[RFC PATCH 5/9] sched: Skip non-root task_groups with disabled RT_GROUP_SCHED

["Michal Koutný" <mkoutny@suse.com>]

First, we want to prevent placement of RT tasks on non-root rt_rqs which
we achieve in the task migration code that'd fall back to
root_task_group's rt_rq.

Second, we want to work with only root_task_group's rt_rq when iterating
all "real" rt_rqs when RT_GROUP is disabled. To achieve this we keep
root_task_group as the first one on the task_groups and break out
quickly.

Signed-off-by: Michal Koutný <mkoutny@suse.com>
---
 kernel/sched/core.c  | 2 +-
 kernel/sched/rt.c    | 9 ++++++---
 kernel/sched/sched.h | 7 +++++++
 3 files changed, 14 insertions(+), 4 deletions(-)

diff --git a/kernel/sched/core.c b/kernel/sched/core.c
index 47898f895a5a3..dfd2778622b8b 100644
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -8958,7 +8958,7 @@ void sched_online_group(struct task_group *tg, struct task_group *parent)
 	unsigned long flags;
 
 	spin_lock_irqsave(&task_group_lock, flags);
-	list_add_rcu(&tg->list, &task_groups);
+	list_add_tail_rcu(&tg->list, &task_groups);
 
 	/* Root should already exist: */
 	WARN_ON(!parent);
diff --git a/kernel/sched/rt.c b/kernel/sched/rt.c
index 41fed8865cb09..923ec978ff756 100644
--- a/kernel/sched/rt.c
+++ b/kernel/sched/rt.c
@@ -497,6 +497,9 @@ typedef struct task_group *rt_rq_iter_t;
 
 static inline struct task_group *next_task_group(struct task_group *tg)
 {
+	if (!rt_group_sched_enabled())
+		return NULL;
+
 	do {
 		tg = list_entry_rcu(tg->list.next,
 			typeof(struct task_group), list);
@@ -509,9 +512,9 @@ static inline struct task_group *next_task_group(struct task_group *tg)
 }
 
 #define for_each_rt_rq(rt_rq, iter, rq)					\
-	for (iter = container_of(&task_groups, typeof(*iter), list);	\
-		(iter = next_task_group(iter)) &&			\
-		(rt_rq = iter->rt_rq[cpu_of(rq)]);)
+	for (iter = &root_task_group;					\
+		iter && (rt_rq = iter->rt_rq[cpu_of(rq)]);		\
+		iter = next_task_group(iter))
 
 #define for_each_sched_rt_entity(rt_se) \
 	for (; rt_se; rt_se = rt_se->parent)
diff --git a/kernel/sched/sched.h b/kernel/sched/sched.h
index 1c457dc1472a3..d8d28c3d1ac5f 100644
--- a/kernel/sched/sched.h
+++ b/kernel/sched/sched.h
@@ -2148,6 +2148,13 @@ static inline void set_task_rq(struct task_struct *p, unsigned int cpu)
 #endif
 
 #ifdef CONFIG_RT_GROUP_SCHED
+	/*
+	 * p->rt.rt_rq is NULL initially and it is easier to assign
+	 * root_task_group's rt_rq than switching in rt_rq_of_se()
+	 * Clobbers tg(!)
+	 */
+	if (!rt_group_sched_enabled())
+		tg = &root_task_group;
 	p->rt.rt_rq  = tg->rt_rq[cpu];
 	p->rt.parent = tg->rt_se[cpu];
 #endif
-- 
2.47.1