sched: Bail out of yield_to when source and target runqueue has one task

sched: Bail out of yield_to when source and target runqueue has one task

In case of undercomitted scenarios, especially in large guests
yield_to overhead is significantly high. when run queue length of
source and target is one, take an opportunity to bail out and return
-ESRCH. This return condition can be further exploited to quickly come
out of PLE handler.

(History: Raghavendra initially worked on break out of kvm ple handler upon
 seeing source runqueue length = 1, but it had to export rq length).
 Peter came up with the elegant idea of return -ESRCH in scheduler core.

Signed-off-by: Peter Zijlstra <[email protected]>
Raghavendra, Checking the rq length of target vcpu condition added.(thanks Avi)
Reviewed-by: Srikar Dronamraju <[email protected]>
Signed-off-by: Raghavendra K T <[email protected]>
Acked-by: Andrew Jones <[email protected]>
Tested-by: Chegu Vinod <[email protected]>
Signed-off-by: Gleb Natapov <[email protected]>