revert ("sched: fair: weight calculations")

Yanmin Zhang reported:

Comparing with kernel 2.6.25, sysbench+mysql(oltp, readonly) has many
regressions with 2.6.26-rc1:

 1) 8-core stoakley: 28%;
 2) 16-core tigerton: 20%;
 3) Itanium Montvale: 50%.

Bisect located this patch:

| 8f1bc385cf is first bad commit
| commit 8f1bc385cf
| Author: Peter Zijlstra <a.p.zijlstra@chello.nl>
| Date:   Sat Apr 19 19:45:00 2008 +0200
|
|     sched: fair: weight calculations

Revert it to the 2.6.25 state.

Bisected-by: Yanmin Zhang <yanmin_zhang@linux.intel.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>

kernel/sched.c

@@ -1368,9 +1368,6 @@ static void __resched_task(struct task_struct *p, int tif_bit)
  */
 #define SRR(x, y) (((x) + (1UL << ((y) - 1))) >> (y))
 
-/*
- * delta *= weight / lw
- */
 static unsigned long
 calc_delta_mine(unsigned long delta_exec, unsigned long weight,
 		struct load_weight *lw)
@@ -1393,6 +1390,12 @@ calc_delta_mine(unsigned long delta_exec, unsigned long weight,
 	return (unsigned long)min(tmp, (u64)(unsigned long)LONG_MAX);
 }
 
+static inline unsigned long
+calc_delta_fair(unsigned long delta_exec, struct load_weight *lw)
+{
+	return calc_delta_mine(delta_exec, NICE_0_LOAD, lw);
+}
+
 static inline void update_load_add(struct load_weight *lw, unsigned long inc)
 {
 	lw->weight += inc;

kernel/sched_fair.c

@@ -333,34 +333,6 @@ int sched_nr_latency_handler(struct ctl_table *table, int write,
 }
 #endif
 
-/*
- * delta *= w / rw
- */
-static inline unsigned long
-calc_delta_weight(unsigned long delta, struct sched_entity *se)
-{
-	for_each_sched_entity(se) {
-		delta = calc_delta_mine(delta,
-				se->load.weight, &cfs_rq_of(se)->load);
-	}
-
-	return delta;
-}
-
-/*
- * delta *= rw / w
- */
-static inline unsigned long
-calc_delta_fair(unsigned long delta, struct sched_entity *se)
-{
-	for_each_sched_entity(se) {
-		delta = calc_delta_mine(delta,
-				cfs_rq_of(se)->load.weight, &se->load);
-	}
-
-	return delta;
-}
-
 /*
  * The idea is to set a period in which each task runs once.
  *
@@ -390,54 +362,47 @@ static u64 __sched_period(unsigned long nr_running)
  */
 static u64 sched_slice(struct cfs_rq *cfs_rq, struct sched_entity *se)
 {
-	return calc_delta_weight(__sched_period(cfs_rq->nr_running), se);
+	u64 slice = __sched_period(cfs_rq->nr_running);
+
+	for_each_sched_entity(se) {
+		cfs_rq = cfs_rq_of(se);
+
+		slice *= se->load.weight;
+		do_div(slice, cfs_rq->load.weight);
+	}
+
+
+	return slice;
 }
 
 /*
  * We calculate the vruntime slice of a to be inserted task
  *
- * vs = s*rw/w = p
+ * vs = s/w = p/rw
  */
 static u64 sched_vslice_add(struct cfs_rq *cfs_rq, struct sched_entity *se)
 {
 	unsigned long nr_running = cfs_rq->nr_running;
+	unsigned long weight;
+	u64 vslice;
 
 	if (!se->on_rq)
 		nr_running++;
 
-	return __sched_period(nr_running);
+	vslice = __sched_period(nr_running);
-}
-
-/*
- * The goal of calc_delta_asym() is to be asymmetrically around NICE_0_LOAD, in
- * that it favours >=0 over <0.
- *
- *   -20         |
- *               |
- *     0 --------+-------
- *             .'
- *    19     .'
- *
- */
-static unsigned long
-calc_delta_asym(unsigned long delta, struct sched_entity *se)
-{
-	struct load_weight lw = {
-		.weight = NICE_0_LOAD,
-		.inv_weight = 1UL << (WMULT_SHIFT-NICE_0_SHIFT)
-	};
 
 	for_each_sched_entity(se) {
-		struct load_weight *se_lw = &se->load;
+		cfs_rq = cfs_rq_of(se);
 
-		if (se->load.weight < NICE_0_LOAD)
+		weight = cfs_rq->load.weight;
-			se_lw = &lw;
+		if (!se->on_rq)
+			weight += se->load.weight;
 
-		delta = calc_delta_mine(delta,
+		vslice *= NICE_0_LOAD;
-				cfs_rq_of(se)->load.weight, se_lw);
+		do_div(vslice, weight);
 	}
 
-	return delta;
+	return vslice;
 }
 
 /*
@@ -454,7 +419,11 @@ __update_curr(struct cfs_rq *cfs_rq, struct sched_entity *curr,
 
 	curr->sum_exec_runtime += delta_exec;
 	schedstat_add(cfs_rq, exec_clock, delta_exec);
-	delta_exec_weighted = calc_delta_fair(delta_exec, curr);
+	delta_exec_weighted = delta_exec;
+	if (unlikely(curr->load.weight != NICE_0_LOAD)) {
+		delta_exec_weighted = calc_delta_fair(delta_exec_weighted,
+							&curr->load);
+	}
 	curr->vruntime += delta_exec_weighted;
 }
 
@@ -661,17 +630,8 @@ 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)) {
+		if (sched_feat(NEW_FAIR_SLEEPERS))
-			unsigned long thresh = sysctl_sched_latency;
+			vruntime -= sysctl_sched_latency;
-
-			/*
-			 * convert the sleeper threshold into virtual time
-			 */
-			if (sched_feat(NORMALIZED_SLEEPER))
-				thresh = calc_delta_fair(thresh, se);
-
-			vruntime -= thresh;
-		}
 
 		/* ensure we never gain time by being placed backwards. */
 		vruntime = max_vruntime(se->vruntime, vruntime);
@@ -1169,10 +1129,11 @@ static unsigned long wakeup_gran(struct sched_entity *se)
 	unsigned long gran = sysctl_sched_wakeup_granularity;
 
 	/*
-	 * More easily preempt - nice tasks, while not making it harder for
+	 * More easily preempt - nice tasks, while not making
-	 * + nice tasks.
+	 * it harder for + nice tasks.
 	 */
-	gran = calc_delta_asym(sysctl_sched_wakeup_granularity, se);
+	if (unlikely(se->load.weight > NICE_0_LOAD))
+		gran = calc_delta_fair(gran, &se->load);
 
 	return gran;
 }