sched: reintroduce SMP tunings again

Yanmin Zhang reported an aim7 regression and bisected it down to:

 |  commit 38ad464d410dadceda1563f36bdb0be7fe4c8938
 |  Author: Ingo Molnar <mingo@elte.hu>
 |  Date:   Mon Oct 15 17:00:02 2007 +0200
 |
 |     sched: uniform tunings
 |
 |     use the same defaults on both UP and SMP.

fix this by reintroducing similar SMP tunings again. This resolves
the regression.

(also update the comments to match the ilog2(nr_cpus) tuning effect)

Signed-off-by: Ingo Molnar <mingo@elte.hu>

1 files changed, 28 insertions, 0 deletions

diff --git a/kernel/sched.c b/kernel/sched.c
index 3f6bd1112900..69cae271c63b 100644
--- a/kernel/sched.c
+++ b/kernel/sched.c
@@ -4992,6 +4992,32 @@ void __cpuinit init_idle(struct task_struct *idle, int cpu)
  */
 cpumask_t nohz_cpu_mask = CPU_MASK_NONE;
 
+/*
+ * Increase the granularity value when there are more CPUs,
+ * because with more CPUs the 'effective latency' as visible
+ * to users decreases. But the relationship is not linear,
+ * so pick a second-best guess by going with the log2 of the
+ * number of CPUs.
+ *
+ * This idea comes from the SD scheduler of Con Kolivas:
+ */
+static inline void sched_init_granularity(void)
+{
+	unsigned int factor = 1 + ilog2(num_online_cpus());
+	const unsigned long limit = 200000000;
+
+	sysctl_sched_min_granularity *= factor;
+	if (sysctl_sched_min_granularity > limit)
+		sysctl_sched_min_granularity = limit;
+
+	sysctl_sched_latency *= factor;
+	if (sysctl_sched_latency > limit)
+		sysctl_sched_latency = limit;
+
+	sysctl_sched_wakeup_granularity *= factor;
+	sysctl_sched_batch_wakeup_granularity *= factor;
+}
+
 #ifdef CONFIG_SMP
 /*
  * This is how migration works:
@@ -6688,10 +6714,12 @@ void __init sched_init_smp(void)
 	/* Move init over to a non-isolated CPU */
 	if (set_cpus_allowed(current, non_isolated_cpus) < 0)
 		BUG();
+	sched_init_granularity();
 }
 #else
 void __init sched_init_smp(void)
 {
+	sched_init_granularity();
 }
 #endif /* CONFIG_SMP */