8351142: Add JFR monitor deflation and statistics events

src/hotspot/share/jfr/metadata/metadata.xml

@@ -116,6 +116,15 @@
     <Field type="InflateCause" name="cause" label="Monitor Inflation Cause" description="Cause of inflation" />
   </Event>
 
+  <Event name="JavaMonitorDeflate" category="Java Application" label="Java Monitor Deflated">
+    <Field type="Class" name="monitorClass" label="Monitor Class" />
+    <Field type="ulong" contentType="address" name="address" label="Monitor Address" relation="JavaMonitorAddress" />
+  </Event>
+
+  <Event name="JavaMonitorStatistics" category="Java Application, Statistics" label="Java Monitor Statistics" period="everyChunk">
+    <Field type="ulong" name="count" label="Monitors in Use" description="Current number of in-use monitors" />
+  </Event>
+
   <Event name="SyncOnValueBasedClass" category="Java Virtual Machine, Diagnostics" label="Value Based Class Synchronization" thread="true" stackTrace="true" startTime="false" experimental="true">
     <Field type="Class" name="valueBasedClass" label="Value Based Class" />
   </Event>

src/hotspot/share/jfr/periodic/jfrPeriodic.cpp

@@ -739,3 +739,9 @@ TRACE_REQUEST_FUNC(NativeMemoryUsage) {
 TRACE_REQUEST_FUNC(NativeMemoryUsageTotal) {
   JfrNativeMemoryEvent::send_total_event(timestamp());
 }
+
+TRACE_REQUEST_FUNC(JavaMonitorStatistics) {
+  EventJavaMonitorStatistics event;
+  event.set_count(ObjectSynchronizer::in_use_list_count());
+  event.commit();
+}

src/hotspot/share/runtime/lightweightSynchronizer.cpp

@@ -369,7 +369,11 @@ static void post_monitor_inflate_event(EventJavaMonitorInflate* event,
                                        const oop obj,
                                        ObjectSynchronizer::InflateCause cause) {
   assert(event != nullptr, "invariant");
-  event->set_monitorClass(obj->klass());
+  const Klass* monitor_klass = obj->klass();
+  if (ObjectMonitor::is_jfr_excluded(monitor_klass)) {
+    return;
+  }
+  event->set_monitorClass(monitor_klass);
   event->set_address((uintptr_t)(void*)obj);
   event->set_cause((u1)cause);
   event->commit();

src/hotspot/share/runtime/objectMonitor.cpp

@@ -734,6 +734,18 @@ bool ObjectMonitor::try_lock_or_add_to_entry_list(JavaThread* current, ObjectWai
   }
 }
 
+static void post_monitor_deflate_event(EventJavaMonitorDeflate* event,
+                                       const oop obj) {
+  assert(event != nullptr, "invariant");
+  const Klass* monitor_klass = obj->klass();
+  if (ObjectMonitor::is_jfr_excluded(monitor_klass)) {
+    return;
+  }
+  event->set_monitorClass(monitor_klass);
+  event->set_address((uintptr_t)(void*)obj);
+  event->commit();
+}
+
 // Deflate the specified ObjectMonitor if not in-use. Returns true if it
 // was deflated and false otherwise.
 //
@@ -753,6 +765,8 @@ bool ObjectMonitor::deflate_monitor(Thread* current) {
     return false;
   }
 
+  EventJavaMonitorDeflate event;
+
   const oop obj = object_peek();
 
   if (obj == nullptr) {
@@ -826,6 +840,10 @@ bool ObjectMonitor::deflate_monitor(Thread* current) {
     install_displaced_markword_in_object(obj);
   }
 
+  if (event.should_commit()) {
+    post_monitor_deflate_event(&event, obj);
+  }
+
   // We leave owner == DEFLATER_MARKER and contentions < 0
   // to force any racing threads to retry.
   return true;  // Success, ObjectMonitor has been deflated.
@@ -1628,12 +1646,6 @@ bool ObjectMonitor::check_owner(TRAPS) {
              "current thread is not owner", false);
 }
 
