profiling: dynamically enable readprofile at runtime

Way too often, I have a machine that exhibits some kind of crappy
behavior.  The CPU looks wedged in the kernel or it is spending way too
much system time and I wonder what is responsible.

I try to run readprofile.  But, of course, Ubuntu doesn't enable it by
default.  Dang!

The reason we boot-time enable it is that it takes a big bufffer that we
generally can only bootmem alloc.  But, does it hurt to at least try and
runtime-alloc it?

To use:
echo 2 > /sys/kernel/profile

Then run readprofile like normal.

This should fix the compile issue with allmodconfig.  I've compile-tested
on a bunch more configs now including a few more architectures.

Signed-off-by: Dave Hansen <dave@linux.vnet.ibm.com>
Acked-by: Ingo Molnar <mingo@elte.hu>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

Documentation/ABI/testing/sysfs-profiling

@@ -0,0 +1,13 @@
+What:		/sys/kernel/profile
+Date:		September 2008
+Contact:	Dave Hansen <dave@linux.vnet.ibm.com>
+Description:
+		/sys/kernel/profile is the runtime equivalent
+		of the boot-time profile= option.
+
+		You can get the same effect running:
+
+			echo 2 > /sys/kernel/profile
+
+		as you would by issuing profile=2 on the boot
+		command line.