add /sys/kernel/realtime entry

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

@@ -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)
@@ -180,6 +189,9 @@ static struct attribute * kernel_attrs[] = {
 	&kexec_crash_loaded_attr.attr,
 	&kexec_crash_size_attr.attr,
 	&vmcoreinfo_attr.attr,
+#endif
+#ifdef CONFIG_PREEMPT_RT_FULL
+	&realtime_attr.attr,
 #endif
 	NULL
 };