runtime: Add a metric that counts currently active tasks

tokio/src/runtime/metrics/runtime.rs

@@ -68,6 +68,25 @@ impl RuntimeMetrics {
         self.handle.inner.num_blocking_threads()
     }
 
+    /// Returns the number of active tasks in the runtime.
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// use tokio::runtime::Handle;
+    ///
+    /// #[tokio::main]
+    /// async fn main() {
+    ///    let metrics = Handle::current().metrics();
+    ///
+    ///     let n = metrics.active_tasks_count();
+    ///     println!("Runtime has {} active tasks", n);
+    /// }
+    /// ```
+    pub fn active_tasks_count(&self) -> usize {
+        self.handle.inner.active_tasks_count()
+    }
+
     /// Returns the number of idle threads, which have spawned by the runtime
     /// for `spawn_blocking` calls.
     ///

tokio/src/runtime/scheduler/current_thread.rs

@@ -430,6 +430,10 @@ cfg_metrics! {
         pub(crate) fn blocking_queue_depth(&self) -> usize {
             self.blocking_spawner.queue_depth()
         }
+
+        pub(crate) fn active_tasks_count(&self) -> usize {
+            self.shared.owned.active_tasks_count()
+        }
     }
 }
 

tokio/src/runtime/scheduler/mod.rs

@@ -135,6 +135,14 @@ cfg_rt! {
                 }
             }
 
+            pub(crate) fn active_tasks_count(&self) -> usize {
+                match self {
+                    Handle::CurrentThread(handle) => handle.active_tasks_count(),
+                    #[cfg(all(feature = "rt-multi-thread", not(tokio_wasi)))]
+                    Handle::MultiThread(handle) => handle.active_tasks_count(),
+                }
+            }
+
             pub(crate) fn scheduler_metrics(&self) -> &SchedulerMetrics {
                 match self {
                     Handle::CurrentThread(handle) => handle.scheduler_metrics(),

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

@@ -69,6 +69,10 @@ cfg_metrics! {
             self.blocking_spawner.num_idle_threads()
         }
 
+        pub(crate) fn active_tasks_count(&self) -> usize {
+            self.shared.owned.active_tasks_count()
+        }
+
         pub(crate) fn scheduler_metrics(&self) -> &SchedulerMetrics {
             &self.shared.scheduler_metrics
         }

tokio/src/runtime/task/list.rs

@@ -10,7 +10,7 @@ use crate::future::Future;
 use crate::loom::cell::UnsafeCell;
 use crate::loom::sync::Mutex;
 use crate::runtime::task::{JoinHandle, LocalNotified, Notified, Schedule, Task};
-use crate::util::linked_list::{Link, LinkedList};
+use crate::util::linked_list::{CountedLinkedList, Link, LinkedList};
 
 use std::marker::PhantomData;
 
@@ -54,9 +54,13 @@ cfg_not_has_atomic_u64! {
 }
 
 pub(crate) struct OwnedTasks<S: 'static> {
-    inner: Mutex<OwnedTasksInner<S>>,
+    inner: Mutex<CountedOwnedTasksInner<S>>,
     id: u64,
 }
+struct CountedOwnedTasksInner<S: 'static> {
+    list: CountedLinkedList<Task<S>, <Task<S> as Link>::Target>,
+    closed: bool,
+}
 pub(crate) struct LocalOwnedTasks<S: 'static> {
     inner: UnsafeCell<OwnedTasksInner<S>>,
     id: u64,
@@ -70,8 +74,8 @@ struct OwnedTasksInner<S: 'static> {
 impl<S: 'static> OwnedTasks<S> {
     pub(crate) fn new() -> Self {
         Self {
-            inner: Mutex::new(OwnedTasksInner {
-                list: LinkedList::new(),
+            inner: Mutex::new(CountedOwnedTasksInner {
+                list: CountedLinkedList::new(),
                 closed: false,
             }),
             id: get_next_id(),
@@ -153,6 +157,10 @@ impl<S: 'static> OwnedTasks<S> {
         }
     }
 
+    pub(crate) fn active_tasks_count(&self) -> usize {
+        self.inner.lock().list.count()
+    }
+
     pub(crate) fn remove(&self, task: &Task<S>) -> Option<Task<S>> {
         let task_id = task.header().get_owner_id();
         if task_id == 0 {

tokio/src/util/linked_list.rs

@@ -228,6 +228,53 @@ impl<L: Link> fmt::Debug for LinkedList<L, L::Target> {
     }
 }
 
+// ===== impl CountedLinkedList ====
+
+// Delegates operations to the base LinkedList implementation, and adds a counter to the elements
+// in the list.
+pub(crate) struct CountedLinkedList<L: Link, T> {
+    list: LinkedList<L, T>,
+    count: usize,
+}
+
+impl<L: Link> CountedLinkedList<L, L::Target> {
+    pub(crate) fn new() -> CountedLinkedList<L, L::Target> {
+        CountedLinkedList {
+            list: LinkedList::new(),
+            count: 0,
+        }
+    }
+
+    pub(crate) fn push_front(&mut self, val: L::Handle) {
+        self.list.push_front(val);
+        self.count += 1;
+    }
+
+    pub(crate) fn pop_back(&mut self) -> Option<L::Handle> {
+        let val = self.list.pop_back();
+        if val.is_some() {
+            self.count -= 1;
+        }
+        val
+    }
+
+    pub(crate) fn is_empty(&self) -> bool {
+        self.list.is_empty()
+    }
+
+    pub(crate) unsafe fn remove(&mut self, node: NonNull<L::Target>) -> Option<L::Handle> {
+        let val = self.list.remove(node);
+        if val.is_some() {
+            self.count -= 1;
+        }
+        val
+    }
+
+    pub(crate) fn count(&self) -> usize {
+        self.count
+    }
+}
+
 #[cfg(any(
     feature = "fs",
     feature = "rt",
@@ -719,6 +766,26 @@ pub(crate) mod tests {
         }
     }
 
+    #[test]
+    fn count() {
+        let mut list = CountedLinkedList::<&Entry, <&Entry as Link>::Target>::new();
+        assert_eq!(0, list.count());
+
+        let a = entry(5);
+        let b = entry(7);
+        list.push_front(a.as_ref());
+        list.push_front(b.as_ref());
+        assert_eq!(2, list.count());
+
+        list.pop_back();
+        assert_eq!(1, list.count());
+
+        unsafe {
+            list.remove(ptr(&b));
+        }
+        assert_eq!(0, list.count());
+    }
+
     /// This is a fuzz test. You run it by entering `cargo fuzz run fuzz_linked_list` in CLI in `/tokio/` module.
     #[cfg(fuzzing)]
     pub fn fuzz_linked_list(ops: &[u8]) {

tokio/tests/rt_metrics.rs

@@ -81,6 +81,23 @@ fn blocking_queue_depth() {
     assert_eq!(0, rt.metrics().blocking_queue_depth());
 }
 
+#[test]
+fn active_tasks_count() {
+    let rt = current_thread();
+    let metrics = rt.metrics();
+    assert_eq!(0, metrics.active_tasks_count());
+    rt.spawn(async move {
+        assert_eq!(1, metrics.active_tasks_count());
+    });
+
+    let rt = threaded();
+    let metrics = rt.metrics();
+    assert_eq!(0, metrics.active_tasks_count());
+    rt.spawn(async move {
+        assert_eq!(1, metrics.active_tasks_count());
+    });
+}
+
 #[test]
 fn remote_schedule_count() {
     use std::thread;