Re: [PATCH v4] sched/freq: move call to cpufreq_update_util

[Peter Zijlstra <peterz@infradead.org>]

On Fri, Nov 15, 2019 at 02:37:27PM +0100, Vincent Guittot wrote:
> On Fri, 15 Nov 2019 at 14:25, Peter Zijlstra <peterz@infradead.org> wrote:

> > Should not all 3 have their windows aligned and thus alway return the
> > exact same value?
> 
> rt and dl yes but not irq
> 
> But having aligned window doesn't mean that they will all decay.
> One can have been updated just before (during a dequeue as an example)
> or at least less than 1ms before

Now, the thing is, if that update happened in sched/rt, then it wouldn't
have called cpufreq anyway. And once we're idle longer than a period,
they'll all decay at once.

Except indeed that IRQ stuff, which runs out of sync.

That is, I'm just not convinced it matters much if we keep rq->cfs
on the list forever (like UP). Because we'll only stop calling when
update_blocked_averages() when everything hit 0, and up until that
point, we'll get one update per period from rq->cfs.

For good measure we can force an update when @done, at that point we
know all 0s.

How is something like this?

---
diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index 545bcb90b4de..a99ac2aa4a23 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -3508,9 +3508,6 @@ update_cfs_rq_load_avg(u64 now, struct cfs_rq *cfs_rq)
 	cfs_rq->load_last_update_time_copy = sa->last_update_time;
 #endif
 
-	if (decayed)
-		cfs_rq_util_change(cfs_rq, 0);
-
 	return decayed;
 }
 
@@ -3620,8 +3617,12 @@ static inline void update_load_avg(struct cfs_rq *cfs_rq, struct sched_entity *s
 		attach_entity_load_avg(cfs_rq, se, SCHED_CPUFREQ_MIGRATION);
 		update_tg_load_avg(cfs_rq, 0);
 
-	} else if (decayed && (flags & UPDATE_TG))
-		update_tg_load_avg(cfs_rq, 0);
+	} else if (decayed) {
+		cfs_rq_util_change(cfs_rq, 0);
+
+		if (flags & UPDATE_TG)
+			update_tg_load_avg(cfs_rq, 0);
+	}
 }
 
 #ifndef CONFIG_64BIT
@@ -7453,7 +7454,7 @@ static void update_blocked_averages(int cpu)
 	struct cfs_rq *cfs_rq, *pos;
 	const struct sched_class *curr_class;
 	struct rq_flags rf;
-	bool done = true;
+	bool done = true, decayed = false;
 
 	rq_lock_irqsave(rq, &rf);
 	update_rq_clock(rq);
@@ -7476,10 +7477,14 @@ static void update_blocked_averages(int cpu)
 	 * list_add_leaf_cfs_rq() for details.
 	 */
 	for_each_leaf_cfs_rq_safe(rq, cfs_rq, pos) {
+		bool last = cfs_rq == &rq->cfs;
 		struct sched_entity *se;
 
-		if (update_cfs_rq_load_avg(cfs_rq_clock_pelt(cfs_rq), cfs_rq))
+		if (update_cfs_rq_load_avg(cfs_rq_clock_pelt(cfs_rq), cfs_rq)) {
 			update_tg_load_avg(cfs_rq, 0);
+			if (last)
+				decayed = true;
+		}
 
 		/* Propagate pending load changes to the parent, if any: */
 		se = cfs_rq->tg->se[cpu];
@@ -7490,7 +7495,7 @@ static void update_blocked_averages(int cpu)
 		 * There can be a lot of idle CPU cgroups.  Don't let fully
 		 * decayed cfs_rqs linger on the list.
 		 */
-		if (cfs_rq_is_decayed(cfs_rq))
+		if (!last && cfs_rq_is_decayed(cfs_rq))
 			list_del_leaf_cfs_rq(cfs_rq);
 
 		/* Don't need periodic decay once load/util_avg are null */
@@ -7498,6 +7503,9 @@ static void update_blocked_averages(int cpu)
 			done = false;
 	}
 
+	if (decayed || done)
+		cpufreq_update_util(rq, 0);
+
 	update_blocked_load_status(rq, !done);
 	rq_unlock_irqrestore(rq, &rf);
 }
@@ -7555,6 +7563,7 @@ static inline void update_blocked_averages(int cpu)
 	struct cfs_rq *cfs_rq = &rq->cfs;
 	const struct sched_class *curr_class;
 	struct rq_flags rf;
+	bool done, decayed;
 
 	rq_lock_irqsave(rq, &rf);
 	update_rq_clock(rq);
@@ -7568,9 +7577,13 @@ static inline void update_blocked_averages(int cpu)
 	update_dl_rq_load_avg(rq_clock_pelt(rq), rq, curr_class == &dl_sched_class);
 	update_irq_load_avg(rq, 0);
 
-	update_cfs_rq_load_avg(cfs_rq_clock_pelt(cfs_rq), cfs_rq);
+	decayed = update_cfs_rq_load_avg(cfs_rq_clock_pelt(cfs_rq), cfs_rq);
+	done = !(cfs_rq_has_blocked(cfs_rq) || others_have_blocked(rq));
 
-	update_blocked_load_status(rq, cfs_rq_has_blocked(cfs_rq) || others_have_blocked(rq));
+	if (decayed || done)
+		cpufreq_update_util(rq, 0);
+
+	update_blocked_load_status(rq, !done);
 	rq_unlock_irqrestore(rq, &rf);
 }