8247179: Mechanism for VM operations to not take part in safepoint coalescing

Reviewed-by: rehn, dholmes

src/hotspot/share/runtime/safepoint.cpp

@@ -1181,8 +1181,6 @@ void SafepointTracing::statistics_exit_log() {
     }
   }
 
-  log_info(safepoint, stats)("VM operations coalesced during safepoint " INT64_FORMAT,
-                              VMThread::get_coalesced_count());
   log_info(safepoint, stats)("Maximum sync time  " INT64_FORMAT" ns",
                               (int64_t)(_max_sync_time));
   log_info(safepoint, stats)("Maximum vm operation time (except for Exit VM operation)  "

src/hotspot/share/runtime/vmThread.cpp

@@ -105,29 +105,6 @@ VM_Operation* VMOperationQueue::queue_remove_front(int prio) {
   return r;
 }
 
-VM_Operation* VMOperationQueue::queue_drain(int prio) {
-  if (queue_empty(prio)) return NULL;
-  DEBUG_ONLY(int length = _queue_length[prio];);
-  assert(length >= 0, "sanity check");
-  _queue_length[prio] = 0;
-  VM_Operation* r = _queue[prio]->next();
-  assert(r != _queue[prio], "cannot remove base element");
-  // remove links to base element from head and tail
-  r->set_prev(NULL);
-  _queue[prio]->prev()->set_next(NULL);
-  // restore queue to empty state
-  _queue[prio]->set_next(_queue[prio]);
-  _queue[prio]->set_prev(_queue[prio]);
-  assert(queue_empty(prio), "drain corrupted queue");
-#ifdef ASSERT
-  int len = 0;
-  VM_Operation* cur;
-  for(cur = r; cur != NULL; cur=cur->next()) len++;
-  assert(len == length, "drain lost some ops");
-#endif
-  return r;
-}
-
 //-----------------------------------------------------------------
 // High-level interface
 void VMOperationQueue::add(VM_Operation *op) {
@@ -199,7 +176,6 @@ VMThread*         VMThread::_vm_thread          = NULL;
 VM_Operation*     VMThread::_cur_vm_operation   = NULL;
 VMOperationQueue* VMThread::_vm_queue           = NULL;
 PerfCounter*      VMThread::_perf_accumulated_vm_operation_time = NULL;
-uint64_t          VMThread::_coalesced_count = 0;
 VMOperationTimeoutTask* VMThread::_timeout_task = NULL;
 
 
@@ -435,7 +411,6 @@ void VMThread::loop() {
   SafepointSynchronize::init(_vm_thread);
 
   while(true) {
-    VM_Operation* safepoint_ops = NULL;
     //
     // Wait for VM operation
     //
@@ -480,13 +455,6 @@ void VMThread::loop() {
           }
         }
         _cur_vm_operation = _vm_queue->remove_next();
-
-        // If we are at a safepoint we will evaluate all the operations that
-        // follow that also require a safepoint
-        if (_cur_vm_operation != NULL &&
-            _cur_vm_operation->evaluate_at_safepoint()) {
-          safepoint_ops = _vm_queue->drain_at_safepoint_priority();
-        }
       }
 
       if (should_terminate()) break;
@@ -512,41 +480,7 @@ void VMThread::loop() {
         }
 
         evaluate_operation(_cur_vm_operation);
-        // now process all queued safepoint ops, iteratively draining
+        _cur_vm_operation = NULL;
-        // the queue until there are none left
-        do {
-          _cur_vm_operation = safepoint_ops;
-          if (_cur_vm_operation != NULL) {
-            do {
-              EventMark em("Executing coalesced safepoint VM operation: %s", _cur_vm_operation->name());
-              log_debug(vmthread)("Evaluating coalesced safepoint VM operation: %s", _cur_vm_operation->name());
-              // evaluate_operation deletes the op object so we have
-              // to grab the next op now
-              VM_Operation* next = _cur_vm_operation->next();
-              evaluate_operation(_cur_vm_operation);
-              _cur_vm_operation = next;
-              _coalesced_count++;
-            } while (_cur_vm_operation != NULL);
-          }
-          // There is a chance that a thread enqueued a safepoint op
-          // since we released the op-queue lock and initiated the safepoint.
-          // So we drain the queue again if there is anything there, as an
-          // optimization to try and reduce the number of safepoints.
-          // As the safepoint synchronizes us with JavaThreads we will see
-          // any enqueue made by a JavaThread, but the peek will not
-          // necessarily detect a concurrent enqueue by a GC thread, but
-          // that simply means the op will wait for the next major cycle of the
-          // VMThread - just as it would if the GC thread lost the race for
-          // the lock.
-          if (_vm_queue->peek_at_safepoint_priority()) {
-            // must hold lock while draining queue
-            MutexLocker mu_queue(VMOperationQueue_lock,
-                                 Mutex::_no_safepoint_check_flag);
-            safepoint_ops = _vm_queue->drain_at_safepoint_priority();
-          } else {
-            safepoint_ops = NULL;
-          }
-        } while(safepoint_ops != NULL);
 
         if (_timeout_task != NULL) {
           _timeout_task->disarm();

src/hotspot/share/runtime/vmThread.hpp

@@ -64,7 +64,6 @@ class VMOperationQueue : public CHeapObj<mtInternal> {
   bool queue_empty                (int prio);
   void queue_add                  (int prio, VM_Operation *op);
   VM_Operation* queue_remove_front(int prio);
-  VM_Operation* queue_drain(int prio);
   // lock-free query: may return the wrong answer but must not break
   bool queue_peek(int prio) { return _queue_length[prio] > 0; }
 
@@ -74,7 +73,6 @@ class VMOperationQueue : public CHeapObj<mtInternal> {
   // Highlevel operations. Encapsulates policy
   void add(VM_Operation *op);
   VM_Operation* remove_next();                        // Returns next or null
-  VM_Operation* drain_at_safepoint_priority() { return queue_drain(SafepointPriority); }
   bool peek_at_safepoint_priority() { return queue_peek(SafepointPriority); }
 };
 
@@ -113,7 +111,6 @@ class VMThread: public NamedThread {
   static bool _terminated;
   static Monitor * _terminate_lock;
   static PerfCounter* _perf_accumulated_vm_operation_time;
-  static uint64_t _coalesced_count;
 
   static VMOperationTimeoutTask* _timeout_task;
 
@@ -148,7 +145,6 @@ class VMThread: public NamedThread {
   // Returns the current vm operation if any.
   static VM_Operation* vm_operation()             { return _cur_vm_operation; }
   static VM_Operation::VMOp_Type vm_op_type()     { return _cur_vm_operation->type(); }
-  static uint64_t get_coalesced_count()           { return _coalesced_count; }
 
   // Returns the single instance of VMThread.
   static VMThread* vm_thread()                    { return _vm_thread; }