Commit 0782e63b authored by Thomas Gleixner's avatar Thomas Gleixner Committed by Ingo Molnar
Browse files

sched: Handle priority boosted tasks proper in setscheduler() 



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 c365c292 ("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: default avatarRonny Meeus <ronny.meeus@gmail.com>
Tested-by: default avatarSteven Rostedt <rostedt@goodmis.org>
Signed-off-by: default avatarThomas Gleixner <tglx@linutronix.de>
Signed-off-by: default avatarPeter Zijlstra (Intel) <peterz@infradead.org>
Reviewed-by: default avatarSteven Rostedt <rostedt@goodmis.org>
Cc: <stable@vger.kernel.org>
Cc: Borislav Petkov <bp@alien8.de>
Cc: H. Peter Anvin <hpa@zytor.com>
Cc: Mike Galbraith <umgwanakikbuti@gmail.com>
Fixes: c365c292 ("sched: Consider pi boosting in setscheduler()")
Link: http://lkml.kernel.org/r/alpine.DEB.2.11.1505051806060.4225@nanos


Signed-off-by: default avatarIngo Molnar <mingo@kernel.org>
parent 3e0283a5

include/linux/sched/rt.h

+4 −3
Original line number Diff line number Diff line
@@ -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)

kernel/locking/rtmutex.c

+7 −5
Original line number Diff line number Diff line
@@ -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;

}



/*

kernel/sched/core.c

+14 −12
Original line number Diff line number Diff line
@@ -3300,14 +3300,17 @@ 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().

	 */

	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))
@@ -3408,7 +3411,7 @@ 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;
@@ -3590,15 +3593,14 @@ 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;
@@ -3612,7 +3614,7 @@ 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);
@@ -7346,7 +7348,7 @@ 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);

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