linux-kernel - Re: [PATCH] sched/rt: Fix double enqueue caused by rt_effective

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Date:   Thu, 8 Jul 2021 12:26:33 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Juri Lelli <juri.lelli@...hat.com>
Cc:     mingo@...hat.com, linux-kernel@...r.kernel.org,
        vincent.guittot@...aro.org, rostedt@...dmis.org,
        dietmar.eggemann@....com, bristot@...hat.com, bsegall@...gle.com,
        mgorman@...e.de, Mark Simmons <msimmons@...hat.com>
Subject: Re: [PATCH] sched/rt: Fix double enqueue caused by rt_effective_prio

On Thu, Jul 08, 2021 at 12:06:27PM +0200, Peter Zijlstra wrote:
> Slightly larger patch, but perhaps a little cleaner.. still pondering if
> we can share a little more between __sched_setscheduler() and
> rt_mutex_setprio().

Best I can seem to come up with...

---
 kernel/sched/core.c | 45 +++++++++++++++++----------------------------
 1 file changed, 17 insertions(+), 28 deletions(-)

diff --git a/kernel/sched/core.c b/kernel/sched/core.c
index ede10642612c..c686f0c70656 100644
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -6341,6 +6341,18 @@ int default_wake_function(wait_queue_entry_t *curr, unsigned mode, int wake_flag
 }
 EXPORT_SYMBOL(default_wake_function);
 
+static void __setscheduler_prio(struct task_struct *p, int prio)
+{
+	if (dl_prio(prio))
+		p->sched_class = &dl_sched_class;
+	else if (rt_prio(prio))
+		p->sched_class = &rt_sched_class;
+	else
+		p->sched_class = &fair_sched_class;
+
+	p->prio = prio;
+}
+
 #ifdef CONFIG_RT_MUTEXES
 
 static inline int __rt_effective_prio(struct task_struct *pi_task, int prio)
@@ -6456,22 +6468,19 @@ void rt_mutex_setprio(struct task_struct *p, struct task_struct *pi_task)
 		} else {
 			p->dl.pi_se = &p->dl;
 		}
-		p->sched_class = &dl_sched_class;
 	} else if (rt_prio(prio)) {
 		if (dl_prio(oldprio))
 			p->dl.pi_se = &p->dl;
 		if (oldprio < prio)
 			queue_flag |= ENQUEUE_HEAD;
-		p->sched_class = &rt_sched_class;
 	} else {
 		if (dl_prio(oldprio))
 			p->dl.pi_se = &p->dl;
 		if (rt_prio(oldprio))
 			p->rt.timeout = 0;
-		p->sched_class = &fair_sched_class;
 	}
 
-	p->prio = prio;
+	__setscheduler_prio(p, prio);
 
 	if (queued)
 		enqueue_task(rq, p, queue_flag);
@@ -6824,29 +6833,6 @@ static void __setscheduler_params(struct task_struct *p,
 	set_load_weight(p, true);
 }
 
-/* Actually do priority change: must hold pi & rq lock. */
-static void __setscheduler(struct rq *rq, struct task_struct *p,
-			   const struct sched_attr *attr, int newprio)
-{
-	/*
-	 * If params can't change scheduling class changes aren't allowed
-	 * either.
-	 */
-	if (attr->sched_flags & SCHED_FLAG_KEEP_PARAMS)
-		return;
-
-	__setscheduler_params(p, attr);
-
-	p->prio = newprio;
-
-	if (dl_prio(p->prio))
-		p->sched_class = &dl_sched_class;
-	else if (rt_prio(p->prio))
-		p->sched_class = &rt_sched_class;
-	else
-		p->sched_class = &fair_sched_class;
-}
-
 /*
  * Check the target process has a UID that matches the current process's:
  */
@@ -7089,7 +7075,10 @@ static int __sched_setscheduler(struct task_struct *p,
 
 	prev_class = p->sched_class;
 
-	__setscheduler(rq, p, attr, newprio);
+	if (!(attr->sched_flags & SCHED_FLAG_KEEP_PARAMS)) {
+		__setscheduler_params(p, attr);
+		__setscheduler_prio(p, newprio);
+	}
 	__setscheduler_uclamp(p, attr);
 
 	if (queued) {