sched/autogroup: Fix failure to set cpu.rt_runtime_us

Because task_group() uses a cache of autogroup_task_group(), whose
output depends on sched_class, switching classes can generate
problems.

In particular, when started as fair, the cache points to the
autogroup, so when switching to RT the tg_rt_schedulable() test fails
for every cpu.rt_{runtime,period}_us change because now the autogroup
has tasks and no runtime.

Furthermore, going back to the previous semantics of varying
task_group() with sched_class has the down-side that the sched_debug
output varies as well, even though the task really is in the
autogroup.

Therefore add an autogroup exception to tg_has_rt_tasks() -- such that
both (all) task_group() usages in sched/core now have one. And remove
all the remnants of the variable task_group() output.

Reported-by: Zefan Li <lizefan@huawei.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Mike Galbraith <umgwanakikbuti@gmail.com>
Cc: Stefan Bader <stefan.bader@canonical.com>
Fixes: 8323f26ce342 ("sched: Fix race in task_group()")
Link: http://lkml.kernel.org/r/20150209112237.GR5029@twins.programming.kicks-ass.net
Signed-off-by: Ingo Molnar <mingo@kernel.org>

kernel/sched/auto_group.c

@@ -87,8 +87,7 @@ static inline struct autogroup *autogroup_create(void)
 	 * so we don't have to move tasks around upon policy change,
 	 * or flail around trying to allocate bandwidth on the fly.
 	 * A bandwidth exception in __sched_setscheduler() allows
-	 * the policy change to proceed.  Thereafter, task_group()
-	 * returns &root_task_group, so zero bandwidth is required.
+	 * the policy change to proceed.
 	 */
 	free_rt_sched_group(tg);
 	tg->rt_se = root_task_group.rt_se;
@@ -115,9 +114,6 @@ bool task_wants_autogroup(struct task_struct *p, struct task_group *tg)
 	if (tg != &root_task_group)
 		return false;
 
-	if (p->sched_class != &fair_sched_class)
-		return false;
-
 	/*
 	 * We can only assume the task group can't go away on us if
 	 * autogroup_move_group() can see us on ->thread_group list.

kernel/sched/core.c

@@ -7577,6 +7577,12 @@ static inline int tg_has_rt_tasks(struct task_group *tg)
 {
 	struct task_struct *g, *p;
 
+	/*
+	 * Autogroups do not have RT tasks; see autogroup_create().
+	 */
+	if (task_group_is_autogroup(tg))
+		return 0;
+
 	for_each_process_thread(g, p) {
 		if (rt_task(p) && task_group(p) == tg)
 			return 1;