--- old/src/hotspot/share/gc/g1/g1ConcurrentRefine.cpp	2017-11-20 16:08:09.989066509 +0100
+++ new/src/hotspot/share/gc/g1/g1ConcurrentRefine.cpp	2017-11-20 16:08:09.576053970 +0100
@@ -104,7 +104,7 @@
     _threads[worker_id] = create_refinement_thread(worker_id, false);
     thread_to_activate = _threads[worker_id];
   }
-  if (thread_to_activate != NULL) {
+  if (thread_to_activate != NULL && !thread_to_activate->is_active()) {
     thread_to_activate->activate();
   }
 }
@@ -418,7 +418,7 @@
 }
 
 void G1ConcurrentRefine::maybe_activate_more_threads(uint worker_id, size_t num_cur_buffers) {
-  if (activation_threshold(worker_id + 1) > num_cur_buffers) {
+  if (num_cur_buffers > activation_threshold(worker_id + 1)) {
     _thread_control.maybe_activate_next(worker_id);
   }
 }