sched/fair: Rewrite group_imb trigger

sched/fair: Rewrite group_imb trigger

Change the group_imb detection from the old 'load-spike' detector to
an actual imbalance detector. We set it from the lower domain balance
pass when it fails to create a balance in the presence of task
affinities.

The advantage is that this should no longer generate the false
positive group_imb conditions generated by transient load spikes from
the normal balancing/bulk-wakeup etc. behaviour.

While I haven't actually observed those they could happen.

I'm not entirely happy with this patch; it somehow feels a little
fragile.

Nor does it solve the biggest issue I have with the group_imb code; it
it still a fragile construct in that once we 'fixed' the imbalance
we'll not detect the group_imb again and could end up re-creating it.

That said, this patch does seem to preserve behaviour for the
described degenerate case. In particular on my 2*6*2 wsm-ep:

  taskset -c 3-11 bash -c 'for ((i=0;i<9;i++)) do while :; do :; done & done'

ends up with 9 spinners, each on their own CPU; whereas if you disable
the group_imb code that typically doesn't happen (you'll get one pair
sharing a CPU most of the time).

Signed-off-by: Peter Zijlstra <peterz@infradead.org>
Link: http://lkml.kernel.org/n/tip-36fpbgl39dv4u51b6yz2ypz5@git.kernel.org
Signed-off-by: Ingo Molnar <mingo@kernel.org>