8296896: Change virtual Thread.yield to use external submit

Reviewed-by: jpai, rpressler

src/java.base/share/classes/java/lang/VirtualThread.java

@@ -224,45 +224,59 @@ final class VirtualThread extends BaseVirtualThread {
     }
 
     /**
-     * Submits the runContinuation task to the scheduler.
-     * @param {@code lazySubmit} to lazy submit
+     * Submits the runContinuation task to the scheduler. For the default scheduler,
+     * and calling it on a worker thread, the task will be pushed to the local queue,
+     * otherwise it will be pushed to a submission queue.
+     *
+     * @throws RejectedExecutionException
+     */
+    private void submitRunContinuation() {
+        try {
+            scheduler.execute(runContinuation);
+        } catch (RejectedExecutionException ree) {
+            submitFailed(ree);
+            throw ree;
+        }
+    }
+
+    /**
+     * Submits the runContinuation task to the scheduler with a lazy submit.
      * @throws RejectedExecutionException
      * @see ForkJoinPool#lazySubmit(ForkJoinTask)
      */
-    private void submitRunContinuation(boolean lazySubmit) {
+    private void lazySubmitRunContinuation(ForkJoinPool pool) {
         try {
-            if (lazySubmit && scheduler instanceof ForkJoinPool pool) {
             pool.lazySubmit(ForkJoinTask.adapt(runContinuation));
-            } else {
-                scheduler.execute(runContinuation);
-            }
         } catch (RejectedExecutionException ree) {
-            // record event
+            submitFailed(ree);
+            throw ree;
+        }
+    }
+
+    /**
+     * Submits the runContinuation task to the scheduler as an external submit.
+     * @throws RejectedExecutionException
+     * @see ForkJoinPool#externalSubmit(ForkJoinTask)
+     */
+    private void externalSubmitRunContinuation(ForkJoinPool pool) {
+        try {
+            pool.externalSubmit(ForkJoinTask.adapt(runContinuation));
+        } catch (RejectedExecutionException ree) {
+            submitFailed(ree);
+            throw ree;
+        }
+    }
+
+    /**
+     * If enabled, emits a JFR VirtualThreadSubmitFailedEvent.
+     */
+    private void submitFailed(RejectedExecutionException ree) {
         var event = new VirtualThreadSubmitFailedEvent();
         if (event.isEnabled()) {
             event.javaThreadId = threadId();
             event.exceptionMessage = ree.getMessage();
             event.commit();
         }
-            throw ree;
-        }
-    }
-
-    /**
-     * Submits the runContinuation task to the scheduler.
-     * @throws RejectedExecutionException
-     */
-    private void submitRunContinuation() {
-        submitRunContinuation(false);
-    }
-
-    /**
-     * Submits the runContinuation task to the scheduler and without signalling
-     * any threads if possible.
-     * @throws RejectedExecutionException
-     */
-    private void lazySubmitRunContinuation() {
-        submitRunContinuation(true);
     }
 
     /**
@@ -437,7 +451,12 @@ final class VirtualThread extends BaseVirtualThread {
             // may have been unparked while parking
             if (parkPermit && compareAndSetState(PARKED, RUNNABLE)) {
                 // lazy submit to continue on the current thread as carrier if possible
-                lazySubmitRunContinuation();
+                if (currentThread() instanceof CarrierThread ct) {
+                    lazySubmitRunContinuation(ct.getPool());
+                } else {
+                    submitRunContinuation();
+                }
+
             }
         } else if (s == YIELDING) {   // Thread.yield
             setState(RUNNABLE);
@@ -445,8 +464,12 @@ final class VirtualThread extends BaseVirtualThread {
             // notify JVMTI that unmount has completed, thread is runnable
             if (notifyJvmtiEvents) notifyJvmtiUnmountEnd(false);
 
