8365972: JFR: ThreadDump and ClassLoaderStatistics events may cause back to back rotations

Reviewed-by: mgronlun

src/jdk.jfr/share/classes/jdk/jfr/internal/PlatformEventType.java

@@ -48,6 +48,7 @@ public final class PlatformEventType extends Type {
     private final boolean isJDK;
     private final boolean isMethodSampling;
     private final boolean isCPUTimeMethodSampling;
+    private final boolean isBackToBackSensitive;
     private final List<SettingDescriptor> settings = new ArrayList<>(5);
     private final boolean dynamicSettings;
     private final int stackTraceOffset;
@@ -81,10 +82,25 @@ public final class PlatformEventType extends Type {
         this.isJVM = Type.isDefinedByJVM(id);
         this.isMethodSampling = determineMethodSampling();
         this.isCPUTimeMethodSampling = isJVM && name.equals(Type.EVENT_NAME_PREFIX + "CPUTimeSample");
+        this.isBackToBackSensitive = determineBackToBackSensitive();
         this.isJDK = isJDK;
         this.stackTraceOffset = determineStackTraceOffset();
     }
 
+    private boolean determineBackToBackSensitive() {
+        if (getName().equals(Type.EVENT_NAME_PREFIX + "ThreadDump")) {
+            return true;
+        }
+        if (getName().equals(Type.EVENT_NAME_PREFIX + "ClassLoaderStatistics")) {
+            return true;
+        }
+        return false;
+    }
+
+    public boolean isBackToBackSensitive() {
+        return isBackToBackSensitive;
+    }
+
     private boolean isExceptionEvent() {
         switch (getName()) {
             case Type.EVENT_NAME_PREFIX + "JavaErrorThrow" :

src/jdk.jfr/share/classes/jdk/jfr/internal/PlatformRecorder.java

@@ -263,7 +263,7 @@ public final class PlatformRecorder {
         if (toDisk) {
             PeriodicEvents.setFlushInterval(streamInterval);
         }
-        PeriodicEvents.doChunkBegin();
+        PeriodicEvents.doChunkBegin(true);
         Duration duration = recording.getDuration();
         if (duration != null) {
             recording.setStopTime(startTime.plus(duration));
@@ -335,7 +335,7 @@ public final class PlatformRecorder {
                 finishChunk(currentChunk, stopTime, null);
             }
             currentChunk = newChunk;
-            PeriodicEvents.doChunkBegin();
+            PeriodicEvents.doChunkBegin(false);
         }
 
         if (toDisk) {
@@ -390,7 +390,7 @@ public final class PlatformRecorder {
             finishChunk(currentChunk, timestamp, null);
         }
         currentChunk = newChunk;
-        PeriodicEvents.doChunkBegin();
+        PeriodicEvents.doChunkBegin(false);
     }
 
     private List<PlatformRecording> getRunningRecordings() {

src/jdk.jfr/share/classes/jdk/jfr/internal/periodic/PeriodicEvents.java

@@ -69,12 +69,14 @@ public final class PeriodicEvents {
         return taskRepository.removeTask(runnable);
     }
 
-    public static void doChunkBegin() {
+    public static void doChunkBegin(boolean startRecording) {
         long timestamp = JVM.counterTime();
         for (EventTask task : taskRepository.getTasks()) {
             var eventType = task.getEventType();
             if (eventType.isEnabled() && eventType.isBeginChunk()) {
-                task.run(timestamp, PeriodicType.BEGIN_CHUNK);
+                if (!eventType.isBackToBackSensitive() || startRecording) {
+                    task.run(timestamp, PeriodicType.BEGIN_CHUNK);
+                }
             }
         }
     }

