Tracing can be very lightweight. Not utterly non-instrusive, but pretty close. As a kernel / games dev, I'm quite aware of timing issues.

You get into serious trouble with "lock free" stuff, where pretty much any additional activity (even register operations) will perturb things and make bugs go away. Hopefully at this point you have help from the hardware. Then again, if you're doing LF, you're in a special place and probably have the chops to deal with it...