[PATCH 4.0 101/148] sched: Handle priority boosted tasks proper in setscheduler()

Greg Kroah-Hartman
2015-06-03T12:22:23Z

From: Thomas Gleixner <tglx@linutronix.de>

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@gmail.com>
Tested-by: Steven Rostedt <rostedt@goodmis.org>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Reviewed-by: Steven Rostedt <rostedt@goodmis.org>
Cc: Borislav Petkov <bp@alien8.de>
Cc: H. Peter Anvin <hpa@zytor.com>
Cc: Mike Galbraith <umgwanakikbuti@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@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

---
include/linux/sched/rt.h | 7 ++++---
kernel/locking/rtmutex.c | 12 +++++++-----
kernel/sched/core.c | 26 ++++++++++++++------------
3 files changed, 25 insertions(+), 20 deletions(-)

--- a/include/linux/sched/rt.h
+++ b/include/linux/sched/rt.h
@@ -18,7 +18,7 @@ static inline int rt_task(struct task_st
#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(struc
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)
--- a/kernel/locking/rtmutex.c
+++ b/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)
+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;
}

/*
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -3295,15 +3295,18 @@ static void __setscheduler_params(struct

/* 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;
@@ -3403,7 +3406,7 @@ 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;
@@ -3585,15 +3588,14 @@ 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;
@@ -3607,7 +3609,7 @@ 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);
@@ -7357,7 +7359,7 @@ static void normalize_task(struct rq *rq
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);