8266073: Regression ~2% in Derby after 8261804

Reviewed-by: ayang, sjohanss

src/hotspot/share/gc/g1/g1ConcurrentMark.cpp

@@ -1562,15 +1562,9 @@ void G1ConcurrentMark::weak_refs_work(bool clear_all_soft_refs) {
     // is not multi-threaded we use the current (VMThread) thread,
     // otherwise we use the work gang from the G1CollectedHeap and
     // we utilize all the worker threads we can.
-    bool processing_is_mt = rp->processing_is_mt();
-    uint active_workers = (processing_is_mt ? _g1h->workers()->active_workers() : 1U);
+    uint active_workers = (ParallelRefProcEnabled ? _g1h->workers()->active_workers() : 1U);
     active_workers = clamp(active_workers, 1u, _max_num_tasks);
 
-    // Parallel processing task executor.
-    G1CMRefProcTaskExecutor par_task_executor(_g1h, this,
-                                              _g1h->workers(), active_workers);
-    AbstractRefProcTaskExecutor* executor = (processing_is_mt ? &par_task_executor : NULL);
-
     // Set the concurrency level. The phase was already set prior to
     // executing the remark task.
     set_concurrency(active_workers);
@@ -1581,6 +1575,11 @@ void G1ConcurrentMark::weak_refs_work(bool clear_all_soft_refs) {
     // Reference lists are balanced (see balance_all_queues() and balance_queues()).
     rp->set_active_mt_degree(active_workers);
 
+    // Parallel processing task executor.
+    G1CMRefProcTaskExecutor par_task_executor(_g1h, this,
+                                              _g1h->workers(), active_workers);
+    AbstractRefProcTaskExecutor* executor = (rp->processing_is_mt() ? &par_task_executor : NULL);
+
     ReferenceProcessorPhaseTimes pt(_gc_timer_cm, rp->max_num_queues());
 
     // Process the weak references.

src/hotspot/share/gc/g1/g1FullGCReferenceProcessorExecutor.cpp

@@ -37,15 +37,11 @@ G1FullGCReferenceProcessingExecutor::G1FullGCReferenceProcessingExecutor(G1FullC
     _collector(collector),
     _reference_processor(collector->reference_processor()),
     _old_mt_degree(_reference_processor->num_queues()) {
-  if (_reference_processor->processing_is_mt()) {
-    _reference_processor->set_active_mt_degree(_collector->workers());
-  }
+  _reference_processor->set_active_mt_degree(_collector->workers());
 }
 
 G1FullGCReferenceProcessingExecutor::~G1FullGCReferenceProcessingExecutor() {
-  if (_reference_processor->processing_is_mt()) {
-    _reference_processor->set_active_mt_degree(_old_mt_degree);
-  }
+  _reference_processor->set_active_mt_degree(_old_mt_degree);
 }
 
 G1FullGCReferenceProcessingExecutor::G1RefProcTaskProxy::G1RefProcTaskProxy(ProcessTask& proc_task,

src/hotspot/share/gc/parallel/psParallelCompact.cpp

@@ -2125,9 +2125,8 @@ void PSParallelCompact::marking_phase(ParCompactionManager* cm,
     ReferenceProcessorStats stats;
     ReferenceProcessorPhaseTimes pt(&_gc_timer, ref_processor()->max_num_queues());
 
+    ref_processor()->set_active_mt_degree(active_gc_threads);
     if (ref_processor()->processing_is_mt()) {
-      ref_processor()->set_active_mt_degree(active_gc_threads);
-
       RefProcTaskExecutor task_executor;
       stats = ref_processor()->process_discovered_references(
         is_alive_closure(), &mark_and_push_closure, &follow_stack_closure,

src/hotspot/share/gc/shared/referenceProcessor.cpp

@@ -1373,7 +1373,7 @@ RefProcMTDegreeAdjuster::RefProcMTDegreeAdjuster(ReferenceProcessor* rp,
                                                  size_t ref_count):
     _rp(rp),
     _saved_num_queues(_rp->num_queues()) {
-  if (!_rp->processing_is_mt() || !_rp->adjust_no_of_processing_threads() || (ReferencesPerThread == 0)) {
+  if (!_rp->adjust_no_of_processing_threads() || (ReferencesPerThread == 0)) {
     return;
   }