-static inline bool is_excluded(const Klass* monitor_klass) {
-  assert(monitor_klass != nullptr, "invariant");
-  NOT_JFR_RETURN_(false);
-  JFR_ONLY(return vmSymbols::jdk_jfr_internal_management_HiddenWait() == monitor_klass->name();)
-}
-
 static void post_monitor_wait_event(EventJavaMonitorWait* event,
                                     ObjectMonitor* monitor,
                                     uint64_t notifier_tid,
@@ -1642,7 +1654,7 @@ static void post_monitor_wait_event(EventJavaMonitorWait* event,
   assert(event != nullptr, "invariant");
   assert(monitor != nullptr, "invariant");
   const Klass* monitor_klass = monitor->object()->klass();
-  if (is_excluded(monitor_klass)) {
+  if (ObjectMonitor::is_jfr_excluded(monitor_klass)) {
     return;
   }
   event->set_monitorClass(monitor_klass);

src/hotspot/share/runtime/objectMonitor.hpp

@@ -463,6 +463,10 @@ class ObjectMonitor : public CHeapObj<mtObjectMonitor> {
   bool      deflate_monitor(Thread* current);
  private:
   void      install_displaced_markword_in_object(const oop obj);
+
+  // JFR support
+public:
+  static bool is_jfr_excluded(const Klass* monitor_klass);
 };
 
 // RAII object to ensure that ObjectMonitor::is_being_async_deflated() is

src/hotspot/share/runtime/objectMonitor.inline.hpp

@@ -27,6 +27,7 @@
 
 #include "runtime/objectMonitor.hpp"
 
+#include "classfile/vmSymbols.hpp"
 #include "logging/log.hpp"
 #include "oops/access.inline.hpp"
 #include "oops/markWord.hpp"
@@ -286,4 +287,10 @@ inline bool ObjectMonitor::object_refers_to(oop obj) const {
   return _object.peek() == obj;
 }
 
+inline bool ObjectMonitor::is_jfr_excluded(const Klass* monitor_klass) {
+  assert(monitor_klass != nullptr, "invariant");
+  NOT_JFR_RETURN_(false);
+  JFR_ONLY(return vmSymbols::jdk_jfr_internal_management_HiddenWait() == monitor_klass->name();)
+}
+
 #endif // SHARE_RUNTIME_OBJECTMONITOR_INLINE_HPP

src/hotspot/share/runtime/synchronizer.cpp

@@ -1312,6 +1312,14 @@ static bool monitors_used_above_threshold(MonitorList* list) {
   return false;
 }
 
+size_t ObjectSynchronizer::in_use_list_count() {
+  return _in_use_list.count();
+}
+
+size_t ObjectSynchronizer::in_use_list_max() {
+  return _in_use_list.max();
+}
+
 size_t ObjectSynchronizer::in_use_list_ceiling() {
   return _in_use_list_ceiling;
 }
@@ -1419,7 +1427,11 @@ static void post_monitor_inflate_event(EventJavaMonitorInflate* event,
                                        const oop obj,
                                        ObjectSynchronizer::InflateCause cause) {
   assert(event != nullptr, "invariant");
-  event->set_monitorClass(obj->klass());
+  const Klass* monitor_klass = obj->klass();
+  if (ObjectMonitor::is_jfr_excluded(monitor_klass)) {
+    return;
+  }
+  event->set_monitorClass(monitor_klass);
   event->set_address((uintptr_t)(void*)obj);
   event->set_cause((u1)cause);
   event->commit();
@@ -1710,8 +1722,8 @@ class ObjectMonitorDeflationLogging: public StackObj {
   elapsedTimer                             _timer;
 
   size_t ceiling() const { return ObjectSynchronizer::in_use_list_ceiling(); }
-  size_t count() const   { return ObjectSynchronizer::_in_use_list.count(); }
-  size_t max() const     { return ObjectSynchronizer::_in_use_list.max(); }
+  size_t count() const   { return ObjectSynchronizer::in_use_list_count(); }
+  size_t max() const     { return ObjectSynchronizer::in_use_list_max(); }
 
 public:
   ObjectMonitorDeflationLogging()

src/hotspot/share/runtime/synchronizer.hpp

@@ -188,6 +188,8 @@ class ObjectSynchronizer : AllStatic {
 
   // Deflate idle monitors:
   static size_t deflate_monitor_list(ObjectMonitorDeflationSafepointer* safepointer);
+  static size_t in_use_list_count();
+  static size_t in_use_list_max();
   static size_t in_use_list_ceiling();
   static void dec_in_use_list_ceiling();
   static void inc_in_use_list_ceiling();

src/jdk.jfr/share/conf/jfr/default.jfc

@@ -101,6 +101,16 @@
       <setting name="threshold">0 ms</setting>
     </event>
 
