sched: Avoid running idle_balance() consecutively

With the introduction of "6dd123a sched: update ld_moved for active
balance from the load balancer" the function load_balance() returns a
non zero number of migrated tasks in anticipation of tasks that will
end up on that CPU via active migration. Unfortunately on kernel
versions 3.14 and beyond this ends up breaking pick_next_task_fair()
which assumes that the load balancer only returns non zero numbers for
tasks already migrated on to the destination CPU. A non zero number
then triggers a rerun of the pick_next_task_fair() logic so that it
can return one of the migrated tasks as the next task. When the load
balancer returns a non zero number for tasks that will be moved via
active migration, the rerun of pick_next_task_fair() finds the CPU to
still have no runnable tasks. This in turn causes a rerun of
idle_balance() and possibly migrating another task. Hence the
destination CPU can unintentionally end up pulling several tasks.

The intent of the change above is still necessary though to indicate
termination of load balance at higher scheduling domains when active
migration occurs. Achieve the same effect by using continue_balancing
instead of faking the number of pulled tasks. This way
pick_next_task_fair() stays happy and load balance stops at higher
scheduling domains.

Change-Id: Id223a3287e5d401e10fbc67316f8551303c7ff96
Signed-off-by: Syed Rameez Mustafa <rameezmustafa@codeaurora.org>

1 files changed, 6 insertions, 3 deletions

diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index 9d046d58050b..6e7ba8bce1fb 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -9553,7 +9553,7 @@ no_move:
 				stop_one_cpu_nowait(cpu_of(busiest),
 					active_load_balance_cpu_stop, busiest,
 					&busiest->active_balance_work);
-				ld_moved++;
+				*continue_balancing = 0;
 			}
 
 			/*
@@ -9761,9 +9761,12 @@ static int idle_balance(struct rq *this_rq)
 
 		/*
 		 * Stop searching for tasks to pull if there are
-		 * now runnable tasks on the balance rq.
+		 * now runnable tasks on the balance rq or if
+		 * continue_balancing has been unset (only possible
+		 * due to active migration).
 		 */
-		if (pulled_task || balance_rq->nr_running > 0)
+		if (pulled_task || balance_rq->nr_running > 0 ||
+						!continue_balancing)
 			break;
 	}
 	rcu_read_unlock();