linux-kernel - Re: [PATCH 1/2] sched/uclamp: Add a new sysctl to control RT default boost value

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Date:   Mon, 20 Apr 2020 10:29:46 +0200
From:   Dietmar Eggemann <dietmar.eggemann@....com>
To:     Qais Yousef <qais.yousef@....com>, Ingo Molnar <mingo@...hat.com>,
        Peter Zijlstra <peterz@...radead.org>
Cc:     Jonathan Corbet <corbet@....net>,
        Juri Lelli <juri.lelli@...hat.com>,
        Vincent Guittot <vincent.guittot@...aro.org>,
        Steven Rostedt <rostedt@...dmis.org>,
        Ben Segall <bsegall@...gle.com>, Mel Gorman <mgorman@...e.de>,
        Luis Chamberlain <mcgrof@...nel.org>,
        Kees Cook <keescook@...omium.org>,
        Iurii Zaikin <yzaikin@...gle.com>,
        Quentin Perret <qperret@...gle.com>,
        Valentin Schneider <valentin.schneider@....com>,
        Patrick Bellasi <patrick.bellasi@...bug.net>,
        Pavan Kondeti <pkondeti@...eaurora.org>,
        linux-doc@...r.kernel.org, linux-kernel@...r.kernel.org,
        linux-fsdevel@...r.kernel.org
Subject: Re: [PATCH 1/2] sched/uclamp: Add a new sysctl to control RT default
 boost value

On 03.04.20 14:30, Qais Yousef wrote:

[...]

> @@ -924,6 +945,14 @@ uclamp_eff_get(struct task_struct *p, enum uclamp_id clamp_id)
>  	return uc_req;
>  }
>  
> +static void uclamp_rt_sync_default_util_min(struct task_struct *p)
> +{
> +	struct uclamp_se *uc_se = &p->uclamp_req[UCLAMP_MIN];
> +
> +	if (!uc_se->user_defined)
> +		uclamp_se_set(uc_se, sysctl_sched_rt_default_uclamp_util_min, false);
> +}
> +
>  unsigned long uclamp_eff_value(struct task_struct *p, enum uclamp_id clamp_id)
>  {
>  	struct uclamp_se uc_eff;
> @@ -1030,6 +1059,12 @@ static inline void uclamp_rq_inc(struct rq *rq, struct task_struct *p)
>  	if (unlikely(!p->sched_class->uclamp_enabled))
>  		return;
>  
> +	/*
> +	 * When sysctl_sched_rt_default_uclamp_util_min value is changed by the
> +	 * user, we apply any new value on the next wakeup, which is here.
> +	 */
> +	uclamp_rt_sync_default_util_min(p);
> +

Does this have to be an extra function? Can we not reuse
uclamp_tg_restrict() by slightly rename it to uclamp_restrict()?

This function will then deal with enforcing restrictions, whether system
and taskgroup hierarchy related or default value (latter only for rt-min
right now since the others are fixed) related.

uclamp_eff_get() -> uclamp_restrict() is called from:

  'enqueue_task(), uclamp_update_active() -> uclamp_rq_inc() -> uclamp_rq_inc_id()' and

  'task_fits_capacity() -> clamp_task_util(), rt_task_fits_capacity() -> uclamp_eff_value()' and

  'schedutil_cpu_util(), find_energy_efficient_cpu() -> uclamp_rq_util_with() -> uclamp_eff_value()'

so there would be more check-points than the one in 'enqueue_task() -> uclamp_rq_inc()' now.

Only lightly tested:

---8<---

From: Dietmar Eggemann <dietmar.eggemann@....com>
Date: Sun, 19 Apr 2020 01:20:17 +0200
Subject: [PATCH] sched/core: uclamp: Move uclamp_rt_sync_default_util_min()
 into uclamp_tg_restrict()

Signed-off-by: Dietmar Eggemann <dietmar.eggemann@....com>
---
 kernel/sched/core.c | 34 +++++++++++++++-------------------
 1 file changed, 15 insertions(+), 19 deletions(-)

diff --git a/kernel/sched/core.c b/kernel/sched/core.c
index 8f4e0d5c7daf..6802113d6d4b 100644
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -899,12 +899,22 @@ unsigned int uclamp_rq_max_value(struct rq *rq, enum uclamp_id clamp_id,
 }
 
 static inline struct uclamp_se
-uclamp_tg_restrict(struct task_struct *p, enum uclamp_id clamp_id)
+uclamp_restrict(struct task_struct *p, enum uclamp_id clamp_id)
 {
-	struct uclamp_se uc_req = p->uclamp_req[clamp_id];
-#ifdef CONFIG_UCLAMP_TASK_GROUP
-	struct uclamp_se uc_max;
+	struct uclamp_se uc_req, __maybe_unused uc_max;
+
+	if (unlikely(rt_task(p)) && clamp_id == UCLAMP_MIN &&
+	    !uc_req.user_defined) {
+		struct uclamp_se *uc_se = &p->uclamp_req[UCLAMP_MIN];
+		int rt_min = sysctl_sched_rt_default_uclamp_util_min;
+
+		if (uc_se->value != rt_min)
+			uclamp_se_set(uc_se, rt_min, false);
+	}
 
+	uc_req = p->uclamp_req[clamp_id];
+
+#ifdef CONFIG_UCLAMP_TASK_GROUP
 	/*
 	 * Tasks in autogroups or root task group will be
 	 * restricted by system defaults.
@@ -933,7 +943,7 @@ uclamp_tg_restrict(struct task_struct *p, enum uclamp_id clamp_id)
 static inline struct uclamp_se
 uclamp_eff_get(struct task_struct *p, enum uclamp_id clamp_id)
 {
-	struct uclamp_se uc_req = uclamp_tg_restrict(p, clamp_id);
+	struct uclamp_se uc_req = uclamp_restrict(p, clamp_id);
 	struct uclamp_se uc_max = uclamp_default[clamp_id];
 
 	/* System default restrictions always apply */
@@ -943,14 +953,6 @@ uclamp_eff_get(struct task_struct *p, enum uclamp_id clamp_id)
 	return uc_req;
 }
 
-static void uclamp_rt_sync_default_util_min(struct task_struct *p)
-{
-	struct uclamp_se *uc_se = &p->uclamp_req[UCLAMP_MIN];
-
-	if (!uc_se->user_defined)
-		uclamp_se_set(uc_se, sysctl_sched_rt_default_uclamp_util_min, false);
-}
-
 unsigned long uclamp_eff_value(struct task_struct *p, enum uclamp_id clamp_id)
 {
 	struct uclamp_se uc_eff;
@@ -1057,12 +1059,6 @@ static inline void uclamp_rq_inc(struct rq *rq, struct task_struct *p)
 	if (unlikely(!p->sched_class->uclamp_enabled))
 		return;
 
-	/*
-	 * When sysctl_sched_rt_default_uclamp_util_min value is changed by the
-	 * user, we apply any new value on the next wakeup, which is here.
-	 */
-	uclamp_rt_sync_default_util_min(p);
-
 	for_each_clamp_id(clamp_id)
 		uclamp_rq_inc_id(rq, p, clamp_id);
 
-- 
2.17.1