JDK-8315149 added an hsperf counter `sun.threads.cpu_time.gc_conc_mark`. As mentioned in https://github.com/openjdk/jdk/pull/15082, the existing G1ConcurrentMarkThread::vtime_accum() and G1ConcurrentMarkThread::vtime_mark_accum() are only used in logging, and serve a similar purpose as the sun.threads.cpu_time.gc_conc_mark counter. Moreover, os::elapsedVTime() could be either CPU time or wall time, depending on the platform, which could be confusing to users.

It would be cleaner to replace the use cases of G1ConcurrentMarkThread::vtime_accum() and G1ConcurrentMarkThread::vtime_mark_accum() with the same underlying value as sun.threads.cpu_time.gc_conc_mark.