Deactivate worker only after global queue re-check

asynchronix/src/runtime/executor.rs

@@ -19,7 +19,7 @@ mod worker;
 #[cfg(all(test, not(asynchronix_loom)))]
 mod tests;
 
-use self::pool::{Pool, PoolState};
+use self::pool::Pool;
 use self::rng::Rng;
 use self::task::{CancelToken, Promise, Runnable};
 use self::worker::Worker;
@@ -41,8 +41,8 @@ static NEXT_EXECUTOR_ID: AtomicUsize = AtomicUsize::new(0);
 /// fairness as a goal in itself. While it does use fair local queues inasmuch
 /// as these tend to perform better in message-passing applications, it uses an
 /// unfair injection queue and a LIFO slot without attempt to mitigate the
-/// effect of badly behaving code (e.g. futures that use spin-locks and hope for
-/// the best by yielding to the executor with something like tokio's
+/// effect of badly behaving code (e.g. futures that spin-lock by yielding to
+/// the executor; there is for this reason no support for something like tokio's
 /// `yield_now`).
 ///
 /// Another way in which it differs from other `async` executors is that it
@@ -50,8 +50,7 @@ static NEXT_EXECUTOR_ID: AtomicUsize = AtomicUsize::new(0);
 /// discrete-time simulator, the simulation of a system at a given time step
 /// will make as much progress as possible until it technically reaches a
 /// deadlock. Only then does the simulator advance the simulated time until the
-/// next "event" extracted from a time-sorted priority queue, sending it to
-/// enable further progress in the computation.
+/// next "event" extracted from a time-sorted priority queue.
 ///
 /// The design of the executor is largely influenced by the tokio and go
 /// schedulers, both of which are optimized for message-passing applications. In
@@ -62,15 +61,14 @@ static NEXT_EXECUTOR_ID: AtomicUsize = AtomicUsize::new(0);
 /// simple by taking advantage of the fact that the injector is not required to
 /// be either LIFO or FIFO.
 ///
-/// Probably the largest difference with tokio is the task system, which boasts
-/// a higher throughput achieved by reducing the need for synchronization.
+/// Probably the largest difference with tokio is the task system, which
+/// achieves a higher throughput by reducing the need for synchronization.
 /// Another difference is that, at the moment, the complete subset of active
-/// worker threads is stored in a single atomic variable. This makes it in
-/// particular possible to rapidly identify free worker threads for stealing
-/// operations. The downside of this approach is that the maximum number of
-/// worker threads is limited to `usize::BITS`, but this is unlikely to
-/// constitute a limitation since system simulation is not typically an
-/// embarrassingly parallel problem.
+/// worker threads is stored in a single atomic variable. This makes it possible
+/// to rapidly identify free worker threads for stealing operations. The
+/// downside of this approach is that the maximum number of worker threads is
+/// limited to `usize::BITS`, but this is unlikely to constitute a limitation
+/// since system simulation is not typically embarrassingly parallel.
 #[derive(Debug)]
 pub(crate) struct Executor {
     pool: Arc<Pool>,
@@ -139,7 +137,7 @@ impl Executor {
 
         // Wait until all workers are blocked on the signal barrier.
         parker.park();
-        assert!(pool.is_idle());
+        assert!(pool.is_pool_idle());
 
         Self {
             pool,
@@ -208,7 +206,7 @@ impl Executor {
             if let Some(worker_panic) = self.pool.take_panic() {
                 panic::resume_unwind(worker_panic);
             }
-            if self.pool.is_idle() {
+            if self.pool.is_pool_idle() {
                 return;
             }
 
@@ -358,26 +356,25 @@ fn run_local_worker(worker: &Worker, id: usize, parker: Parker) {
 
         loop {
             // Signal barrier: park until notified to continue or terminate.
-            if worker.pool.set_worker_inactive(id) == PoolState::Idle {
-                // If this worker was the last active worker, it is necessary to
-                // check again whether the global queue is not populated. This
-                // could happen if the executor thread pushed a task to the
-                // global queue but could not activate a new worker because all
-                // workers were then activated.
-                if !worker.pool.global_queue.is_empty() {
-                    worker.pool.set_worker_active(id);
-                } else {
+            if worker.pool.try_set_worker_inactive(id) {
+                parker.park();
+            } else {
+                // This worker is the last active worker so it is necessary to
+                // check if the global queue is populated. This could happen if
+                // the executor thread pushed a task to the global queue but
+                // could not activate a new worker because all workers were then
+                // activated.
+                if worker.pool.global_queue.is_empty() {
+                    worker.pool.set_all_workers_inactive();
                     worker.pool.executor_unparker.unpark();
                     parker.park();
                 }
-            } else {
-                parker.park();
             }
+
             if worker.pool.termination_is_triggered() {
                 return;
             }
 
