8223951: Shenandoah: Only need to update thread roots during final update refs

Reviewed-by: shade

src/hotspot/share/gc/shenandoah/shenandoahConcurrentMark.cpp

@@ -304,6 +304,34 @@ void ShenandoahConcurrentMark::update_roots(ShenandoahPhaseTimings::Phase root_p
 #endif
 }
 
+class ShenandoahUpdateThreadRootsTask : public AbstractGangTask {
+private:
+  ShenandoahThreadRoots           _thread_roots;
+  ShenandoahPhaseTimings::Phase   _phase;
+public:
+  ShenandoahUpdateThreadRootsTask(bool is_par, ShenandoahPhaseTimings::Phase phase) :
+    AbstractGangTask("Shenandoah Update Thread Roots"),
+    _thread_roots(is_par),
+    _phase(phase) {
+    ShenandoahHeap::heap()->phase_timings()->record_workers_start(_phase);
+  }
+
+  ~ShenandoahUpdateThreadRootsTask() {
+    ShenandoahHeap::heap()->phase_timings()->record_workers_end(_phase);
+  }
+  void work(uint worker_id) {
+    ShenandoahUpdateRefsClosure cl;
+    _thread_roots.oops_do(&cl, NULL, worker_id);
+  }
+};
+
+void ShenandoahConcurrentMark::update_thread_roots(ShenandoahPhaseTimings::Phase root_phase) {
+  WorkGang* workers = _heap->workers();
+  bool is_par = workers->active_workers() > 1;
+  ShenandoahUpdateThreadRootsTask task(is_par, root_phase);
+  workers->run_task(&task);
+}
+
 void ShenandoahConcurrentMark::initialize(uint workers) {
   _heap = ShenandoahHeap::heap();
 

src/hotspot/share/gc/shenandoah/shenandoahConcurrentMark.hpp

@@ -79,6 +79,7 @@ public:
 
   void mark_roots(ShenandoahPhaseTimings::Phase root_phase);
   void update_roots(ShenandoahPhaseTimings::Phase root_phase);
+  void update_thread_roots(ShenandoahPhaseTimings::Phase root_phase);
 
 // ---------- Weak references
 //

src/hotspot/share/gc/shenandoah/shenandoahHeap.cpp

@@ -2200,9 +2200,11 @@ void ShenandoahHeap::op_final_updaterefs() {
     verifier()->verify_roots_no_forwarded_except(ShenandoahRootVerifier::ThreadRoots);
   }
 
-  concurrent_mark()->update_roots(is_degenerated_gc_in_progress() ?
-                                  ShenandoahPhaseTimings::degen_gc_update_roots:
-                                  ShenandoahPhaseTimings::final_update_refs_roots);
+  if (is_degenerated_gc_in_progress()) {
+    concurrent_mark()->update_roots(ShenandoahPhaseTimings::degen_gc_update_roots);
+  } else {
+    concurrent_mark()->update_thread_roots(ShenandoahPhaseTimings::final_update_refs_roots);
+  }
 
   ShenandoahGCPhase final_update_refs(ShenandoahPhaseTimings::final_update_refs_recycle);