add support for tree based vruntime averages.
Signed-off-by: NPeter Zijlstra <a.p.zijlstra@chello.nl>
Signed-off-by: NIngo Molnar <mingo@elte.hu>
Signed-off-by: NMike Galbraith <efault@gmx.de>
Reviewed-by: NThomas Gleixner <tglx@linutronix.de>