sched: Fix select_idle_sibling() logic in select_task_rq_fair()

sched: Fix select_idle_sibling() logic in select_task_rq_fair()

Issues in the current select_idle_sibling() logic in select_task_rq_fair()
in the context of a task wake-up:

a) Once we select the idle sibling, we use that domain (spanning the cpu that
   the task is currently woken-up and the idle sibling that we found) in our
   wake_affine() decisions. This domain is completely different from the
   domain(we are supposed to use) that spans the cpu that the task currently
   woken-up and the cpu where the task previously ran.

b) We do select_idle_sibling() check only for the cpu that the task is
   currently woken-up on. If select_task_rq_fair() selects the previously run
   cpu for waking the task, doing a select_idle_sibling() check
   for that cpu also helps and we don't do this currently.

c) In the scenarios where the cpu that the task is woken-up is busy but
   with its HT siblings are idle, we are selecting the task be woken-up
   on the idle HT sibling instead of a core that it previously ran
   and currently completely idle. i.e., we are not taking decisions based on
   wake_affine() but directly selecting an idle sibling that can cause
   an imbalance at the SMT/MC level which will be later corrected by the
   periodic load balancer.

Fix this by first going through the load imbalance calculations using
wake_affine() and once we make a decision of woken-up cpu vs previously-ran cpu,
then choose a possible idle sibling for waking up the task on.

Signed-off-by: Suresh Siddha <suresh.b.siddha@intel.com>
Signed-off-by: Peter Zijlstra <a.p.zijlstra@chello.nl>
LKML-Reference: <1270079265.7835.8.camel@sbs-t61.sc.intel.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>