There is presently one use of G1StressConcRegionFreeingDelayMillis, in g1/concurrentMark.cpp:

#ifndef PRODUCT
      if (G1StressConcRegionFreeing) {
        for (uintx i = 0; i < G1StressConcRegionFreeingDelayMillis; ++i) {
          os::sleep(Thread::current(), (jlong) 1, false);
        }
      }
#endif

Why isn't this just

      if (G1StressConcRegionFreeing &&
          G1StressConcRegionFreeingDelayMillis > 0) {
        os::sleep(Thread::current(),
                       G1StressConcRegionFreeingDelayMillis,
                       false);
      }

or maybe even "true" for the third argument to sleep, to allow it to be interruptable.