-            // lazy submit to continue on the current thread as carrier if possible
-            lazySubmitRunContinuation();
+            // external submit if there are no tasks in the local task queue
+            if (currentThread() instanceof CarrierThread ct && ct.getQueuedTaskCount() == 0) {
+                externalSubmitRunContinuation(ct.getPool());
+            } else {
+                submitRunContinuation();
+            }
         }
     }
 

test/jdk/java/lang/Thread/virtual/YieldQueuing.java

@@ -0,0 +1,126 @@
+/*
+ * Copyright (c) 2022, 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.
+ */
+
+/**
+ * @test
+ * @summary Test Thread.yield submits the virtual thread task to the expected queue
+ * @requires vm.continuations
+ * @enablePreview
+ * @run junit/othervm -Djdk.virtualThreadScheduler.maxPoolSize=1 YieldQueuing
+ */
+
+import java.util.List;
+import java.util.concurrent.CopyOnWriteArrayList;
+import java.util.concurrent.atomic.AtomicBoolean;
+import java.util.concurrent.locks.LockSupport;
+
+import org.junit.jupiter.api.Test;
+import static org.junit.jupiter.api.Assertions.*;
+
+class YieldQueuing {
+
+    /**
+     * Test Thread.yield submits the task for the current virtual thread to a scheduler
+     * submission queue when there are no tasks in the local queue.
+     */
+    @Test
+    void testYieldWithEmptyLocalQueue() throws Exception {
+        var list = new CopyOnWriteArrayList<String>();
+
+        var threadsStarted = new AtomicBoolean();
+
+        var threadA = Thread.ofVirtual().unstarted(() -> {
+            // pin thread until task for B is in submission queue
+            while (!threadsStarted.get()) {
+                Thread.onSpinWait();
+            }
+
+            list.add("A");
+            Thread.yield();      // push task for A to submission queue, B should run
+            list.add("A");
+        });
+
+        var threadB = Thread.ofVirtual().unstarted(() -> {
+            list.add("B");
+        });
+
+        // push tasks for A and B to submission queue
+        threadA.start();
+        threadB.start();
+
+        // release A
+        threadsStarted.set(true);
+
+        // wait for result
+        threadA.join();
+        threadB.join();
+        assertEquals(list, List.of("A", "B", "A"));
+    }
+
+    /**
+     * Test Thread.yield submits the task for the current virtual thread to the local
+     * queue when there are tasks in the local queue.
+     */
+    @Test
+    void testYieldWithNonEmptyLocalQueue() throws Exception {
+        var list = new CopyOnWriteArrayList<String>();
+
+        var threadsStarted = new AtomicBoolean();
+
+        var threadA = Thread.ofVirtual().unstarted(() -> {
+            // pin thread until tasks for B and C are in submission queue
+            while (!threadsStarted.get()) {
+                Thread.onSpinWait();
+            }
+
+            list.add("A");
+            LockSupport.park();   // B should run
+            list.add("A");
+        });
+
+        var threadB = Thread.ofVirtual().unstarted(() -> {
+            list.add("B");
+            LockSupport.unpark(threadA);  // push task for A to local queue
+            Thread.yield();               // push task for B to local queue, A should run
+            list.add("B");
+        });
+
+        var threadC = Thread.ofVirtual().unstarted(() -> {
+            list.add("C");
+        });
+
+        // push tasks for A, B and C to submission queue
+        threadA.start();
+        threadB.start();
+        threadC.start();
+
+        // release A
+        threadsStarted.set(true);
+
+        // wait for result
+        threadA.join();
+        threadB.join();
+        threadC.join();
+        assertEquals(list, List.of("A", "B", "A", "B", "C"));
+    }
+}

test/jdk/java/lang/Thread/virtual/stress/YieldALot.java

@@ -26,7 +26,7 @@
  * @summary Stress test Thread.yield
  * @requires vm.debug != true
  * @enablePreview
- * @run main YieldALot 500000
+ * @run main YieldALot 350000
  */
 
 /*