[RFC][PATCH 1/2] sched: Move idle_balance() to post_schedule

[Steven Rostedt]

The idle_balance() code is called to do task load balancing just before
going to idle. This makes sense as the CPU is about to sleep anyway.
But currently it's called in the middle of the scheduler and in a place
that must have interrupts disabled. That means, while the load balancing
is going on, if a task wakes up on this CPU, it wont get to run while
the interrupts are disabled. The idle task doing the balancing will be
clueless about it.

There's no real reason that the idle_balance() needs to be called in the
middle of schedule anyway. The only benefit is that if a task is pulled
to this CPU, it can be scheduled without the need to schedule the idle
task. But load balancing and migrating the task makes a switch to idle
and back negligible.

By using the post_schedule function pointer of the sched class, the
unlikely branch in the hot path of the scheduler can be removed, and
the idle task itself can do the load balancing.

Another advantage of this, is that by moving the idle_balance to the
post_schedule routine, interrupts can now be enabled in the load balance
allowing for interrupts and wakeups to still occur on that CPU while a
balance is taking place. The enabling of interrupts will come as a separate
patch.

Signed-off-by: Steven Rostedt <rostedt@xxxxxxxxxxx>

Index: linux-trace.git/kernel/sched/core.c
===================================================================
--- linux-trace.git.orig/kernel/sched/core.c
+++ linux-trace.git/kernel/sched/core.c
@@ -2926,9 +2926,6 @@ need_resched:
 
 	pre_schedule(rq, prev);
 
-	if (unlikely(!rq->nr_running))
-		idle_balance(cpu, rq);
-
 	put_prev_task(rq, prev);
 	next = pick_next_task(rq);
 	clear_tsk_need_resched(prev);
Index: linux-trace.git/kernel/sched/idle_task.c
===================================================================
--- linux-trace.git.orig/kernel/sched/idle_task.c
+++ linux-trace.git/kernel/sched/idle_task.c
@@ -25,6 +25,7 @@ static void check_preempt_curr_idle(stru
 static struct task_struct *pick_next_task_idle(struct rq *rq)
 {
 	schedstat_inc(rq, sched_goidle);
+	rq->post_schedule = 1;
 	return rq->idle;
 }
 
@@ -69,6 +70,12 @@ static unsigned int get_rr_interval_idle
 	return 0;
 }
 
+
+static void post_schedule_idle(struct rq *rq)
+{
+	idle_balance(smp_processor_id(), rq);
+}
+
 /*
  * Simple, special scheduling class for the per-CPU idle tasks:
  */
@@ -91,6 +98,8 @@ const struct sched_class idle_sched_clas
 	.set_curr_task          = set_curr_task_idle,
 	.task_tick		= task_tick_idle,
 
+	.post_schedule		= post_schedule_idle,
+
 	.get_rr_interval	= get_rr_interval_idle,
 
 	.prio_changed		= prio_changed_idle,

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