sched: fix SysRq-N (normalize RT tasks)

Gene Heskett reported the following problem while testing CFS: SysRq-N
is not always effective in normalizing tasks back to SCHED_OTHER.

The reason for that turns out to be the following bug:

 - normalize_rt_tasks() uses for_each_process() to iterate through all
   tasks in the system.  The problem is, this method does not iterate
   through all tasks, it iterates through all thread groups.

The proper mechanism to enumerate over all threads is to use a
do_each_thread() + while_each_thread() loop.
Reported-by: NGene Heskett <gene.heskett@gmail.com>
Signed-off-by: NIngo Molnar <mingo@elte.hu>
Signed-off-by: NLinus Torvalds <torvalds@linux-foundation.org>

kernel/sched.c

@@ -7071,12 +7071,13 @@ EXPORT_SYMBOL(__might_sleep);
 void normalize_rt_tasks(void)
 {
 	struct prio_array *array;
-	struct task_struct *p;
+	struct task_struct *g, *p;
 	unsigned long flags;
 	struct rq *rq;
 
 	read_lock_irq(&tasklist_lock);
-	for_each_process(p) {
+
+	do_each_thread(g, p) {
 		if (!rt_task(p))
 			continue;
 
@@ -7094,7 +7095,8 @@ void normalize_rt_tasks(void)
 
 		__task_rq_unlock(rq);
 		spin_unlock_irqrestore(&p->pi_lock, flags);
-	}
+	} while_each_thread(g, p);
+
 	read_unlock_irq(&tasklist_lock);
 }