Re: [PATCH v2] sched/rt: fix call to cpufreq_update_util

[Peter Zijlstra]

On Thu, May 17, 2018 at 09:16:43AM +0200, Vincent Guittot wrote:
> With commit
>
>  8f111bc357aa ("cpufreq/schedutil: Rewrite CPUFREQ_RT support")
>
> schedutil governor uses rq->rt.rt_nr_running to detect whether a RT task is
> currently running on the CPU and to set frequency to max if necessary.
>
> cpufreq_update_util() is called in enqueue/dequeue_top_rt_rq() but
> rq->rt.rt_nr_running as not been updated yet when dequeue_top_rt_rq() is
> called so schedutil still considers that a RT task is running when the
> last task is dequeued. The update of rq->rt.rt_nr_running happens later
> in dequeue_rt_stack()
> 
> Fixes: 8f111bc357aa ('cpufreq/schedutil: Rewrite CPUFREQ_RT support')
> Cc: <stable@xxxxxxxxxxxxxxx> # v4.16+
> Signed-off-by: Vincent Guittot <vincent.guittot@xxxxxxxxxx>
> ---
> - v2:
>   - remove blank line
> 
>  kernel/sched/rt.c | 6 +++---
>  1 file changed, 3 insertions(+), 3 deletions(-)
> 
> diff --git a/kernel/sched/rt.c b/kernel/sched/rt.c
> index 7aef6b4..a9f1119 100644
> --- a/kernel/sched/rt.c
> +++ b/kernel/sched/rt.c
> @@ -1000,9 +1000,6 @@ 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
> @@ -1288,6 +1285,9 @@ static void dequeue_rt_stack(struct sched_rt_entity *rt_se, unsigned int flags)
>  		if (on_rt_rq(rt_se))
>  			__dequeue_rt_entity(rt_se, flags);
>  	}
> +
> +	/* Kick cpufreq (see the comment in kernel/sched/sched.h). */
> +	cpufreq_update_util(rq_of_rt_rq(rt_rq_of_se(back)), 0);
>  }

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.

And talking about throttle; I think we wanted to check rt_queued in that
case.

How about the below?

---
 kernel/sched/cpufreq_schedutil.c |  2 +-
 kernel/sched/rt.c                | 24 ++++++++++++++----------
 kernel/sched/sched.h             |  5 +++++
 3 files changed, 20 insertions(+), 11 deletions(-)

diff --git a/kernel/sched/cpufreq_schedutil.c b/kernel/sched/cpufreq_schedutil.c
index e13df951aca7..1751f9630e49 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 7aef6b4e885a..a1108a7da777 100644
--- a/kernel/sched/rt.c
+++ b/kernel/sched/rt.c
@@ -979,8 +979,14 @@ static void update_curr_rt(struct rq *rq)
 		if (sched_rt_runtime(rt_rq) != RUNTIME_INF) {
 			raw_spin_lock(&rt_rq->rt_runtime_lock);
 			rt_rq->rt_time += delta_exec;
-			if (sched_rt_runtime_exceeded(rt_rq))
+			if (sched_rt_runtime_exceeded(rt_rq)) {
+				struct rq *rq = rq_of_rt_rq(rt_rq);
+
+				if (&rq->rt == rt_rq)
+					cpufreq_update_util(rq, 0);
+
 				resched_curr(rq);
+			}
 			raw_spin_unlock(&rt_rq->rt_runtime_lock);
 		}
 	}
@@ -1000,9 +1006,6 @@ 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
@@ -1019,9 +1022,6 @@ enqueue_top_rt_rq(struct rt_rq *rt_rq)
 
 	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);
 }
 
 #if defined CONFIG_SMP
@@ -1330,6 +1330,8 @@ enqueue_task_rt(struct rq *rq, struct task_struct *p, int flags)
 
 	if (!task_current(rq, p) && p->nr_cpus_allowed > 1)
 		enqueue_pushable_task(rq, p);
+
+	cpufreq_update_util(rq, 0);
 }
 
 static void dequeue_task_rt(struct rq *rq, struct task_struct *p, int flags)
@@ -1340,6 +1342,8 @@ static void dequeue_task_rt(struct rq *rq, struct task_struct *p, int flags)
 	dequeue_rt_entity(rt_se, flags);
 
 	dequeue_pushable_task(rq, p);
+
+	cpufreq_update_util(rq, 0);
 }
 
 /*
@@ -1533,6 +1537,9 @@ pick_next_task_rt(struct rq *rq, struct task_struct *prev, struct rq_flags *rf)
 	struct task_struct *p;
 	struct rt_rq *rt_rq = &rq->rt;
 
+	if (!rt_rq->rt_queued)
+		return NULL;
+
 	if (need_pull_rt_task(rq, prev)) {
 		/*
 		 * This is OK, because current is on_cpu, which avoids it being
@@ -1560,9 +1567,6 @@ pick_next_task_rt(struct rq *rq, struct task_struct *prev, struct rq_flags *rf)
 	if (prev->sched_class == &rt_sched_class)
 		update_curr_rt(rq);
 
-	if (!rt_rq->rt_queued)
-		return NULL;
-
 	put_prev_task(rq, prev);
 
 	p = _pick_next_task_rt(rq);
diff --git a/kernel/sched/sched.h b/kernel/sched/sched.h
index c9895d35c5f7..bec7f2eecaf3 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 rq_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 */