tokio: optimize taskdumps when waiting for dump result

Don't just repeatedly yield the thread; wait for it to be notified.

ref: #5717 (comment)

tokio/src/runtime/handle.rs

@@ -349,24 +349,27 @@ cfg_taskdump! {
                     // perform the trace in a separate thread so that the
                     // trace itself does not appear in the taskdump.
                     let handle = handle.clone();
-                    spawn_thread(|| {
+                    spawn_thread(async {
                         let handle = handle;
-                        handle.dump()
+                        handle.dump().await
                     }).await
                 },
             }
         }
     }
 
     /// Spawn a new thread and asynchronously await on its result.
-    async fn spawn_thread<F, T>(f: F) -> T
+    async fn spawn_thread<F>(f: F) -> <F as Future>::Output
     where
-        F: FnOnce() -> T + Send + 'static,
-        T: Send + 'static
+        F: Future + Send + 'static,
+        <F as Future>::Output: Send + 'static
     {
         let (tx, rx) = crate::sync::oneshot::channel();
         crate::loom::thread::spawn(|| {
-            let _ = tx.send(f());
+            let rt = crate::runtime::Builder::new_current_thread().build().unwrap();
+            rt.block_on(async {
+                let _ = tx.send(f.await);
+            });
         });
         rx.await.unwrap()
     }

tokio/src/runtime/scheduler/multi_thread/handle.rs

@@ -97,23 +97,23 @@ cfg_metrics! {
 
 cfg_taskdump! {
     impl Handle {
-        pub(crate) fn dump(&self) -> crate::runtime::Dump {
+        pub(crate) async fn dump(&self) -> crate::runtime::Dump {
             let trace_status = &self.shared.trace_status;
 
             // If a dump is in progress, block.
-            trace_status.start_trace_request(&self);
+            trace_status.start_trace_request(&self).await;
 
             let result = loop {
                 if let Some(result) = trace_status.take_result() {
                     break result;
                 } else {
                     self.notify_all();
-                    crate::loom::thread::yield_now();
+                    trace_status.result_ready.notified().await;
                 }
             };
 
             // Allow other queued dumps to proceed.
-            trace_status.end_trace_request(&self);
+            trace_status.end_trace_request(&self).await;
 
             result
         }

tokio/src/runtime/scheduler/multi_thread/worker.rs

@@ -214,6 +214,7 @@ cfg_taskdump! {
         pub(super) trace_requested: AtomicBool,
         trace_start: Barrier,
         trace_end: Barrier,
+        pub(super) result_ready: crate::sync::Notify,
         pub(super) trace_result: Mutex<Option<Dump>>,
     }
 
@@ -223,6 +224,7 @@ cfg_taskdump! {
                 trace_requested: AtomicBool::new(false),
                 trace_start: Barrier::new(remotes_len),
                 trace_end: Barrier::new(remotes_len),
+                result_ready: crate::sync::Notify::new(),
                 trace_result: Mutex::new(None),
             }
         }
@@ -231,29 +233,34 @@ cfg_taskdump! {
             self.trace_requested.load(Ordering::Relaxed)
         }
 
-        pub(super) fn start_trace_request(&self, handle: &Handle) {
+        pub(super) async fn start_trace_request(&self, handle: &Handle) {
             while self.trace_requested.compare_exchange(false,
                 true,
                 Ordering::Acquire,
                 Ordering::Relaxed).is_err()
             {
                 handle.notify_all();
-                crate::loom::thread::yield_now();
+                crate::task::yield_now().await;
             }
         }
 
+        fn stash_result(&self, dump: Dump) {
+            let _ = self.trace_result.lock().insert(dump);
+            self.result_ready.notify_one();
+        }
+
         pub(super) fn take_result(&self) -> Option<Dump> {
             self.trace_result.lock().take()
         }
 
-        pub(super) fn end_trace_request(&self, handle: &Handle) {
+        pub(super) async fn end_trace_request(&self, handle: &Handle) {
             while self.trace_requested.compare_exchange(true,
                 false,
                 Ordering::Acquire,
                 Ordering::Relaxed).is_err()
             {
                 handle.notify_all();
-                crate::loom::thread::yield_now();
+                crate::task::yield_now().await;
             }
         }
     }
@@ -1121,7 +1128,7 @@ impl Handle {
             let result = dump::Dump::new(traces);
 
             // stash the result
-            let _ = self.shared.trace_status.trace_result.lock().insert(result);
+            self.shared.trace_status.stash_result(result);
 
             // allow other workers to proceed
             self.shared.trace_status.trace_end.wait();