sched: fix weight calculations

The conversion between virtual and real time is as follows:

  dvt = rw/w * dt <=> dt = w/rw * dvt

Since we want the fair sleeper granularity to be in real time, we actually
need to do:

  dvt = - rw/w * l

This bug could be related to the regression reported by Yanmin Zhang:

| Comparing with kernel 2.6.25, sysbench+mysql(oltp, readonly) has lots
| of regressions with 2.6.26-rc1:
|
| 1) 8-core stoakley: 28%;
| 2) 16-core tigerton: 20%;
| 3) Itanium Montvale: 50%.

Reported-by: "Zhang, Yanmin" <yanmin_zhang@linux.intel.com>
Signed-off-by: Mike Galbraith <efault@gmx.de>
Signed-off-by: Peter Zijlstra <a.p.zijlstra@chello.nl>
Signed-off-by: Ingo Molnar <mingo@elte.hu>

kernel/sched_fair.c

@@ -662,10 +662,15 @@ place_entity(struct cfs_rq *cfs_rq, struct sched_entity *se, int initial)
 	if (!initial) {
 		/* sleeps upto a single latency don't count. */
 		if (sched_feat(NEW_FAIR_SLEEPERS)) {
+			unsigned long thresh = sysctl_sched_latency;
+
+			/*
+			 * convert the sleeper threshold into virtual time
+			 */
 			if (sched_feat(NORMALIZED_SLEEPER))
-				vruntime -= calc_delta_weight(sysctl_sched_latency, se);
-			else
-				vruntime -= sysctl_sched_latency;
+				thresh = calc_delta_fair(thresh, se);
+
+			vruntime -= thresh;
 		}
 
 		/* ensure we never gain time by being placed backwards. */