The logging of how many concurrent GC threads we have:

  log_trace(gc)("ConcGCThreads: %u", ConcGCThreads);

is done at the end of these two methods:

Arguments::set_cms_and_parnew_gc_flags()
Arguments::set_g1_gc_flags()

But we have not yet set up ConcGCThreads at that point.

$ java -Xlog:gc=trace -XX:+PrintFlagsFinal -version | grep ConcGCThreads
[0.002s][trace][gc] ConcGCThreads: 0
     uint ConcGCThreads := 6 {product}