-            // We may spin for a little while: start counting.
             let mut search_start = Instant::now();
 
             // Process the tasks one by one.
@@ -388,17 +385,17 @@ fn run_local_worker(worker: &Worker, id: usize, parker: Parker) {
 
                     // There is a _very_ remote possibility that, even though
                     // the local queue is empty, it has temporarily too little
-                    // spare capacity for the bucket. This could happen because
-                    // a concurrent steal operation could be preempted for all
-                    // the time it took to pop and process the remaining tasks
-                    // and hasn't released the stolen capacity yet.
+                    // spare capacity for the bucket. This could happen if a
+                    // concurrent steal operation was preempted for all the time
+                    // it took to pop and process the remaining tasks and it
+                    // hasn't released the stolen capacity yet.
                     //
                     // Unfortunately, we cannot just skip checking the global
                     // queue altogether when there isn't enough spare capacity
-                    // in the local queue, as this could lead to a race: suppose
-                    // that (1) this thread has earlier pushed tasks onto the
-                    // global queue, and (2) the stealer has processed all
-                    // stolen tasks before this thread sees the capacity
+                    // in the local queue because this could lead to a race:
+                    // suppose that (1) this thread has earlier pushed tasks
+                    // onto the global queue, and (2) the stealer has processed
+                    // all stolen tasks before this thread sees the capacity
                     // restored and at the same time (3) the stealer does not
                     // yet see the tasks this thread pushed to the global queue;
                     // in such scenario, both this thread and the stealer thread

asynchronix/src/runtime/executor/pool.rs

@@ -7,12 +7,13 @@ use super::injector::Injector;
 use super::rng;
 use super::{GlobalQueue, Stealer};
 
+/// A view of the thread pool shared between the executor and all workers.
 #[derive(Debug)]
 pub(crate) struct Pool {
     pub(crate) global_queue: GlobalQueue,
     pub(crate) executor_id: usize,
     pub(crate) executor_unparker: parking::Unparker,
-    state: PoolRegistry,
+    registry: PoolRegistry,
     stealers: Box<[Stealer]>,
     worker_unparkers: Box<[parking::Unparker]>,
     searching_workers: AtomicUsize,
@@ -34,7 +35,7 @@ impl Pool {
             global_queue: Injector::new(),
             executor_id,
             executor_unparker,
-            state: PoolRegistry::new(worker_unparkers.len()),
+            registry: PoolRegistry::new(worker_unparkers.len()),
             stealers: stealers.into_boxed_slice(),
             worker_unparkers,
             searching_workers: AtomicUsize::new(0),
@@ -47,45 +48,48 @@ impl Pool {
     ///
     /// Unparking the worker threads is the responsibility of the caller.
     pub(crate) fn set_all_workers_active(&self) {
-        self.state.set_all_active();
+        self.registry.set_all_active();
     }
 
-    /// Marks the specified worker as active.
+    /// Marks all pool workers as inactive.
     ///
-    /// Unparking the worker thread is the responsibility of the caller.
-    pub(crate) fn set_worker_active(&self, worker_id: usize) {
-        self.state.set_active(worker_id);
+    /// Unparking the executor threads is the responsibility of the caller.
+    pub(crate) fn set_all_workers_inactive(&self) {
+        self.registry.set_all_inactive();
     }
 
