This patch implements 'nice' support across physical cpus on SMP.

It introduces an extra runqueue variable prio_bias which is the sum of the
(inverted) static priorities of all the tasks on the runqueue.

This is then used to bias busy rebalancing between runqueues to obtain good
distribution of tasks of different nice values.  By biasing the balancing only
during busy rebalancing we can avoid having any significant loss of throughput
by not affecting the carefully tuned idle balancing already in place.  If all
tasks are running at the same nice level this code should also have minimal
effect.  The code is optimised out in the !CONFIG_SMP case.

Signed-off-by: Con Kolivas <kernel@kolivas.org>
Cc: Ingo Molnar <mingo@elte.hu>
Cc: Nick Piggin <nickpiggin@yahoo.com.au>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>