Re: [BUG: NULL pointer dereference] cgroups and RT scheduling interact badly.

[Peter Zijlstra <peterz@infradead.org>]

On Tue, 2008-06-17 at 14:25 +0200, Daniel K. wrote:
> Peter Zijlstra wrote:
> > How's this [patch] work for you? (includes the previuos patchlet too)
> 
> Thanks,
> 
> this patch fixed the obvious problem, namely
> 
> # echo $$ > /dev/cgroup/burn/oops/tasks
> # schedtool -R -p 1 -e burnP6 &
> 
> now works again. However, the last step below
> 
> # echo $$ > /dev/cgroup/tasks
> # burnP6 &
> [1] 3414
> # echo 3414 > /dev/cgroup/burn/oops/tasks
> # schedtool -R -p 1 3414
> 
> gives this new and shiny Oops instead.

Whilst I'm gracious for your testing, I truly hope you're done breaking
my stuff ;-)

How's this for you?

Signed-off-by: Peter Zijlstra <a.p.zijlstra@chello.nl>
---
diff --git a/kernel/sched.c b/kernel/sched.c
index eaf6751..efbb7d9 100644
--- a/kernel/sched.c
+++ b/kernel/sched.c
@@ -7626,7 +7626,6 @@ static void init_tg_rt_entry(struct task_group *tg, struct rt_rq *rt_rq,
 	else
 		rt_se->rt_rq = parent->my_q;
 
-	rt_se->rt_rq = &rq->rt;
 	rt_se->my_q = rt_rq;
 	rt_se->parent = parent;
 	INIT_LIST_HEAD(&rt_se->run_list);
diff --git a/kernel/sched_rt.c b/kernel/sched_rt.c
index 3432d57..2e73cac 100644
--- a/kernel/sched_rt.c
+++ b/kernel/sched_rt.c
@@ -449,13 +449,13 @@ void dec_rt_tasks(struct sched_rt_entity *rt_se, struct rt_rq *rt_rq)
 #endif
 }
 
-static void enqueue_rt_entity(struct sched_rt_entity *rt_se)
+static void __enqueue_rt_entity(struct sched_rt_entity *rt_se)
 {
 	struct rt_rq *rt_rq = rt_rq_of_se(rt_se);
 	struct rt_prio_array *array = &rt_rq->active;
 	struct rt_rq *group_rq = group_rt_rq(rt_se);
 
-	if (group_rq && rt_rq_throttled(group_rq))
+	if (group_rq && (rt_rq_throttled(group_rq) || !group_rq->rt_nr_running))
 		return;
 
 	list_add_tail(&rt_se->run_list, array->queue + rt_se_prio(rt_se));
@@ -464,7 +464,7 @@ static void enqueue_rt_entity(struct sched_rt_entity *rt_se)
 	inc_rt_tasks(rt_se, rt_rq);
 }
 
-static void dequeue_rt_entity(struct sched_rt_entity *rt_se)
+static void __dequeue_rt_entity(struct sched_rt_entity *rt_se)
 {
 	struct rt_rq *rt_rq = rt_rq_of_se(rt_se);
 	struct rt_prio_array *array = &rt_rq->active;
@@ -480,11 +480,10 @@ static void dequeue_rt_entity(struct sched_rt_entity *rt_se)
  * Because the prio of an upper entry depends on the lower
  * entries, we must remove entries top - down.
  */
-static void dequeue_rt_stack(struct task_struct *p)
+static void dequeue_rt_stack(struct sched_rt_entity *rt_se)
 {
-	struct sched_rt_entity *rt_se, *back = NULL;
+	struct sched_rt_entity *back = NULL;
 
-	rt_se = &p->rt;
 	for_each_sched_rt_entity(rt_se) {
 		rt_se->back = back;
 		back = rt_se;
@@ -492,7 +491,26 @@ static void dequeue_rt_stack(struct task_struct *p)
 
 	for (rt_se = back; rt_se; rt_se = rt_se->back) {
 		if (on_rt_rq(rt_se))
-			dequeue_rt_entity(rt_se);
+			__dequeue_rt_entity(rt_se);
+	}
+}
+
+static void enqueue_rt_entity(struct sched_rt_entity *rt_se)
+{
+	dequeue_rt_stack(rt_se);
+	for_each_sched_rt_entity(rt_se)
+		__enqueue_rt_entity(rt_se);
+}
+
+static void dequeue_rt_entity(struct sched_rt_entity *rt_se)
+{
+	dequeue_rt_stack(rt_se);
+
+	for_each_sched_rt_entity(rt_se) {
+		struct rt_rq *rt_rq = group_rt_rq(rt_se);
+
+		if (rt_rq && rt_rq->rt_nr_running)
+			__enqueue_rt_entity(rt_se);
 	}
 }
 
@@ -506,32 +524,15 @@ static void enqueue_task_rt(struct rq *rq, struct task_struct *p, int wakeup)
 	if (wakeup)
 		rt_se->timeout = 0;
 
-	dequeue_rt_stack(p);
-
-	/*
-	 * enqueue everybody, bottom - up.
-	 */
-	for_each_sched_rt_entity(rt_se)
-		enqueue_rt_entity(rt_se);
+	enqueue_rt_entity(rt_se);
 }
 
 static void dequeue_task_rt(struct rq *rq, struct task_struct *p, int sleep)
 {
 	struct sched_rt_entity *rt_se = &p->rt;
-	struct rt_rq *rt_rq;
 
 	update_curr_rt(rq);
-
-	dequeue_rt_stack(p);
-
-	/*
-	 * re-enqueue all non-empty rt_rq entities.
-	 */
-	for_each_sched_rt_entity(rt_se) {
-		rt_rq = group_rt_rq(rt_se);
-		if (rt_rq && rt_rq->rt_nr_running)
-			enqueue_rt_entity(rt_se);
-	}
+	dequeue_rt_entity(rt_se);
 }
 
 /*