[PATCH] SysRq-n changes RT tasks to normal

Teach sysrq-N to switch all rt-policy tasks to SCHED_OTHER.  For recovering
from (and diagnosing) userspace bugs.
Acked-by: Ingo Molnar <mingo@elte.hu>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>

drivers/char/sysrq.c

@@ -216,6 +216,16 @@ static struct sysrq_key_op sysrq_kill_op = {
 
 /* END SIGNAL SYSRQ HANDLERS BLOCK */
 
+static void sysrq_handle_unrt(int key, struct pt_regs *pt_regs,
+				struct tty_struct *tty)
+{
+	normalize_rt_tasks();
+}
+static struct sysrq_key_op sysrq_unrt_op = {
+	.handler	= sysrq_handle_unrt,
+	.help_msg	= "Nice",
+	.action_msg	= "Nice All RT Tasks"
+};
 
 /* Key Operations table and lock */
 static spinlock_t sysrq_key_table_lock = SPIN_LOCK_UNLOCKED;
@@ -250,7 +260,7 @@ static struct sysrq_key_op *sysrq_key_table[SYSRQ_KEY_TABLE_LENGTH] = {
 #endif
 /* l */	NULL,
 /* m */	&sysrq_showmem_op,
-/* n */	NULL,
+/* n */	&sysrq_unrt_op,
 /* o */	NULL, /* This will often be registered
 		 as 'Off' at init time */
 /* p */	&sysrq_showregs_op,

include/linux/sched.h

@@ -1105,6 +1105,12 @@ static inline void arch_pick_mmap_layout(struct mm_struct *mm)
 extern long sched_setaffinity(pid_t pid, cpumask_t new_mask);
 extern long sched_getaffinity(pid_t pid, cpumask_t *mask);
 
+#ifdef CONFIG_MAGIC_SYSRQ
+
+extern void normalize_rt_tasks(void);
+
+#endif
+
 #endif /* __KERNEL__ */
 
 #endif

kernel/sched.c

@@ -4607,3 +4607,35 @@ void __might_sleep(char *file, int line)
 }
 EXPORT_SYMBOL(__might_sleep);
 #endif
+
+#ifdef CONFIG_MAGIC_SYSRQ
+void normalize_rt_tasks(void)
+{
+	struct task_struct *p;
+	prio_array_t *array;
+	unsigned long flags;
+	runqueue_t *rq;
+
+	read_lock_irq(&tasklist_lock);
+	for_each_process (p) {
+		if (!rt_task(p))
+			continue;
+
+		rq = task_rq_lock(p, &flags);
+
+		array = p->array;
+		if (array)
+			deactivate_task(p, task_rq(p));
+		__setscheduler(p, SCHED_NORMAL, 0);
+		if (array) {
+			__activate_task(p, task_rq(p));
+			resched_task(rq->curr);
+		}
+
+		task_rq_unlock(rq, &flags);
+	}
+	read_unlock_irq(&tasklist_lock);
+}
+
+EXPORT_SYMBOL(normalize_rt_tasks);
+#endif /* CONFIG_MAGIC_SYSRQ */