rt: reduce no-op wakeups in the multi-threaded scheduler

tokio/src/runtime/thread_pool/idle.rs

@@ -64,7 +64,7 @@ impl Idle {
 
         // A worker should be woken up, atomically increment the number of
         // searching workers as well as the number of unparked workers.
-        State::unpark_one(&self.state);
+        State::unpark_one(&self.state, 1);
 
         // Get the worker to unpark
         let ret = sleepers.pop();
@@ -111,19 +111,23 @@ impl Idle {
 
     /// Unpark a specific worker. This happens if tasks are submitted from
     /// within the worker's park routine.
-    pub(super) fn unpark_worker_by_id(&self, worker_id: usize) {
+    ///
+    /// Returns `true` if the worker was parked before calling the method.
+    pub(super) fn unpark_worker_by_id(&self, worker_id: usize) -> bool {
         let mut sleepers = self.sleepers.lock();
 
         for index in 0..sleepers.len() {
             if sleepers[index] == worker_id {
                 sleepers.swap_remove(index);
 
                 // Update the state accordingly while the lock is held.
-                State::unpark_one(&self.state);
+                State::unpark_one(&self.state, 0);
 
-                return;
+                return true;
             }
         }
+
+        false
     }
 
     /// Returns `true` if `worker_id` is contained in the sleep set.
@@ -151,8 +155,8 @@ impl State {
         State(cell.load(ordering))
     }
 
-    fn unpark_one(cell: &AtomicUsize) {
-        cell.fetch_add(1 | (1 << UNPARK_SHIFT), SeqCst);
+    fn unpark_one(cell: &AtomicUsize, num_searching: usize) {
+        cell.fetch_add(num_searching | (1 << UNPARK_SHIFT), SeqCst);
     }
 
     fn inc_num_searching(cell: &AtomicUsize, ordering: Ordering) {

tokio/src/runtime/thread_pool/worker.rs

@@ -511,8 +511,9 @@ impl Context {
         // Place `park` back in `core`
         core.park = Some(park);
 
-        // If there are tasks available to steal, notify a worker
-        if core.run_queue.is_stealable() {
+        // If there are tasks available to steal, but this worker is not
+        // looking for tasks to steal, notify another worker.
+        if !core.is_searching && core.run_queue.is_stealable() {
             self.worker.shared.notify_parked();
         }
 
@@ -621,8 +622,11 @@ impl Core {
         // If a task is in the lifo slot, then we must unpark regardless of
         // being notified
         if self.lifo_slot.is_some() {
-            worker.shared.idle.unpark_worker_by_id(worker.index);
-            self.is_searching = true;
+            // When a worker wakes, it should only transition to the "searching"
+            // state when the wake originates from another worker *or* a new task
+            // is pushed. We do *not* want the worker to transition to "searching"
+            // when it wakes when the I/O driver receives new events.
+            self.is_searching = !worker.shared.idle.unpark_worker_by_id(worker.index);
             return true;
         }