[PATCH v3 2/5] sched/fair: Skip detach and attach new group task

[Yuyang Du]

Vincent reported that the first task to a new task group's cfs_rq will
be attached in attach_task_cfs_rq() and once more when it is enqueued
(see https://lkml.org/lkml/2016/5/25/388).

Actually, it is much worse. The load is currently attached mostly twice
every time when we switch to fair class or change task groups. These two
scenarios are concerned, which we will descripbe in the following
respectively

1) Switch to fair class:

The sched class change is done like this:

	if (queued)
	  enqueue_task();
	check_class_changed()
	  switched_from()
	  switched_to()

If the task is on_rq, it should have already been enqueued, which
MAY have attached the load to the cfs_rq, if so, we shouldn't attach
it again in switched_to(), otherwise, we will attach it twice. This is
what the current situation is.

So to cover both the on_rq and !on_rq cases, as well as both the task
was switched from fair and otherwise, the simplest solution is to reset
the task's last_update_time to 0, when the task is switched from fair.
Then let task enqueue do the load attachment.

2) Change between fair task groups:

The task groups are changed like this:

	if (queued)
          dequeue_task()
	task_move_group()
	if (queued)
          enqueue_task()

Unlike the switch to fair class, if the task is on_rq, it will be enqueued
after we move task groups, so the simplest solution is to reset the
task's last_update_time when we do task_move_group(), and then let
enqueue_task() do the load attachment.

Reported-by: Vincent Guittot <vincent.guittot@xxxxxxxxxx>
Signed-off-by: Yuyang Du <yuyang.du@xxxxxxxxx>
---
 kernel/sched/fair.c |   47 +++++++++++++++++++++--------------------------
 1 file changed, 21 insertions(+), 26 deletions(-)

diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index 3270598..89513b6 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -2959,7 +2959,8 @@ static inline void update_load_avg(struct sched_entity *se, int update_tg)
 		update_tg_load_avg(cfs_rq, 0);
 }
 
-static void attach_entity_load_avg(struct cfs_rq *cfs_rq, struct sched_entity *se)
+/* Virtually synchronize task with its cfs_rq */
+static inline void attach_entity_load_avg(struct cfs_rq *cfs_rq, struct sched_entity *se)
 {
 	se->avg.last_update_time = cfs_rq->avg.last_update_time;
 	cfs_rq->avg.load_avg += se->avg.load_avg;
@@ -2970,19 +2971,6 @@ static void attach_entity_load_avg(struct cfs_rq *cfs_rq, struct sched_entity *s
 	cfs_rq_util_change(cfs_rq);
 }
 
-static inline void attach_age_load_task(struct rq *rq, struct task_struct *p)
-{
-	struct sched_entity *se = &p->se;
-
-	if (!sched_feat(ATTACH_AGE_LOAD))
-		return;
-
-	if (se->avg.last_update_time) {
-		__update_load_avg(cfs_rq_of(se)->avg.last_update_time, cpu_of(rq),
-				  &se->avg, 0, 0, NULL);
-	}
-}
-
 static void detach_entity_load_avg(struct cfs_rq *cfs_rq, struct sched_entity *se)
 {
 	__update_load_avg(cfs_rq->avg.last_update_time, cpu_of(rq_of(cfs_rq)),
@@ -3057,6 +3045,11 @@ static inline u64 cfs_rq_last_update_time(struct cfs_rq *cfs_rq)
 }
 #endif
 
+static inline void reset_task_last_update_time(struct task_struct *p)
+{
+	p->se.avg.last_update_time = 0;
+}
+
 /*
  * Task first catches up with cfs_rq, and then subtract
  * itself from the cfs_rq (task must be off the queue now).
@@ -3109,10 +3102,8 @@ dequeue_entity_load_avg(struct cfs_rq *cfs_rq, struct sched_entity *se) {}
 static inline void remove_entity_load_avg(struct sched_entity *se) {}
 
 static inline void
-attach_entity_load_avg(struct cfs_rq *cfs_rq, struct sched_entity *se) {}
-static inline void
 detach_entity_load_avg(struct cfs_rq *cfs_rq, struct sched_entity *se) {}
-static inline void attach_age_load_task(struct rq *rq, struct task_struct *p) {}
+static inline void reset_task_last_update_time(struct task_struct *p) {}
 
 static inline int idle_balance(struct rq *rq)
 {
@@ -8400,9 +8391,6 @@ static void attach_task_cfs_rq(struct task_struct *p)
 	se->depth = se->parent ? se->parent->depth + 1 : 0;
 #endif
 
-	/* Synchronize task with its cfs_rq */
-	attach_entity_load_avg(cfs_rq, se);
-
 	if (!vruntime_normalized(p))
 		se->vruntime += cfs_rq->min_vruntime;
 }
@@ -8410,16 +8398,18 @@ static void attach_task_cfs_rq(struct task_struct *p)
 static void switched_from_fair(struct rq *rq, struct task_struct *p)
 {
 	detach_task_cfs_rq(p);
+	reset_task_last_update_time(p);
+	/*
+	 * If we change back to fair class, we will attach the sched
+	 * avgs when we are enqueued, which will be done only once. We
+	 * won't have the chance to consistently age the avgs before
+	 * attaching them, so we have to continue with the last updated
+	 * sched avgs when we were detached.
+	 */
 }
 
 static void switched_to_fair(struct rq *rq, struct task_struct *p)
 {
-	/*
-	 * If we change between classes, age the averages before attaching them.
-	 * XXX: we could have just aged the entire load away if we've been
-	 * absent from the fair class for too long.
-	 */
-	attach_age_load_task(rq, p);
 	attach_task_cfs_rq(p);
 
 	if (task_on_rq_queued(p)) {
@@ -8472,6 +8462,11 @@ static void task_move_group_fair(struct task_struct *p)
 	detach_task_cfs_rq(p);
 	set_task_rq(p, task_cpu(p));
 	attach_task_cfs_rq(p);
+	/*
+	 * This assures we will attach the sched avgs when we are enqueued,
+	 * which will be done only once.
+	 */
+	reset_task_last_update_time(p);
 }
 
 void free_fair_sched_group(struct task_group *tg)
-- 
1.7.9.5