In the change for JDK-8152312 I always launch the WorkGang workers, even when ParallelGCThreads == 1:

void PreservedMarksSet::restore_internal(WorkGang* workers,
                                         volatile size_t* total_size_addr) {
  assert(workers != NULL, "pre-condition");
  ParRestoreTask task(workers->active_workers(), this, total_size_addr);
  workers->run_task(&task);
}

HotSpot convention is to call task.work(0) directly when ParallelGCThreads == 1.