rt(threaded): cap LIFO slot polls

.github/workflows/ci.yml

@@ -60,6 +60,7 @@ jobs:
       - wasm32-wasi
       - check-external-types
       - check-fuzzing
+      - check-unstable-mt-counters
     steps:
       - run: exit 0
 
@@ -233,6 +234,31 @@ jobs:
           # the unstable cfg to RustDoc
           RUSTDOCFLAGS: --cfg tokio_unstable --cfg tokio_taskdump
 
+  check-unstable-mt-counters:
+    name: check tokio full --internal-mt-counters
+    needs: basics
+    runs-on: ${{ matrix.os }}
+    strategy:
+      matrix:
+        include:
+          - os: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v3
+      - name: Install Rust ${{ env.rust_stable }}
+        uses: dtolnay/rust-toolchain@master
+        with:
+            toolchain: ${{ env.rust_stable }}
+      - uses: Swatinem/rust-cache@v2
+      # Run `tokio` with "unstable" and "taskdump" cfg flags.
+      - name: check tokio full --cfg unstable --cfg internal-mt-counters
+        run: cargo test --all-features
+        working-directory: tokio
+        env:
+          RUSTFLAGS: --cfg tokio_unstable --cfg tokio_internal_mt_counters -Dwarnings
+          # in order to run doctests for unstable features, we must also pass
+          # the unstable cfg to RustDoc
+          RUSTDOCFLAGS: --cfg tokio_unstable --cfg tokio_internal_mt_counters
+
   miri:
     name: miri
     needs: basics

tokio/src/runtime/coop.rs

@@ -119,17 +119,6 @@ cfg_rt_multi_thread! {
     pub(crate) fn set(budget: Budget) {
         let _ = context::budget(|cell| cell.set(budget));
     }
-
-    /// Consume one unit of progress from the current task's budget.
-    pub(crate) fn consume_one() {
-        let _ = context::budget(|cell| {
-            let mut budget = cell.get();
-            if let Some(ref mut counter) = budget.0 {
-                *counter = counter.saturating_sub(1);
-            }
-            cell.set(budget);
-        });
-    }
 }
 
 cfg_rt! {

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

@@ -6,17 +6,23 @@ mod imp {
     static NUM_MAINTENANCE: AtomicUsize = AtomicUsize::new(0);
     static NUM_NOTIFY_LOCAL: AtomicUsize = AtomicUsize::new(0);
     static NUM_UNPARKS_LOCAL: AtomicUsize = AtomicUsize::new(0);
+    static NUM_LIFO_SCHEDULES: AtomicUsize = AtomicUsize::new(0);
+    static NUM_LIFO_CAPPED: AtomicUsize = AtomicUsize::new(0);
 
     impl Drop for super::Counters {
         fn drop(&mut self) {
             let notifies_local = NUM_NOTIFY_LOCAL.load(Relaxed);
             let unparks_local = NUM_UNPARKS_LOCAL.load(Relaxed);
             let maintenance = NUM_MAINTENANCE.load(Relaxed);
+            let lifo_scheds = NUM_LIFO_SCHEDULES.load(Relaxed);
+            let lifo_capped = NUM_LIFO_CAPPED.load(Relaxed);
 
             println!("---");
             println!("notifies (local): {}", notifies_local);
             println!(" unparks (local): {}", unparks_local);
             println!("     maintenance: {}", maintenance);
+            println!("  LIFO schedules: {}", lifo_scheds);
+            println!("     LIFO capped: {}", lifo_capped);
         }
     }
 
@@ -31,13 +37,23 @@ mod imp {
     pub(crate) fn inc_num_maintenance() {
         NUM_MAINTENANCE.fetch_add(1, Relaxed);
     }
+
+    pub(crate) fn inc_lifo_schedules() {
+        NUM_LIFO_SCHEDULES.fetch_add(1, Relaxed);
+    }
+
+    pub(crate) fn inc_lifo_capped() {
+        NUM_LIFO_CAPPED.fetch_add(1, Relaxed);
+    }
 }
 
 #[cfg(not(tokio_internal_mt_counters))]
 mod imp {
     pub(crate) fn inc_num_inc_notify_local() {}
     pub(crate) fn inc_num_unparks_local() {}
     pub(crate) fn inc_num_maintenance() {}
+    pub(crate) fn inc_lifo_schedules() {}
+    pub(crate) fn inc_lifo_capped() {}
 }
 
 #[derive(Debug)]

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

@@ -90,10 +90,14 @@ struct Core {
     /// When a task is scheduled from a worker, it is stored in this slot. The
     /// worker will check this slot for a task **before** checking the run
     /// queue. This effectively results in the **last** scheduled task to be run
-    /// next (LIFO). This is an optimization for message passing patterns and
-    /// helps to reduce latency.
+    /// next (LIFO). This is an optimization for improving locality which
+    /// benefits message passing patterns and helps to reduce latency.
     lifo_slot: Option<Notified>,
 
