sched: introduce se->vruntime

introduce se->vruntime as a sum of weighted delta-exec's, and use that
as the key into the tree.

the idea to use absolute virtual time as the basic metric of scheduling
has been first raised by William Lee Irwin, advanced by Tong Li and first
prototyped by Roman Zippel in the "Really Fair Scheduler" (RFS) patchset.

also see:

   http://lkml.org/lkml/2007/9/2/76

for a simpler variant of this patch.

Signed-off-by: Ingo Molnar <mingo@elte.hu>
Signed-off-by: Peter Zijlstra <a.p.zijlstra@chello.nl>
Signed-off-by: Mike Galbraith <efault@gmx.de>
Reviewed-by: Thomas Gleixner <tglx@linutronix.de>

kernel/sched.c

@@ -182,6 +182,7 @@ struct cfs_rq {
 
 	s64 fair_clock;
 	u64 exec_clock;
+	u64 min_vruntime;
 	s64 wait_runtime;
 	u64 sleeper_bonus;
 	unsigned long wait_runtime_overruns, wait_runtime_underruns;