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>

1 files changed, 8 insertions, 3 deletions

diff --git a/kernel/sched_fair.c b/kernel/sched_fair.c
index c863663d204d..e24ecd39c4b8 100644
--- a/kernel/sched_fair.c
+++ b/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. */