+    <event name="jdk.JavaMonitorDeflate">
+      <setting name="enabled">false</setting>
+      <setting name="threshold">0 ms</setting>
+    </event>
+
+    <event name="jdk.JavaMonitorStatistics">
+      <setting name="enabled">true</setting>
+      <setting name="period">everyChunk</setting>
+    </event>
+
     <event name="jdk.SyncOnValueBasedClass">
       <setting name="enabled">true</setting>
       <setting name="stackTrace">true</setting>

src/jdk.jfr/share/conf/jfr/profile.jfc

@@ -101,6 +101,16 @@
       <setting name="threshold">0 ms</setting>
     </event>
 
+    <event name="jdk.JavaMonitorDeflate">
+      <setting name="enabled">false</setting>
+      <setting name="threshold">0 ms</setting>
+    </event>
+
+    <event name="jdk.JavaMonitorStatistics">
+      <setting name="enabled">true</setting>
+      <setting name="period">everyChunk</setting>
+    </event>
+
     <event name="jdk.SyncOnValueBasedClass">
       <setting name="enabled">true</setting>
       <setting name="stackTrace">true</setting>

test/jdk/jdk/jfr/event/runtime/TestJavaMonitorDeflateEvent.java

@@ -0,0 +1,90 @@
+/*
+ * Copyright (c) 2016, 2025, Oracle and/or its affiliates. All rights reserved.
+ * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
+ *
+ * This code is free software; you can redistribute it and/or modify it
+ * under the terms of the GNU General Public License version 2 only, as
+ * published by the Free Software Foundation.
+ *
+ * This code is distributed in the hope that it will be useful, but WITHOUT
+ * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
+ * FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public License
+ * version 2 for more details (a copy is included in the LICENSE file that
+ * accompanied this code).
+ *
+ * You should have received a copy of the GNU General Public License version
+ * 2 along with this work; if not, write to the Free Software Foundation,
+ * Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA.
+ *
+ * Please contact Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA
+ * or visit www.oracle.com if you need additional information or have any
+ * questions.
+ */
+
+package jdk.jfr.event.runtime;
+
+import static jdk.test.lib.Asserts.assertFalse;
+
+import java.nio.file.Paths;
+import java.time.Duration;
+import java.util.List;
+import java.util.concurrent.CopyOnWriteArrayList;
+
+import jdk.jfr.Recording;
+import jdk.jfr.consumer.RecordingStream;
+import jdk.jfr.consumer.RecordedClass;
+import jdk.jfr.consumer.RecordedEvent;
+import jdk.test.lib.jfr.EventNames;
+import jdk.test.lib.jfr.Events;
+import jdk.test.lib.thread.TestThread;
+import jdk.test.lib.thread.XRun;
+
+/**
+ * @test TestJavaMonitorDeflateEvent
+ * @requires vm.flagless
+ * @requires vm.hasJFR
+ * @library /test/lib
+ * @run main/othervm -XX:+UnlockDiagnosticVMOptions -XX:GuaranteedAsyncDeflationInterval=100 jdk.jfr.event.runtime.TestJavaMonitorDeflateEvent
+ */
+public class TestJavaMonitorDeflateEvent {
+
+    private static final String FIELD_KLASS_NAME = "monitorClass.name";
+    private static final String FIELD_ADDRESS    = "address";
+
+    private static final String EVENT_NAME = EventNames.JavaMonitorDeflate;
+
+    static class Lock {
+    }
+
+    public static void main(String[] args) throws Exception {
+        final Lock lock = new Lock();
+        final String lockClassName = lock.getClass().getName().replace('.', '/');
+
+        List<RecordedEvent> events = new CopyOnWriteArrayList<>();
+        try (RecordingStream rs = new RecordingStream()) {
+            rs.enable(EVENT_NAME).withoutThreshold();
+            rs.onEvent(EVENT_NAME, e -> {
+                Object clazz = e.getValue(FIELD_KLASS_NAME);
+                if (clazz.equals(lockClassName)) {
+                    events.add(e);
+                    rs.close();
+                }
+            });
+            rs.startAsync();
+
+            synchronized (lock) {
+                // Causes lock inflation.
+                lock.wait(1);
+            }
+
+            // Wait for deflater thread to act.
+            rs.awaitTermination();
+
+            System.out.println(events);
+            assertFalse(events.isEmpty());
+            for (RecordedEvent ev : events) {
+                Events.assertField(ev, FIELD_ADDRESS).notEqual(0L);
+            }
+        }
+    }
+}

