rt(threaded): adjust transition_from_parked behavior after introduc…

…ing `disable_lifo_slot` feature (tokio-rs#5753)

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

@@ -105,9 +105,9 @@ pub(crate) fn local<T: 'static>() -> (Steal<T>, Local<T>) {
 }
 
 impl<T> Local<T> {
-    /// Returns true if the queue has entries that can be stolen.
-    pub(crate) fn is_stealable(&self) -> bool {
-        !self.inner.is_empty()
+    /// Returns the number of entries in the queue
+    pub(crate) fn len(&self) -> usize {
+        self.inner.len() as usize
     }
 
     /// How many tasks can be pushed into the queue

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

@@ -718,9 +718,7 @@ impl Context {
         // Place `park` back in `core`
         core.park = Some(park);
 
-        // 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() {
+        if core.should_notify_others() {
             self.worker.handle.notify_parked_local();
         }
 
@@ -846,12 +844,25 @@ impl Core {
         worker.handle.transition_worker_from_searching();
     }
 
+    fn has_tasks(&self) -> bool {
+        self.lifo_slot.is_some() || self.run_queue.has_tasks()
+    }
+
+    fn should_notify_others(&self) -> bool {
+        // If there are tasks available to steal, but this worker is not
+        // looking for tasks to steal, notify another worker.
+        if self.is_searching {
+            return false;
+        }
+        self.lifo_slot.is_some() as usize + self.run_queue.len() > 1
+    }
+
     /// Prepares the worker state for parking.
     ///
     /// Returns true if the transition happened, false if there is work to do first.
     fn transition_to_parked(&mut self, worker: &Worker) -> bool {
         // Workers should not park if they have work to do
-        if self.lifo_slot.is_some() || self.run_queue.has_tasks() || self.is_traced {
+        if self.has_tasks() || self.is_traced {
             return false;
         }
 
@@ -877,9 +888,9 @@ impl Core {
 
     /// Returns `true` if the transition happened.
     fn transition_from_parked(&mut self, worker: &Worker) -> bool {
-        // If a task is in the lifo slot, then we must unpark regardless of
+        // If a task is in the lifo slot/run queue, then we must unpark regardless of
         // being notified
-        if self.lifo_slot.is_some() {
+        if self.has_tasks() {
             // 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"