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 c365c292d0590 "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.me...@gmail.com>
Signed-off-by: Thomas Gleixner <t...@linutronix.de>
---
kernel/locking/rtmutex.c | 10 ++++++----
kernel/sched/core.c | 11 +++++------
2 files changed, 11 insertions(+), 10 deletions(-)
Index: tip/kernel/locking/rtmutex.c
===================================================================
--- tip.orig/kernel/locking/rtmutex.c
+++ tip/kernel/locking/rtmutex.c
@@ -265,15 +265,17 @@ struct task_struct *rt_mutex_get_top_tas
}
/*
- * 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)
{
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;
}
/*
Index: tip/kernel/sched/core.c
===================================================================
--- tip.orig/kernel/sched/core.c
+++ tip/kernel/sched/core.c
@@ -3414,7 +3414,7 @@ static int __sched_setscheduler(struct t
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;
@@ -3596,15 +3596,14 @@ 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_check_prio(p, newprio);
+ if (new_effective_prio == oldprio) {
__setscheduler_params(p, attr);
task_rq_unlock(rq, p, &flags);
return 0;
--
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