gerrit_Virtual_Submit
Tested-by: Peter Zu <pzu@nvidia.com>
Reviewed-by: Yu-Huan Hsu <yhsu@nvidia.com>
Signed-off-by: franciscofranco <franciscofranco.1990@gmail.com>