Merge branch 'sched-fixes-for-linus' of git://git.kernel.org/pub/scm/linux/kernel/git/tip/linux-2.6-tip

* 'sched-fixes-for-linus' of git://git.kernel.org/pub/scm/linux/kernel/git/tip/linux-2.6-tip:
  sched: Fix rebalance interval calculation
  sched, doc: Beef up load balancing description
  sched: Leave sched_setscheduler() earlier if possible, do not disturb SCHED_FIFO tasks

Documentation/scheduler/sched-domains.txt

@@ -1,8 +1,7 @@
-Each CPU has a "base" scheduling domain (struct sched_domain). These are
-accessed via cpu_sched_domain(i) and this_sched_domain() macros. The domain
+Each CPU has a "base" scheduling domain (struct sched_domain). The domain
 hierarchy is built from these base domains via the ->parent pointer. ->parent
-MUST be NULL terminated, and domain structures should be per-CPU as they
-are locklessly updated.
+MUST be NULL terminated, and domain structures should be per-CPU as they are
+locklessly updated.
 
 Each scheduling domain spans a number of CPUs (stored in the ->span field).
 A domain's span MUST be a superset of it child's span (this restriction could
@@ -26,11 +25,26 @@ is treated as one entity. The load of a group is defined as the sum of the
 load of each of its member CPUs, and only when the load of a group becomes
 out of balance are tasks moved between groups.
 
-In kernel/sched.c, rebalance_tick is run periodically on each CPU. This
-function takes its CPU's base sched domain and checks to see if has reached
-its rebalance interval. If so, then it will run load_balance on that domain.
-rebalance_tick then checks the parent sched_domain (if it exists), and the
-parent of the parent and so forth.
+In kernel/sched.c, trigger_load_balance() is run periodically on each CPU
+through scheduler_tick(). It raises a softirq after the next regularly scheduled
+rebalancing event for the current runqueue has arrived. The actual load
+balancing workhorse, run_rebalance_domains()->rebalance_domains(), is then run
+in softirq context (SCHED_SOFTIRQ).
+
+The latter function takes two arguments: the current CPU and whether it was idle
+at the time the scheduler_tick() happened and iterates over all sched domains
+our CPU is on, starting from its base domain and going up the ->parent chain.
+While doing that, it checks to see if the current domain has exhausted its
+rebalance interval. If so, it runs load_balance() on that domain. It then checks
+the parent sched_domain (if it exists), and the parent of the parent and so
+forth.
+
+Initially, load_balance() finds the busiest group in the current sched domain.
+If it succeeds, it looks for the busiest runqueue of all the CPUs' runqueues in
+that group. If it manages to find such a runqueue, it locks both our initial
+CPU's runqueue and the newly found busiest one and starts moving tasks from it
+to our runqueue. The exact number of tasks amounts to an imbalance previously
+computed while iterating over this sched domain's groups.
 
 *** Implementing sched domains ***
 The "base" domain will "span" the first level of the hierarchy. In the case

kernel/sched.c

@@ -5011,6 +5011,17 @@ static int __sched_setscheduler(struct task_struct *p, int policy,
 		return -EINVAL;
 	}
 
+	/*
+	 * If not changing anything there's no need to proceed further:
+	 */
+	if (unlikely(policy == p->policy && (!rt_policy(policy) ||
+			param->sched_priority == p->rt_priority))) {
+
+		__task_rq_unlock(rq);
+		raw_spin_unlock_irqrestore(&p->pi_lock, flags);
+		return 0;
+	}
+
 #ifdef CONFIG_RT_GROUP_SCHED
 	if (user) {
 		/*

kernel/sched_fair.c

@@ -22,6 +22,7 @@
 
 #include <linux/latencytop.h>
 #include <linux/sched.h>
+#include <linux/cpumask.h>
 
 /*
  * Targeted preemption latency for CPU-bound tasks:
@@ -3850,8 +3851,8 @@ static void rebalance_domains(int cpu, enum cpu_idle_type idle)
 		interval = msecs_to_jiffies(interval);
 		if (unlikely(!interval))
 			interval = 1;
-		if (interval > HZ*NR_CPUS/10)
-			interval = HZ*NR_CPUS/10;
+		if (interval > HZ*num_online_cpus()/10)
+			interval = HZ*num_online_cpus()/10;
 
 		need_serialize = sd->flags & SD_SERIALIZE;