SystemTap is a very useful and powerful tool that enables one to insert kernel debug into a running kernel. Today I wanted to inspect the I/O read/write operations occurring when running some ACPI AML, so it was a case of hacking up a few lines of system tap to dump out the relevant state (e.g. which port being accessed, width of the I/O operation in bits and value being written or read).
So instead of spinning a bespoke kernel with a few lines of debug in, I use SystemTap to quickly write a re-usable script. Simple and easy!
I've put the SystemTap script in my git repository for any who are interested.