sched/fair: Improve PELT stuff some more

Vincent noted that the update_tg_load_avg() usage in commit:

  3d30544f ("sched/fair: Apply more PELT fixes")

isn't entirely sufficient. We need to call this function every time
cfs_rq->avg.load changes, this includes when update_cfs_rq_load_avg()
returns true, but {attach,detach}_entity_load_avg() themselves also
change it. This means we need to unconditionally call
update_tg_load_avg().

Also, add more comments.
Reported-by: Vincent Guittot <vincent.guittot@linaro.org>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Mike Galbraith <efault@gmx.de>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: linux-kernel@vger.kernel.org
Signed-off-by: Ingo Molnar <mingo@kernel.org>

kernel/sched/fair.c

@@ -726,7 +726,6 @@ void post_init_entity_util_avg(struct sched_entity *se)
 	struct sched_avg *sa = &se->avg;
 	long cap = (long)(SCHED_CAPACITY_SCALE - cfs_rq->avg.util_avg) / 2;
 	u64 now = cfs_rq_clock_task(cfs_rq);
-	int tg_update;
 
 	if (cap > 0) {
 		if (cfs_rq->avg.util_avg != 0) {
@@ -759,9 +758,8 @@ void post_init_entity_util_avg(struct sched_entity *se)
 		}
 	}
 
-	tg_update = update_cfs_rq_load_avg(now, cfs_rq, false);
+	update_cfs_rq_load_avg(now, cfs_rq, false);
 	attach_entity_load_avg(cfs_rq, se);
-	if (tg_update)
 	update_tg_load_avg(cfs_rq, false);
 }
 
@@ -2803,9 +2801,21 @@ __update_load_avg(u64 now, int cpu, struct sched_avg *sa,
 }
 
 #ifdef CONFIG_FAIR_GROUP_SCHED
-/*
- * Updating tg's load_avg is necessary before update_cfs_share (which is done)
- * and effective_load (which is not done because it is too costly).
+/**
+ * update_tg_load_avg - update the tg's load avg
+ * @cfs_rq: the cfs_rq whose avg changed
+ * @force: update regardless of how small the difference
+ *
+ * This function 'ensures': tg->load_avg := \Sum tg->cfs_rq[]->avg.load.
+ * However, because tg->load_avg is a global value there are performance
+ * considerations.
+ *
+ * In order to avoid having to look at the other cfs_rq's, we use a
+ * differential update where we store the last value we propagated. This in
+ * turn allows skipping updates if the differential is 'small'.
+ *
+ * Updating tg's load_avg is necessary before update_cfs_share() (which is
+ * done) and effective_load() (which is not done because it is too costly).
  */
 static inline void update_tg_load_avg(struct cfs_rq *cfs_rq, int force)
 {
@@ -2931,10 +2941,10 @@ static inline void cfs_rq_util_change(struct cfs_rq *cfs_rq)
  *
  * cfs_rq->avg is used for task_h_load() and update_cfs_share() for example.
  *
- * Returns true if the load decayed or we removed utilization. It is expected
- * that one calls update_tg_load_avg() on this condition, but after you've
- * modified the cfs_rq avg (attach/detach), such that we propagate the new
- * avg up.
+ * Returns true if the load decayed or we removed load.
+ *
+ * Since both these conditions indicate a changed cfs_rq->avg.load we should
+ * call update_tg_load_avg() when this function returns true.
  */
 static inline int
 update_cfs_rq_load_avg(u64 now, struct cfs_rq *cfs_rq, bool update_freq)
@@ -8442,7 +8452,6 @@ static void detach_task_cfs_rq(struct task_struct *p)
 	struct sched_entity *se = &p->se;
 	struct cfs_rq *cfs_rq = cfs_rq_of(se);
 	u64 now = cfs_rq_clock_task(cfs_rq);
-	int tg_update;
 
 	if (!vruntime_normalized(p)) {
 		/*
@@ -8454,9 +8463,8 @@ static void detach_task_cfs_rq(struct task_struct *p)
 	}
 
 	/* Catch up with the cfs_rq and remove our load when we leave */
-	tg_update = update_cfs_rq_load_avg(now, cfs_rq, false);
+	update_cfs_rq_load_avg(now, cfs_rq, false);
 	detach_entity_load_avg(cfs_rq, se);
-	if (tg_update)
 	update_tg_load_avg(cfs_rq, false);
 }
 
@@ -8465,7 +8473,6 @@ static void attach_task_cfs_rq(struct task_struct *p)
 	struct sched_entity *se = &p->se;
 	struct cfs_rq *cfs_rq = cfs_rq_of(se);
 	u64 now = cfs_rq_clock_task(cfs_rq);
-	int tg_update;
 
 #ifdef CONFIG_FAIR_GROUP_SCHED
 	/*
@@ -8476,9 +8483,8 @@ static void attach_task_cfs_rq(struct task_struct *p)
 #endif
 
 	/* Synchronize task with its cfs_rq */
-	tg_update = update_cfs_rq_load_avg(now, cfs_rq, false);
+	update_cfs_rq_load_avg(now, cfs_rq, false);
 	attach_entity_load_avg(cfs_rq, se);
-	if (tg_update)
 	update_tg_load_avg(cfs_rq, false);
 
 	if (!vruntime_normalized(p))