+    /// When `true`, locally scheduled tasks go to the LIFO slot. When `false`,
+    /// they go to the back of the `run_queue`.
+    lifo_enabled: bool,
+
     /// The worker-local run queue.
     run_queue: queue::Local<Arc<Handle>>,
 
@@ -191,6 +195,12 @@ type Notified = task::Notified<Arc<Handle>>;
 // Tracks thread-local state
 scoped_thread_local!(static CURRENT: Context);
 
+/// Value picked out of thin-air. Running the LIFO slot a handful of times
+/// seemms sufficient to benefit from locality. More than 3 times probably is
+/// overweighing. The value can be tuned in the future with data that shows
+/// improvements.
+const MAX_LIFO_POLLS_PER_TICK: usize = 3;
+
 pub(super) fn create(
     size: usize,
     park: Parker,
@@ -214,6 +224,7 @@ pub(super) fn create(
         cores.push(Box::new(Core {
             tick: 0,
             lifo_slot: None,
+            lifo_enabled: !config.disable_lifo_slot,
             run_queue,
             is_searching: false,
             is_shutdown: false,
@@ -470,6 +481,7 @@ impl Context {
         // Run the task
         coop::budget(|| {
             task.run();
+            let mut lifo_polls = 0;
 
             // As long as there is budget remaining and a task exists in the
             // `lifo_slot`, then keep running.
@@ -491,35 +503,55 @@ impl Context {
                 // Check for a task in the LIFO slot
                 let task = match core.lifo_slot.take() {
                     Some(task) => task,
-                    None => return Ok(core),
+                    None => {
+                        self.reset_lifo_enabled(&mut core);
+                        return Ok(core);
+                    }
                 };
 
-                // Polling a task doesn't necessarily consume any budget, if it
-                // doesn't use any Tokio leaf futures. To prevent such tasks
-                // from using the lifo slot in an infinite loop, we consume an
-                // extra unit of budget between each iteration of the loop.
-                coop::consume_one();
-
-                if coop::has_budget_remaining() {
-                    // Run the LIFO task, then loop
-                    core.metrics.start_poll();
-                    *self.core.borrow_mut() = Some(core);
-                    let task = self.worker.handle.shared.owned.assert_owner(task);
-                    task.run();
-                } else {
+                if !coop::has_budget_remaining() {
                     // Not enough budget left to run the LIFO task, push it to
                     // the back of the queue and return.
                     core.run_queue.push_back_or_overflow(
                         task,
                         self.worker.inject(),
                         &mut core.metrics,
                     );
+                    // If we hit this point, the LIFO slot should be enabled.
+                    // There is no need to reset it.
+                    debug_assert!(core.lifo_enabled);
                     return Ok(core);
                 }
+
+                // Track that we are about to run a task from the LIFO slot.
+                lifo_polls += 1;
+                super::counters::inc_lifo_schedules();
+
+                // Disable the LIFO slot if we reach our limit
+                //
+                // In ping-ping style workloads where task A notifies task B,
+                // which notifies task A again, continuously prioritizing the
+                // LIFO slot can cause starvation as these two tasks will
+                // repeatedly schedule the other. To mitigate this, we limit the
+                // number of times the LIFO slot is prioritized.
+                if lifo_polls >= MAX_LIFO_POLLS_PER_TICK {
+                    core.lifo_enabled = false;
+                    super::counters::inc_lifo_capped();
+                }
+
+                // Run the LIFO task, then loop
+                core.metrics.start_poll();
+                *self.core.borrow_mut() = Some(core);
+                let task = self.worker.handle.shared.owned.assert_owner(task);
+                task.run();
             }
         })
     }
 
+    fn reset_lifo_enabled(&self, core: &mut Core) {
+        core.lifo_enabled = !self.worker.handle.shared.config.disable_lifo_slot;
+    }
+
     fn maintenance(&self, mut core: Box<Core>) -> Box<Core> {
         if core.tick % self.worker.handle.shared.config.event_interval == 0 {
             super::counters::inc_num_maintenance();
@@ -840,7 +872,7 @@ impl Handle {
         // task must always be pushed to the back of the queue, enabling other
         // tasks to be executed. If **not** a yield, then there is more
         // flexibility and the task may go to the front of the queue.
-        let should_notify = if is_yield || self.shared.config.disable_lifo_slot {
+        let should_notify = if is_yield || !core.lifo_enabled {
             core.run_queue
                 .push_back_or_overflow(task, &self.shared.inject, &mut core.metrics);
             true