sched: Optimize select_best_cpu() to reduce execution time

select_best_cpu() is a crucial wakeup routine that determines the
time taken by the scheduler to wake up a task. Optimize this routine
to get higher performance. The following changes have been made as
part of the optimization listed in order of how they built on top of
one another:

* Several routines called by select_best_cpu() recalculate task load
  and CPU load even though these are already known quantities. For
  example mostly_idle_cpu_sync() calculates CPU load; task_will_fit()
  calculates task load before spill_threshold_crossed() recalculates
  both. Remove these redundant calculations by moving the task load
  and CPU load computations to the select_best_cpu() 'for' loop and
  passing to any functions that need the information.

* Rewrite best_small_task_cpu() to avoid the existing two pass
  approach. The two pass approach was only in place to find the
  minimum power cluster for small task placement. This information
  can easily be established by looking at runqueue capacities. The
  cluster with not the highest capacity constitutes the minimum power
  cluster. A special CPU mask is called the mpc_mask required to safeguard
  against undue side effects on SMP systems. Also terminate the function
  early if the previous CPU is found to be mostly_idle.

* Reorganize code to ensure that no unnecessary computations or
  variable assignments are done. For example there is no need to
  compute CPU load if that information does not end up getting used
  in any iteration of the 'for' loop.

* The tick logic for EA migrations unnecessarily checks for the power
  of all CPUs only for skip_cpu() to throw away the result later.
  Ensure that for EA we only check CPUs within the same cluster
  and avoid running select_best_cpu() whenever possible.

CRs-fixed: 849655
Change-Id: I4e722912fcf3fe4e365a826d4d92a4dd45c05ef3
Signed-off-by: Syed Rameez Mustafa <rameezmustafa@codeaurora.org>
[joonwoop@codeaurora.org: fixed cpufreq_notifier_policy() to set mpc_mask.
 added a comment about prerequisite of lower_power_cpu_available().
 s/struct rq * rq/struct rq *rq/. s/TASK_NICE/task_nice/]
Signed-off-by: Joonwoo Park <joonwoop@codeaurora.org>

0 files changed, 0 insertions, 0 deletions