sched: Avoid frequent migration of running task

Power values for cpus can drop quite considerably when it goes idle.
As a result, the best choice for running a single task in a cluster
can vary quite rapidly. As the task keeps hopping cpus, other cpus go
idle and start being seen as more favorable target for running a task,
leading to task migrating almost every scheduler tick!

Prevent this by keeping track of when a task started running on a cpu
and allowing task migration in tick path (migration_needed()) on
account of energy efficiency reasons only if the task has run
sufficiently long (as determined by sysctl_sched_min_runtime
variable).

Note that currently sysctl_sched_min_runtime setting is considered
only in scheduler_tick()->migration_needed() path and not in
idle_balance() path. In other words, a task could be migrated to
another cpu which did a idle_balance(). This limitation should not
affect high-frequency migrations seen typically (when a single
high-demand task runs on high-performance cpu).

CRs-Fixed: 756570
Change-Id: I96413b7a81b623193c3bbcec6f3fa9dfec367d99
Signed-off-by: Srivatsa Vaddagiri <vatsa@codeaurora.org>
[joonwoop@codeaurora.org: fixed conflict in set_task_cpu() and
 __schedule().]
Signed-off-by: Joonwoo Park <joonwoop@codeaurora.org>

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