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

@@ -0,0 +1,122 @@
+/*
+ * Copyright (c) 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 java.io.IOException;
+import java.nio.file.Files;
+import java.nio.file.Path;
+import java.time.Instant;
+import java.util.Collections;
+import java.util.LinkedHashSet;
+import java.util.Set;
+
+import jdk.jfr.Configuration;
+import jdk.jfr.Event;
+import jdk.jfr.Recording;
+import jdk.jfr.StackTrace;
+import jdk.jfr.consumer.RecordedClassLoader;
+import jdk.jfr.consumer.RecordingStream;
+
+/**
+ * @test
+ * @summary The test verifies that jdk.ClassLoaderStatistics and
+ *          jdk.ThreadThreadDump are not emitted at the beginning of a chunk
+ *          when the period is everyChunk, as is the case in default.jfc
+ * @requires vm.flagless
+ * @requires vm.hasJFR
+ * @library /test/lib /test/jdk
+ * @run main/othervm jdk.jfr.event.runtime.TestBackToBackSensitive
+ */
+public class TestBackToBackSensitive {
+    @StackTrace(false)
+    static class FillEvent extends Event {
+        String message;
+    }
+
+    public static void main(String... arg) throws Exception {
+        Set<Instant> threadDumps = Collections.synchronizedSet(new LinkedHashSet<>());
+        Set<Instant> classLoaderStatistics = Collections.synchronizedSet(new LinkedHashSet<>());
+        Set<Instant> physicalMemory = Collections.synchronizedSet(new LinkedHashSet<>());
+
+        Configuration configuration = Configuration.getConfiguration("default");
+        try (RecordingStream r1 = new RecordingStream(configuration)) {
+            r1.setMaxSize(Long.MAX_VALUE);
+            r1.onEvent("jdk.ThreadDump", e -> threadDumps.add(e.getStartTime()));
+            r1.onEvent("jdk.ClassLoaderStatistics", e -> {
+                RecordedClassLoader cl = e.getValue("classLoader");
+                if (cl != null) {
+                    if (cl.getType().getName().contains("PlatformClassLoader")) {
+                        classLoaderStatistics.add(e.getStartTime());
+                    }
+                }
+            });
+            r1.onEvent("jdk.PhysicalMemory", e -> physicalMemory.add(e.getStartTime()));
+            // Start chunk 1
+            r1.startAsync();
+            try (Recording r2 = new Recording()) {
+                // Start chunk 2
+                r2.start();
+                // Starts chunk 3
+                r2.stop();
+            }
+            // Start chunk 4 by filling up chunk 3
+            for (int i = 0; i < 1_500_000; i++) {
+                FillEvent f = new FillEvent();
+                f.commit();
+            }
+            r1.stop();
+            long chunkFiles = filesInRepository();
+            System.out.println("Number of chunk files: " + chunkFiles);
+            // When jdk.ClassLoaderStatistics and jdk.ThreadThreadDump are expected to be
+            // emitted:
+            // Chunk 1: begin, end
+            // Chunk 2: begin, end
+            // Chunk 3: end
+            // Chunk 4: end
+            assertCount("jdk.ThreadDump", threadDumps, 2 + 2 + (chunkFiles - 2));
+            assertCount("jdk.ClassLoaderStatistics", classLoaderStatistics, 2 + 2 + (chunkFiles - 2));
+            // When jdk.PhysicalMemory is expected to be emitted:
+            // Chunk 1: begin, end
+            // Chunk 2: begin, end
+            // Chunk 3: begin, end
+            // Chunk 4: begin, end
+            assertCount("jdk.PhysicalMemory", physicalMemory, 2 * chunkFiles);
+        }
+    }
+
+    private static long filesInRepository() throws IOException {
+        Path repository = Path.of(System.getProperty("jdk.jfr.repository"));
+        return Files.list(repository).filter(p -> p.toString().endsWith(".jfr")).count();
+    }
+
+    private static void assertCount(String eventName, Set<Instant> timestamps, long expected) throws Exception {
+        System.out.println("Timestamps for " + eventName + ":");
+        for (Instant timestamp : timestamps) {
+            System.out.println(timestamp);
+        }
+        int count = timestamps.size();
+        if (count != expected) {
+            throw new Exception("Expected " + expected + "  timestamps for event " + eventName + ", but got " + count);
+        }
+    }
+}