sched/fair: Fix reordering of enqueue/dequeue_task_fair()

Even when a cgroup is throttled, the group se of a child cgroup can still
be enqueued and its gse->on_rq stays true. When a task is enqueued on such
child, we still have to update the load_avg and increase
h_nr_running of the throttled cfs. Nevertheless, the 1st
for_each_sched_entity() loop is skipped because of gse->on_rq == true and the
2nd loop because the cfs is throttled whereas we have to update both
load_avg with the old h_nr_running and increase h_nr_running in such case.

The same sequence can happen during dequeue when se moves to parent before
breaking in the 1st loop.

Note that the update of load_avg will effectively happen only once in order
to sync up to the throttled time. Next call for updating load_avg will stop
early because the clock stays unchanged.
Signed-off-by: Vincent Guittot <vincent.guittot@linaro.org>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Signed-off-by: Ingo Molnar <mingo@kernel.org>
Fixes: 6d4d2246 ("sched/fair: Reorder enqueue/dequeue_task_fair path")
Link: https://lkml.kernel.org/r/20200306084208.12583-1-vincent.guittot@linaro.org

kernel/sched/fair.c

@@ -5460,16 +5460,16 @@ enqueue_task_fair(struct rq *rq, struct task_struct *p, int flags)
 	for_each_sched_entity(se) {
 		cfs_rq = cfs_rq_of(se);
 
-		/* end evaluation on encountering a throttled cfs_rq */
-		if (cfs_rq_throttled(cfs_rq))
-			goto enqueue_throttle;
-
 		update_load_avg(cfs_rq, se, UPDATE_TG);
 		se_update_runnable(se);
 		update_cfs_group(se);
 
 		cfs_rq->h_nr_running++;
 		cfs_rq->idle_h_nr_running += idle_h_nr_running;
+
+		/* end evaluation on encountering a throttled cfs_rq */
+		if (cfs_rq_throttled(cfs_rq))
+			goto enqueue_throttle;
 	}
 
 enqueue_throttle:
@@ -5558,16 +5558,17 @@ static void dequeue_task_fair(struct rq *rq, struct task_struct *p, int flags)
 	for_each_sched_entity(se) {
 		cfs_rq = cfs_rq_of(se);
 
-		/* end evaluation on encountering a throttled cfs_rq */
-		if (cfs_rq_throttled(cfs_rq))
-			goto dequeue_throttle;
-
 		update_load_avg(cfs_rq, se, UPDATE_TG);
 		se_update_runnable(se);
 		update_cfs_group(se);
 
 		cfs_rq->h_nr_running--;
 		cfs_rq->idle_h_nr_running -= idle_h_nr_running;
+
+		/* end evaluation on encountering a throttled cfs_rq */
+		if (cfs_rq_throttled(cfs_rq))
+			goto dequeue_throttle;
+
 	}
 
 dequeue_throttle: