* [PATCH RT 2/4] sched: Queue RT tasks to head when prio drops
[not found] <20121222171201.588460638@goodmis.org>
2012-12-22 17:12 ` [PATCH RT 1/4] sched: Adjust sched_reset_on_fork when nothing else changes Steven Rostedt
@ 2012-12-22 17:12 ` Steven Rostedt
2012-12-22 17:12 ` [PATCH RT 3/4] sched: Consider pi boosting in setscheduler Steven Rostedt
2 siblings, 0 replies; 4+ messages in thread
From: Steven Rostedt @ 2012-12-22 17:12 UTC (permalink / raw)
To: linux-kernel, linux-rt-users
Cc: Thomas Gleixner, Carsten Emde, John Kacur, stable, stable-rt
[-- Attachment #1: 0002-sched-Queue-RT-tasks-to-head-when-prio-drops.patch --]
[-- Type: text/plain, Size: 2376 bytes --]
From: Thomas Gleixner <tglx@linutronix.de>
The following scenario does not work correctly:
Runqueue of CPUx contains two runnable and pinned tasks:
T1: SCHED_FIFO, prio 80
T2: SCHED_FIFO, prio 80
T1 is on the cpu and executes the following syscalls (classic priority
ceiling scenario):
sys_sched_setscheduler(pid(T1), SCHED_FIFO, .prio = 90);
...
sys_sched_setscheduler(pid(T1), SCHED_FIFO, .prio = 80);
...
Now T1 gets preempted by T3 (SCHED_FIFO, prio 95). After T3 goes back
to sleep the scheduler picks T2. Surprise!
The same happens w/o actual preemption when T1 is forced into the
scheduler due to a sporadic NEED_RESCHED event. The scheduler invokes
pick_next_task() which returns T2. So T1 gets preempted and scheduled
out.
This happens because sched_setscheduler() dequeues T1 from the prio 90
list and then enqueues it on the tail of the prio 80 list behind T2.
This violates the POSIX spec and surprises user space which relies on
the guarantee that SCHED_FIFO tasks are not scheduled out unless they
give the CPU up voluntarily or are preempted by a higher priority
task. In the latter case the preempted task must get back on the CPU
after the preempting task schedules out again.
We fixed a similar issue already in commit 60db48c (sched: Queue a
deboosted task to the head of the RT prio queue). The same treatment
is necessary for sched_setscheduler(). So enqueue to head of the prio
bucket list if the priority of the task is lowered.
It might be possible that existing user space relies on the current
behaviour, but it can be considered highly unlikely due to the corner
case nature of the application scenario.
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Cc: stable@vger.kernel.org
Cc: stable-rt@vger.kernel.org
---
kernel/sched.c | 9 +++++++--
1 file changed, 7 insertions(+), 2 deletions(-)
diff --git a/kernel/sched.c b/kernel/sched.c
index 945009e..ad42cb2 100644
--- a/kernel/sched.c
+++ b/kernel/sched.c
@@ -5387,8 +5387,13 @@ recheck:
if (running)
p->sched_class->set_curr_task(rq);
- if (on_rq)
- activate_task(rq, p, 0);
+ if (on_rq) {
+ /*
+ * We enqueue to tail when the priority of a task is
+ * increased (user space view).
+ */
+ activate_task(rq, p, oldprio <= p->prio ? ENQUEUE_HEAD : 0);
+ }
check_class_changed(rq, p, prev_class, oldprio);
task_rq_unlock(rq, p, &flags);
--
1.7.10.4
^ permalink raw reply related [flat|nested] 4+ messages in thread* [PATCH RT 3/4] sched: Consider pi boosting in setscheduler
[not found] <20121222171201.588460638@goodmis.org>
2012-12-22 17:12 ` [PATCH RT 1/4] sched: Adjust sched_reset_on_fork when nothing else changes Steven Rostedt
2012-12-22 17:12 ` [PATCH RT 2/4] sched: Queue RT tasks to head when prio drops Steven Rostedt
@ 2012-12-22 17:12 ` Steven Rostedt
2 siblings, 0 replies; 4+ messages in thread
From: Steven Rostedt @ 2012-12-22 17:12 UTC (permalink / raw)
To: linux-kernel, linux-rt-users
Cc: Thomas Gleixner, Carsten Emde, John Kacur, stable, stable-rt
[-- Attachment #1: 0003-sched-Consider-pi-boosting-in-setscheduler.patch --]
[-- Type: text/plain, Size: 5278 bytes --]
From: Thomas Gleixner <tglx@linutronix.de>
If a PI boosted task policy/priority is modified by a setscheduler()
call we unconditionally dequeue and requeue the task if it is on the
runqueue even if the new priority is lower than the current effective
boosted priority. This can result in undesired reordering of the
priority bucket list.
If the new priority is less or equal than the current effective we
just store the new parameters in the task struct and leave the
scheduler class and the runqueue untouched. This is handled when the
task deboosts itself. Only if the new priority is higher than the
effective boosted priority we apply the change immediately.
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Cc: stable@vger.kernel.org
Cc: stable-rt@vger.kernel.org
---
include/linux/sched.h | 5 +++++
kernel/rtmutex.c | 12 ++++++++++++
kernel/sched.c | 39 +++++++++++++++++++++++++++++++--------
3 files changed, 48 insertions(+), 8 deletions(-)
diff --git a/include/linux/sched.h b/include/linux/sched.h
index a179dd0..8772834 100644
--- a/include/linux/sched.h
+++ b/include/linux/sched.h
@@ -2089,6 +2089,7 @@ static inline void sched_autogroup_exit(struct signal_struct *sig) { }
#ifdef CONFIG_RT_MUTEXES
extern void task_setprio(struct task_struct *p, int prio);
extern int rt_mutex_getprio(struct task_struct *p);
+extern int rt_mutex_check_prio(struct task_struct *task, int newprio);
static inline void rt_mutex_setprio(struct task_struct *p, int prio)
{
task_setprio(p, prio);
@@ -2103,6 +2104,10 @@ 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)
+{
+ return 0;
+}
# define rt_mutex_adjust_pi(p) do { } while (0)
static inline bool tsk_is_pi_blocked(struct task_struct *tsk)
{
diff --git a/kernel/rtmutex.c b/kernel/rtmutex.c
index d58db99..7667c5e 100644
--- a/kernel/rtmutex.c
+++ b/kernel/rtmutex.c
@@ -124,6 +124,18 @@ int rt_mutex_getprio(struct task_struct *task)
}
/*
+ * Called by sched_setscheduler() to check whether the priority change
+ * is overruled by a possible priority boosting.
+ */
+int rt_mutex_check_prio(struct task_struct *task, int newprio)
+{
+ if (!task_has_pi_waiters(task))
+ return 0;
+
+ return task_top_pi_waiter(task)->pi_list_entry.prio <= newprio;
+}
+
+/*
* Adjust the priority of a task, after its pi_waiters got modified.
*
* This can be both boosting and unboosting. task->pi_lock must be held.
diff --git a/kernel/sched.c b/kernel/sched.c
index ad42cb2..858f5df 100644
--- a/kernel/sched.c
+++ b/kernel/sched.c
@@ -4989,7 +4989,8 @@ EXPORT_SYMBOL(sleep_on_timeout);
* This function changes the 'effective' priority of a task. It does
* not touch ->normal_prio like __setscheduler().
*
- * Used by the rt_mutex code to implement priority inheritance logic.
+ * Used by the rt_mutex code to implement priority inheritance
+ * logic. Call site only calls if the priority of the task changed.
*/
void task_setprio(struct task_struct *p, int prio)
{
@@ -5206,20 +5207,25 @@ static struct task_struct *find_process_by_pid(pid_t pid)
return pid ? find_task_by_vpid(pid) : current;
}
-/* Actually do priority change: must hold rq lock. */
-static void
-__setscheduler(struct rq *rq, struct task_struct *p, int policy, int prio)
+static void __setscheduler_params(struct task_struct *p, int policy, int prio)
{
p->policy = policy;
p->rt_priority = prio;
p->normal_prio = normal_prio(p);
+ set_load_weight(p);
+}
+
+/* Actually do priority change: must hold rq lock. */
+static void
+__setscheduler(struct rq *rq, struct task_struct *p, int policy, int prio)
+{
+ __setscheduler_params(p, policy, prio);
/* we are holding p->pi_lock already */
p->prio = rt_mutex_getprio(p);
if (rt_prio(p->prio))
p->sched_class = &rt_sched_class;
else
p->sched_class = &fair_sched_class;
- set_load_weight(p);
}
/*
@@ -5244,6 +5250,7 @@ static bool check_same_owner(struct task_struct *p)
static int __sched_setscheduler(struct task_struct *p, int policy,
const struct sched_param *param, bool user)
{
+ int newprio = MAX_RT_PRIO - 1 - param->sched_priority;
int retval, oldprio, oldpolicy = -1, on_rq, running;
unsigned long flags;
const struct sched_class *prev_class;
@@ -5372,6 +5379,25 @@ recheck:
task_rq_unlock(rq, p, &flags);
goto recheck;
}
+
+ p->sched_reset_on_fork = reset_on_fork;
+ 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
+ * 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)) {
+ __setscheduler_params(p, policy, param->sched_priority);
+ task_rq_unlock(rq, p, &flags);
+ return 0;
+ }
+
on_rq = p->on_rq;
running = task_current(rq, p);
if (on_rq)
@@ -5379,9 +5405,6 @@ recheck:
if (running)
p->sched_class->put_prev_task(rq, p);
- p->sched_reset_on_fork = reset_on_fork;
-
- oldprio = p->prio;
prev_class = p->sched_class;
__setscheduler(rq, p, policy, param->sched_priority);
--
1.7.10.4
^ permalink raw reply related [flat|nested] 4+ messages in thread