[3.19.y-ckt stable] Patch "sched: Handle priority boosted tasks proper in setscheduler()" has been added to staging queue

Kamal Mostafa kamal at canonical.com
Tue Jun 16 00:21:58 UTC 2015


This is a note to let you know that I have just added a patch titled

    sched: Handle priority boosted tasks proper in setscheduler()

to the linux-3.19.y-queue branch of the 3.19.y-ckt extended stable tree 
which can be found at:

    http://kernel.ubuntu.com/git/ubuntu/linux.git/log/?h=linux-3.19.y-queue

This patch is scheduled to be released in version 3.19.8-ckt2.

If you, or anyone else, feels it should not be added to this tree, please 
reply to this email.

For more information about the 3.19.y-ckt tree, see
https://wiki.ubuntu.com/Kernel/Dev/ExtendedStable

Thanks.
-Kamal

------

>From 029e3800943041bb0ec69b7e816524e0ed09f849 Mon Sep 17 00:00:00 2001
From: Thomas Gleixner <tglx at linutronix.de>
Date: Tue, 5 May 2015 19:49:49 +0200
Subject: sched: Handle priority boosted tasks proper in setscheduler()

commit 0782e63bc6fe7e2d3408d250df11d388b7799c6b upstream.

Ronny reported that the following scenario is not handled correctly:

	T1 (prio = 10)
	   lock(rtmutex);

	T2 (prio = 20)
	   lock(rtmutex)
	      boost T1

	T1 (prio = 20)
	   sys_set_scheduler(prio = 30)
	   T1 prio = 30
	   ....
	   sys_set_scheduler(prio = 10)
	   T1 prio = 30

The last step is wrong as T1 should now be back at prio 20.

Commit c365c292d059 ("sched: Consider pi boosting in setscheduler()")
only handles the case where a boosted tasks tries to lower its
priority.

Fix it by taking the new effective priority into account for the
decision whether a change of the priority is required.

Reported-by: Ronny Meeus <ronny.meeus at gmail.com>
Tested-by: Steven Rostedt <rostedt at goodmis.org>
Signed-off-by: Thomas Gleixner <tglx at linutronix.de>
Signed-off-by: Peter Zijlstra (Intel) <peterz at infradead.org>
Reviewed-by: Steven Rostedt <rostedt at goodmis.org>
Cc: Borislav Petkov <bp at alien8.de>
Cc: H. Peter Anvin <hpa at zytor.com>
Cc: Mike Galbraith <umgwanakikbuti at gmail.com>
Fixes: c365c292d059 ("sched: Consider pi boosting in setscheduler()")
Link: http://lkml.kernel.org/r/alpine.DEB.2.11.1505051806060.4225@nanos
Signed-off-by: Ingo Molnar <mingo at kernel.org>
Signed-off-by: Kamal Mostafa <kamal at canonical.com>
---
 include/linux/sched/rt.h |  7 ++++---
 kernel/locking/rtmutex.c | 12 +++++++-----
 kernel/sched/core.c      | 26 ++++++++++++++------------
 3 files changed, 25 insertions(+), 20 deletions(-)

diff --git a/include/linux/sched/rt.h b/include/linux/sched/rt.h
index 6341f5b..a30b172 100644
--- a/include/linux/sched/rt.h
+++ b/include/linux/sched/rt.h
@@ -18,7 +18,7 @@ static inline int rt_task(struct task_struct *p)
 #ifdef CONFIG_RT_MUTEXES
 extern int rt_mutex_getprio(struct task_struct *p);
 extern void rt_mutex_setprio(struct task_struct *p, int prio);
-extern int rt_mutex_check_prio(struct task_struct *task, int newprio);
+extern int rt_mutex_get_effective_prio(struct task_struct *task, int newprio);
 extern struct task_struct *rt_mutex_get_top_task(struct task_struct *task);
 extern void rt_mutex_adjust_pi(struct task_struct *p);
 static inline bool tsk_is_pi_blocked(struct task_struct *tsk)
@@ -31,9 +31,10 @@ static inline int rt_mutex_getprio(struct task_struct *p)
 	return p->normal_prio;
 }

