While developing JDK-8340827 it has been very useful to have some regular information on the cpu usage of the different thread groups used by the G1 collector, like:

[604.423s][debug][gc,cpu ] GC(197) GC worker threads CPU time: 735795.00
[604.423s][debug][gc,cpu ] GC(197) Marking worker threads CPU time: 295592.69
[604.423s][debug][gc,cpu ] GC(197) Refinement worker thread CPU time: 337.47

Not only for comparison after changes, but also to determine whether there has been significant cpu usage in a particular component and rule out issues in that component (i.e. some heuristics going wild).

Add such messages separately from that other change, prettifying them a little (add unit, maybe add number of worker threads in use etc).