Just to remind myself and others who are interested in kernel instrumentation. Long time ago, perhaps we did it via manual code modification followed by kernel recompilation, or...uhm, have you ever done syscall hijacking or something like that (ok, it sounds dirty...) ?
Now, here comes SystemTap. Some people call it Dtrace-clone. You can find it here. And here you can read a simple example on how to trace boot process. Hungry for more something alike? Go to a section of Daniel P. Berrange website dedicated to boot instrumnentation.