linux-kernel - Re: [PATCH v2] sched/rt: fix call to cpufreq_update

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Date:   Thu, 17 May 2018 22:12:07 +0200
From:   Vincent Guittot <vincent.guittot@...aro.org>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     mingo@...nel.org, linux-kernel@...r.kernel.org,
        patrick.bellasi@....com, viresh.kumar@...aro.org,
        tglx@...utronix.de, efault@....de, juri.lelli@...hat.com,
        "# v4 . 16+" <stable@...r.kernel.org>
Subject: Re: [PATCH v2] sched/rt: fix call to cpufreq_update_util

Le Thursday 17 May 2018 à 11:32:06 (+0200), Peter Zijlstra a écrit :
> On Thu, May 17, 2018 at 11:05:30AM +0200, Peter Zijlstra wrote:
> > Hurm.. I think this is also wrong. See how dequeue_rt_stack() is also
> > called from the enqueue path. Also, nothing calls cpufreq_update_util()
> > on the throttle path now.

Yes I missed the point that rt_entities were dequeued then re-enqueued when
a task was either enqueued or dequeued.

That's also means that enqueue_top_rt_rq() is always called when a task is
enqueued or dequeued and also when groups are throttled or unthrottled
In fact, the only point where it's not called, is when root rt_rq is
throttled. So I have prepared the patch below which call cpufreq_update util
in enqueue_top_rt_rq() and also when we throttle the root rt_rq.
According to the tests I have done , it seems to work for enqueue/dequeue and
throttle/unthrottle use cases.

I have re-used your rt_rq_is_runnable() which takes into account rt_queued

---
 kernel/sched/cpufreq_schedutil.c |  2 +-
 kernel/sched/rt.c                | 16 ++++++++++------
 kernel/sched/sched.h             |  5 +++++
 3 files changed, 16 insertions(+), 7 deletions(-)

diff --git a/kernel/sched/cpufreq_schedutil.c b/kernel/sched/cpufreq_schedutil.c
index e13df95..1751f96 100644
--- a/kernel/sched/cpufreq_schedutil.c
+++ b/kernel/sched/cpufreq_schedutil.c
@@ -185,7 +185,7 @@ static unsigned long sugov_aggregate_util(struct sugov_cpu *sg_cpu)
 	struct rq *rq = cpu_rq(sg_cpu->cpu);
 	unsigned long util;
 
-	if (rq->rt.rt_nr_running) {
+	if (rt_rq_is_runnable(&rq->rt)) {
 		util = sg_cpu->max;
 	} else {
 		util = sg_cpu->util_dl;
diff --git a/kernel/sched/rt.c b/kernel/sched/rt.c
index 7aef6b4..d6b9517 100644
--- a/kernel/sched/rt.c
+++ b/kernel/sched/rt.c
@@ -508,8 +508,11 @@ static void sched_rt_rq_dequeue(struct rt_rq *rt_rq)
 
 	rt_se = rt_rq->tg->rt_se[cpu];
 
-	if (!rt_se)
+	if (!rt_se) {
 		dequeue_top_rt_rq(rt_rq);
+		/* Kick cpufreq (see the comment in kernel/sched/sched.h). */
+		cpufreq_update_util(rq_of_rt_rq(rt_rq), 0);
+	}
 	else if (on_rt_rq(rt_se))
 		dequeue_rt_entity(rt_se, 0);
 }
@@ -1001,8 +1004,6 @@ dequeue_top_rt_rq(struct rt_rq *rt_rq)
 	sub_nr_running(rq, rt_rq->rt_nr_running);
 	rt_rq->rt_queued = 0;
 
-	/* Kick cpufreq (see the comment in kernel/sched/sched.h). */
-	cpufreq_update_util(rq, 0);
 }
 
 static void
@@ -1014,11 +1015,14 @@ enqueue_top_rt_rq(struct rt_rq *rt_rq)
 
 	if (rt_rq->rt_queued)
 		return;
-	if (rt_rq_throttled(rt_rq) || !rt_rq->rt_nr_running)
+
+	if (rt_rq_throttled(rt_rq))
 		return;
 
-	add_nr_running(rq, rt_rq->rt_nr_running);
-	rt_rq->rt_queued = 1;
+	if (rt_rq->rt_nr_running) {
+		add_nr_running(rq, rt_rq->rt_nr_running);
+		rt_rq->rt_queued = 1;
+	}
 
 	/* Kick cpufreq (see the comment in kernel/sched/sched.h). */
 	cpufreq_update_util(rq, 0);
diff --git a/kernel/sched/sched.h b/kernel/sched/sched.h
index c9895d3..bbebcfe 100644
--- a/kernel/sched/sched.h
+++ b/kernel/sched/sched.h
@@ -609,6 +609,11 @@ struct rt_rq {
 #endif
 };
 
+static inline bool rt_rq_is_runnable(struct rt_rq *rt_rq)
+{
+	return rt_rq->rt_queued && rt_rq->rt_nr_running;
+}
+
 /* Deadline class' related fields in a runqueue */
 struct dl_rq {
 	/* runqueue is an rbtree, ordered by deadline */
-- 
2.7.4





> > 
> > And talking about throttle; I think we wanted to check rt_queued in that
> > case.
> 
> Bah, clearly you also want to update on unthrottle.. but I think there's
> more buggered wrt throtteling.