Subject: add /sys/kernel/realtime entry
From: Clark Williams <williams@redhat.com>
Date: Sat Jul 30 21:55:53 2011 -0500

Add a /sys/kernel entry to indicate that the kernel is a
realtime kernel.

Clark says that he needs this for udev rules, udev needs to evaluate
if its a PREEMPT_RT kernel a few thousand times and parsing uname
output is too slow or so.

Are there better solutions? Should it exist and return 0 on !-rt?

Signed-off-by: Clark Williams <williams@redhat.com>
Signed-off-by: Peter Zijlstra <a.p.zijlstra@chello.nl>
---
 kernel/ksysfs.c |   12 ++++++++++++
 1 file changed, 12 insertions(+)

--- a/kernel/ksysfs.c
+++ b/kernel/ksysfs.c
@@ -132,6 +132,15 @@ KERNEL_ATTR_RO(vmcoreinfo);
 
 #endif /* CONFIG_KEXEC */
 
+#if defined(CONFIG_PREEMPT_RT_FULL)
+static ssize_t  realtime_show(struct kobject *kobj,
+			      struct kobj_attribute *attr, char *buf)
+{
+	return sprintf(buf, "%d\n", 1);
+}
+KERNEL_ATTR_RO(realtime);
+#endif
+
 /* whether file capabilities are enabled */
 static ssize_t fscaps_show(struct kobject *kobj,
 				  struct kobj_attribute *attr, char *buf)
@@ -197,6 +206,9 @@ static struct attribute * kernel_attrs[]
 	&vmcoreinfo_attr.attr,
 #endif
 	&rcu_expedited_attr.attr,
+#ifdef CONFIG_PREEMPT_RT_FULL
+	&realtime_attr.attr,
+#endif
 	NULL
 };