[PATCH v3] sched: fix wrong rq's runnable_avg update with rt task

From: Vincent Guittot
Date: Wed Feb 13 2013 - 03:55:00 EST

When a RT task is scheduled on an idle CPU, the update of the rq's load is
not done because CFS's functions are not called. Then, the idle_balance,
which is called just before entering the idle function, updates the
rq's load and makes the assumption that the elapsed time since the last
update, was only running time.

The rq's load of a CPU that only runs a periodic RT task, is close to
LOAD_AVG_MAX whatever the running duration of the RT task is.

A new idle_exit function is called when the prev task is the idle function
so the elapsed time will be accounted as idle time in the rq's load.

Changes since V2:
- remove useless definition for UP platform
- rebased on top of Steven Rostedt's patches :

Changes since V1:
- move code out of schedule function and create a pre_schedule callback for
idle class instead.

Signed-off-by: Vincent Guittot <vincent.guittot@xxxxxxxxxx>
kernel/sched/fair.c | 10 ++++++++++
kernel/sched/idle_task.c | 7 +++++++
kernel/sched/sched.h | 1 +
3 files changed, 18 insertions(+)

diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index 0fcdbff..6af5db3 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -1562,6 +1562,16 @@ static inline void dequeue_entity_load_avg(struct cfs_rq *cfs_rq,
se->avg.decay_count = atomic64_read(&cfs_rq->decay_counter);
} /* migrations, e.g. sleep=0 leave decay_count == 0 */
+ * Update the rq's load with the elapsed idle time before a task is
+ * scheduled. if the newly scheduled task is not a CFS task, idle_exit will
+ * be the only way to update the runnable statistic.
+ */
+void idle_exit(int this_cpu, struct rq *this_rq)
+ update_rq_runnable_avg(this_rq, 0);
static inline void update_entity_load_avg(struct sched_entity *se,
int update_cfs_rq) {}
diff --git a/kernel/sched/idle_task.c b/kernel/sched/idle_task.c
index 66b5220..6e7e63c 100644
--- a/kernel/sched/idle_task.c
+++ b/kernel/sched/idle_task.c
@@ -14,6 +14,12 @@ select_task_rq_idle(struct task_struct *p, int sd_flag, int flags)
return task_cpu(p); /* IDLE tasks as never migrated */

+static void pre_schedule_idle(struct rq *rq, struct task_struct *prev)
+ /* Update rq's load with elapsed idle time */
+ idle_exit(smp_processor_id(), rq);
static void post_schedule_idle(struct rq *rq)
idle_balance(smp_processor_id(), rq);
@@ -95,6 +101,7 @@ const struct sched_class idle_sched_class = {

.select_task_rq = select_task_rq_idle,
+ .pre_schedule = pre_schedule_idle,
.post_schedule = post_schedule_idle,

diff --git a/kernel/sched/sched.h b/kernel/sched/sched.h
index fc88644..5f26c93f 100644
--- a/kernel/sched/sched.h
+++ b/kernel/sched/sched.h
@@ -877,6 +877,7 @@ extern const struct sched_class idle_sched_class;

extern void trigger_load_balance(struct rq *rq, int cpu);
extern void idle_balance(int this_cpu, struct rq *this_rq);
+extern void idle_exit(int this_cpu, struct rq *this_rq);

#else /* CONFIG_SMP */


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