sched: Use the best-buddy 'ideal cpu' in balancing decisions

sched: Use the best-buddy 'ideal cpu' in balancing decisions

Now that we have a notion of (one of the) best CPUs we interrelate
with in terms of memory usage, use that information to improve
can_migrate_task() balancing decisions: allow the migration to
occur even if we locally cache-hot, if we are on another node
and want to migrate towards our best buddy's node.

( Note that this is not hard affinity - if imbalance persists long
  enough then the scheduler will eventually balance tasks anyway,
  to maximize CPU utilization. )

Cc: Peter Zijlstra <a.p.zijlstra@chello.nl>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: Andrea Arcangeli <aarcange@redhat.com>
Cc: Rik van Riel <riel@redhat.com>
Cc: Mel Gorman <mgorman@suse.de>
Cc: Hugh Dickins <hughd@google.com>
Signed-off-by: Ingo Molnar <mingo@kernel.org>