I tried running with -XX:+PrintSafepointStatistics. I got lines like:

    Safepoint#1: Threads:(avg: 8.0, max: 8), Time: (avg: 0.01344, min: 0.01344, max:0.01344), Suspends: (current: 0, cummulative: 0), Callbacks: (current: 0, cumm: 1075838976), Self-blocked: (current: 0, cummulative:: 0)

That Callbacks:cumm looks like an uninitialized variable.