8371720: G1: Move concurrent mark initialization to first concurrent start pause

Reviewed-by: iwalulya

src/hotspot/share/gc/g1/g1CollectedHeap.cpp

@@ -2723,13 +2723,14 @@ void G1CollectedHeap::do_collection_pause_at_safepoint(size_t allocation_word_si
 
   _bytes_used_during_gc = 0;
 
-  _cm->fully_initialize();
-
   policy()->decide_on_concurrent_start_pause();
   // Record whether this pause may need to trigger a concurrent operation. Later,
   // when we signal the G1ConcurrentMarkThread, the collector state has already
   // been reset for the next pause.
   bool should_start_concurrent_mark_operation = collector_state()->is_in_concurrent_start_gc();
+  if (should_start_concurrent_mark_operation) {
+    _cm->fully_initialize();
+  }
 
   // Perform the collection.
   G1YoungCollector collector(gc_cause(), allocation_word_size);

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

@@ -569,6 +569,11 @@ public:
 
   uint worker_id_offset() const { return _worker_id_offset; }
 
+  // Fully allocates and initializes data structures for the concurrent cycle.
+  // Methods that use concurrent cycle state such as the concurrent mark threads,
+  // tasks, marking stack, statistics, TAMS or TARS require this initialization.
+  // Callers that run before the first concurrent start pause, which calls this,
+  // should guard calls with is_fully_initialized().
   void fully_initialize();
   bool is_fully_initialized() const { return _cm_thread != nullptr; }