There are instances where we keep searching for an idle CPU despite
already having a sched-idle CPU (in find_idlest_group_cpu(),
select_idle_smt() and select_idle_cpu() and then there are places where
we don't necessarily do that and return a sched-idle CPU as soon as we
find one (in select_idle_sibling()). This looks a bit inconsistent and
it may be worth having the same policy everywhere.

On the other hand, choosing a sched-idle CPU over a idle one shall be
beneficial from performance and power point of view as well, as we don't
need to get the CPU online from a deep idle state which wastes quite a
lot of time and energy and delays the scheduling of the newly woken up
task.

This patch tries to simplify code around sched-idle CPU selection and
make it consistent throughout.

Testing is done with the help of rt-app on hikey board (ARM64 octa-core,
2 clusters, 0-3 and 4-7). The cpufreq governor was set to performance to
avoid any side affects from CPU frequency. Following are the tests
performed:

Test 1: 1-cfs-task:

 A single SCHED_NORMAL task is pinned to CPU5 which runs for 2333 us
 out of 7777 us (so gives time for the cluster to go in deep idle
 state).

Test 2: 1-cfs-1-idle-task:

 A single SCHED_NORMAL task is pinned on CPU5 and single SCHED_IDLE
 task is pinned on CPU6 (to make sure cluster 1 doesn't go in deep idle
 state).

Test 3: 1-cfs-8-idle-task:

 A single SCHED_NORMAL task is pinned on CPU5 and eight SCHED_IDLE
 tasks are created which run forever (not pinned anywhere, so they run
 on all CPUs). Checked with kernelshark that as soon as NORMAL task
 sleeps, the SCHED_IDLE task starts running on CPU5.

And here are the results on mean latency (in us), using the "st" tool.

  $ st 1-cfs-task/rt-app-cfs_thread-0.log
  N       min     max     sum     mean    stddev
  642     90      592     197180  307.134 109.906

  $ st 1-cfs-1-idle-task/rt-app-cfs_thread-0.log
  N       min     max     sum     mean    stddev
  642     67      311     113850  177.336 41.4251

  $ st 1-cfs-8-idle-task/rt-app-cfs_thread-0.log
  N       min     max     sum     mean    stddev
  643     29      173     41364   64.3297 13.2344

The mean latency when we need to:

 - wakeup from deep idle state is 307 us.
 - wakeup from shallow idle state is 177 us.
 - preempt a SCHED_IDLE task is 64 us.
Signed-off-by: Viresh Kumar <viresh.kumar@linaro.org>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Reviewed-by: Vincent Guittot <vincent.guittot@linaro.org>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Link: https://lkml.kernel.org/r/b90cbcce608cef4e02a7bbfe178335f76d201bab.1573728344.git.viresh.kumar@linaro.orgSigned-off-by: Ingo Molnar <mingo@kernel.org>