Provides a selftest and examples of using the interface.

[peterz: updated to not use sched_debug]
Signed-off-by: NChris Hyser <chris.hyser@oracle.com>
Signed-off-by: NPeter Zijlstra (Intel) <peterz@infradead.org>
Tested-by: NDon Hiatt <dhiatt@digitalocean.com>
Tested-by: NHongyu Ning <hongyu.ning@linux.intel.com>
Tested-by: NVincent Guittot <vincent.guittot@linaro.org>
Link: https://lkml.kernel.org/r/20210422123309.100860030@infradead.org