UPSTREAM: sched/fair: Fix FTQ noise bench regression

commit bc4278987e3874da62edf585fe8b3bdd9b53f638 upstream.

A regression of the FTQ noise has been reported by Ying Huang,
on the following hardware:

  8 threads Intel(R) Core(TM)i7-4770 CPU @ 3.40GHz with 8G memory

... which was caused by this commit:

  commit 4e5160766fcc ("sched/fair: Propagate asynchrous detach")

The only part of the patch that can increase the noise is the update
of blocked load of group entity in update_blocked_averages().

We can optimize this call and skip the update of group entity if its load
and utilization are already null and there is no pending propagation of load
in the task group.

This optimization partly restores the noise score. A more agressive
optimization has been tried but has shown worse score.

Reported-by:  <ying.huang@linux.intel.com>
Signed-off-by: Vincent Guittot <vincent.guittot@linaro.org>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Mike Galbraith <efault@gmx.de>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: dietmar.eggemann@arm.com
Cc: ying.huang@intel.com
Fixes: 4e5160766fcc ("sched/fair: Propagate asynchrous detach")
Link: http://lkml.kernel.org/r/1489758442-2877-1-git-send-email-vincent.guittot@linaro.org


[ Fixed typos, improved layout. ]
Signed-off-by: Ingo Molnar <mingo@kernel.org>

Change-Id: Iba93bbda71125ce14519aa30b1beac2bf3c52e1e
Fixes: Change-Id: I5d1a9f273111c0bb7503ca3c9ad2406d12867cb5
       ("UPSTREAM: sched/fair: Propagate asynchrous detach")
Signed-off-by: Amit Pundir <amit.pundir@linaro.org>