-static inline int rt_mutex_check_prio(struct task_struct *task, int newprio)
+static inline int rt_mutex_get_effective_prio(struct task_struct *task,
+					      int newprio)
 {
-	return 0;
+	return newprio;
 }

 static inline struct task_struct *rt_mutex_get_top_task(struct task_struct *task)
diff --git a/kernel/locking/rtmutex.c b/kernel/locking/rtmutex.c
index 41d53e5..1929ee2 100644
--- a/kernel/locking/rtmutex.c
+++ b/kernel/locking/rtmutex.c
@@ -265,15 +265,17 @@ struct task_struct *rt_mutex_get_top_task(struct task_struct *task)
 }

 /*
- * Called by sched_setscheduler() to check whether the priority change
- * is overruled by a possible priority boosting.
+ * Called by sched_setscheduler() to get the priority which will be
+ * effective after the change.
  */
-int rt_mutex_check_prio(struct task_struct *task, int newprio)
+int rt_mutex_get_effective_prio(struct task_struct *task, int newprio)
 {
 	if (!task_has_pi_waiters(task))
-		return 0;
+		return newprio;

-	return task_top_pi_waiter(task)->task->prio <= newprio;
+	if (task_top_pi_waiter(task)->task->prio <= newprio)
+		return task_top_pi_waiter(task)->task->prio;
+	return newprio;
 }

 /*
diff --git a/kernel/sched/core.c b/kernel/sched/core.c
index d24220d..c90b977 100644
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -3340,15 +3340,18 @@ static void __setscheduler_params(struct task_struct *p,

 /* Actually do priority change: must hold pi & rq lock. */
 static void __setscheduler(struct rq *rq, struct task_struct *p,
-			   const struct sched_attr *attr)
+			   const struct sched_attr *attr, bool keep_boost)
 {
 	__setscheduler_params(p, attr);

 	/*
-	 * If we get here, there was no pi waiters boosting the
-	 * task. It is safe to use the normal prio.
+	 * Keep a potential priority boosting if called from
+	 * sched_setscheduler().
 	 */
-	p->prio = normal_prio(p);
+	if (keep_boost)
+		p->prio = rt_mutex_get_effective_prio(p, normal_prio(p));
+	else
+		p->prio = normal_prio(p);

 	if (dl_prio(p->prio))
 		p->sched_class = &dl_sched_class;
@@ -3434,7 +3437,7 @@ static int __sched_setscheduler(struct task_struct *p,
 	int newprio = dl_policy(attr->sched_policy) ? MAX_DL_PRIO - 1 :
 		      MAX_RT_PRIO - 1 - attr->sched_priority;
 	int retval, oldprio, oldpolicy = -1, queued, running;
-	int policy = attr->sched_policy;
+	int new_effective_prio, policy = attr->sched_policy;
 	unsigned long flags;
 	const struct sched_class *prev_class;
 	struct rq *rq;
@@ -3616,15 +3619,14 @@ change:
 	oldprio = p->prio;

 	/*
-	 * Special case for priority boosted tasks.
-	 *
-	 * If the new priority is lower or equal (user space view)
-	 * than the current (boosted) priority, we just store the new
+	 * Take priority boosted tasks into account. If the new
+	 * effective priority is unchanged, we just store the new
 	 * normal parameters and do not touch the scheduler class and
 	 * the runqueue. This will be done when the task deboost
 	 * itself.
 	 */
-	if (rt_mutex_check_prio(p, newprio)) {
+	new_effective_prio = rt_mutex_get_effective_prio(p, newprio);
+	if (new_effective_prio == oldprio) {
 		__setscheduler_params(p, attr);
 		task_rq_unlock(rq, p, &flags);
 		return 0;
@@ -3638,7 +3640,7 @@ change:
 		put_prev_task(rq, p);

 	prev_class = p->sched_class;
-	__setscheduler(rq, p, attr);
+	__setscheduler(rq, p, attr, true);

 	if (running)
 		p->sched_class->set_curr_task(rq);
@@ -7400,7 +7402,7 @@ static void normalize_task(struct rq *rq, struct task_struct *p)
 	queued = task_on_rq_queued(p);
 	if (queued)
 		dequeue_task(rq, p, 0);
-	__setscheduler(rq, p, &attr);
+	__setscheduler(rq, p, &attr, false);
 	if (queued) {
 		enqueue_task(rq, p, 0);
 		resched_curr(rq);
--
1.9.1





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