test/jdk/jdk/jfr/event/runtime/TestJavaMonitorStatisticsEvent.java

@@ -0,0 +1,101 @@
+/*
+ * Copyright (c) 2016, 2025, Oracle and/or its affiliates. All rights reserved.
+ * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
+ *
+ * This code is free software; you can redistribute it and/or modify it
+ * under the terms of the GNU General Public License version 2 only, as
+ * published by the Free Software Foundation.
+ *
+ * This code is distributed in the hope that it will be useful, but WITHOUT
+ * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
+ * FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public License
+ * version 2 for more details (a copy is included in the LICENSE file that
+ * accompanied this code).
+ *
+ * You should have received a copy of the GNU General Public License version
+ * 2 along with this work; if not, write to the Free Software Foundation,
+ * Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA.
+ *
+ * Please contact Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA
+ * or visit www.oracle.com if you need additional information or have any
+ * questions.
+ */
+
+package jdk.jfr.event.runtime;
+
+import static jdk.test.lib.Asserts.assertFalse;
+import static jdk.test.lib.Asserts.assertTrue;
+
+import java.nio.file.Paths;
+import java.time.Duration;
+import java.util.List;
+import java.util.concurrent.CopyOnWriteArrayList;
+
+import jdk.jfr.Recording;
+import jdk.jfr.consumer.RecordingStream;
+import jdk.jfr.consumer.RecordedClass;
+import jdk.jfr.consumer.RecordedEvent;
+import jdk.test.lib.jfr.EventNames;
+import jdk.test.lib.jfr.Events;
+import jdk.test.lib.thread.TestThread;
+import jdk.test.lib.thread.XRun;
+
+/**
+ * @test TestJavaMonitorStatisticsEvent
+ * @requires vm.flagless
+ * @requires vm.hasJFR
+ * @library /test/lib
+ * @run main/othervm jdk.jfr.event.runtime.TestJavaMonitorStatisticsEvent
+ */
+public class TestJavaMonitorStatisticsEvent {
+
+    private static final String FIELD_COUNT = "count";
+
+    private static final String EVENT_NAME = EventNames.JavaMonitorStatistics;
+    private static final int NUM_LOCKS = 512;
+
+    static class Lock {
+    }
+
+    static final Lock[] LOCKS = new Lock[NUM_LOCKS];
+
+    static void lockNext(int idx, Runnable action) throws InterruptedException {
+        if (idx >= NUM_LOCKS) {
+            action.run();
+            return;
+        }
+        synchronized (LOCKS[idx]) {
+            LOCKS[idx].wait(1);
+            lockNext(idx + 1, action);
+        }
+    }
+
+    public static void main(String[] args) throws Exception {
+        List<RecordedEvent> events = new CopyOnWriteArrayList<>();
+        try (RecordingStream rs = new RecordingStream()) {
+            rs.enable(EVENT_NAME).with("period", "everyChunk");
+            rs.onEvent(EVENT_NAME, e -> events.add(e));
+            rs.startAsync();
+
+            // Recursively lock all, causing NUM_LOCKS monitors to exist.
+            // Stop the recording when holding all the locks, so that we
+            // get at least one event with NUM_LOCKS max.
+            for (int c = 0; c < NUM_LOCKS; c++) {
+                LOCKS[c] = new Lock();
+            }
+            lockNext(0, () -> rs.stop());
+
+            System.out.println(events);
+            assertFalse(events.isEmpty());
+
+            long globalCount = Long.MIN_VALUE;
+            for (RecordedEvent ev : events) {
+                long evCount = Events.assertField(ev, FIELD_COUNT).getValue();
+                assertTrue(evCount >= 0, "Count should be non-negative: " + evCount);
+                globalCount = Math.max(globalCount, evCount);
+            }
+
+            assertTrue(globalCount >= NUM_LOCKS, "Global count should be at least " + NUM_LOCKS + ": " + globalCount);
+        }
+    }
+}

test/lib/jdk/test/lib/jfr/EventNames.java

@@ -59,6 +59,8 @@ public class EventNames {
     public static final String JavaMonitorEnter = PREFIX + "JavaMonitorEnter";
     public static final String JavaMonitorWait = PREFIX + "JavaMonitorWait";
     public static final String JavaMonitorInflate = PREFIX + "JavaMonitorInflate";
+    public static final String JavaMonitorDeflate = PREFIX + "JavaMonitorDeflate";
+    public static final String JavaMonitorStatistics = PREFIX + "JavaMonitorStatistics";
     public static final String SyncOnValueBasedClass = PREFIX + "SyncOnValueBasedClass";
     public static final String ClassLoad = PREFIX + "ClassLoad";
     public static final String ClassDefine = PREFIX + "ClassDefine";