-    /// Marks the specified worker as idle.
+    /// Marks the specified worker as inactive unless it is the last active
+    /// worker.
     ///
     /// Parking the worker thread is the responsibility of the caller.
     ///
-    /// If this was the last active worker, the main executor thread is
-    /// unparked.
-    pub(crate) fn set_worker_inactive(&self, worker_id: usize) -> PoolState {
-        self.state.set_inactive(worker_id)
+    /// If this was the last active worker, `false` is returned and it is
+    /// guaranteed that all memory operations performed by threads that called
+    /// `activate_worker` will be visible.
+    pub(crate) fn try_set_worker_inactive(&self, worker_id: usize) -> bool {
+        self.registry.try_set_inactive(worker_id)
     }
 
-    /// Unparks an idle worker if any is found, or do nothing otherwise.
+    /// Unparks an idle worker if any is found and mark it as active, or do
+    /// nothing otherwise.
     ///
     /// For performance reasons, no synchronization is established if no worker
     /// is found, meaning that workers in other threads may later transition to
-    /// idle state without observing the tasks scheduled by the caller to this
-    /// method. If this is not tolerable (for instance if this method is called
-    /// from a non-worker thread), use the more expensive `activate_worker`.
+    /// idle state without observing the tasks scheduled by this caller. If this
+    /// is not tolerable (for instance if this method is called from a
+    /// non-worker thread), use the more expensive `activate_worker`.
     pub(crate) fn activate_worker_relaxed(&self) {
-        if let Some(worker_id) = self.state.set_one_active_relaxed() {
+        if let Some(worker_id) = self.registry.set_one_active_relaxed() {
             self.searching_workers.fetch_add(1, Ordering::Relaxed);
             self.worker_unparkers[worker_id].unpark();
         }
     }
 
-    /// Unparks an idle worker if any is found, or ensure that at least the last
-    /// worker to transition to idle state will observe all tasks previously
-    /// scheduled by the caller to this method.
+    /// Unparks an idle worker if any is found and mark it as active, or ensure
+    /// that at least the last active worker will observe all memory operations
+    /// performed before this call when calling `try_set_worker_inactive`.
     pub(crate) fn activate_worker(&self) {
-        if let Some(worker_id) = self.state.set_one_active() {
+        if let Some(worker_id) = self.registry.set_one_active() {
             self.searching_workers.fetch_add(1, Ordering::Relaxed);
             self.worker_unparkers[worker_id].unpark();
         }
@@ -95,8 +99,8 @@ impl Pool {
     ///
     /// If `true` is returned, it is guaranteed that all operations performed by
     /// the now-inactive workers become visible in this thread.
-    pub(crate) fn is_idle(&self) -> bool {
-        self.state.pool_state() == PoolState::Idle
+    pub(crate) fn is_pool_idle(&self) -> bool {
+        self.registry.is_idle()
     }
 
     /// Increments the count of workers actively searching for tasks.
@@ -119,7 +123,7 @@ impl Pool {
     pub(crate) fn trigger_termination(&self) {
         self.terminate_signal.store(true, Ordering::Relaxed);
 
-        self.state.set_all_active();
+        self.registry.set_all_active();
         for unparker in &*self.worker_unparkers {
             unparker.unpark();
         }
@@ -158,7 +162,7 @@ impl Pool {
         rng: &'_ rng::Rng,
     ) -> ShuffledStealers<'a> {
         // All active workers except the specified one are candidate for stealing.
-        let mut candidates = self.state.get_active();
+        let mut candidates = self.registry.get_active();
         if let Some(excluded_worker_id) = excluded_worker_id {
             candidates &= !(1 << excluded_worker_id);
         }
@@ -167,6 +171,8 @@ impl Pool {
     }
 }
 
+/// An iterator over active workers that yields their associated stealer,
+/// starting from a randomly selected worker.
 pub(crate) struct ShuffledStealers<'a> {
     stealers: &'a [Stealer],
     // A bit-rotated bit field of the remaining candidate workers to steal from.
@@ -186,8 +192,7 @@ impl<'a> ShuffledStealers<'a> {
             // Right-rotate the candidates so that the bit corresponding to the
             // randomly selected worker becomes the LSB.
             let candidate_count = stealers.len();
-            let lower_mask = (1 << next_candidate) - 1;
-            let lower_bits = candidates & lower_mask;
+            let lower_bits = candidates & ((1 << next_candidate) - 1);
             let candidates =
                 (candidates >> next_candidate) | (lower_bits << (candidate_count - next_candidate));
 
@@ -266,53 +271,66 @@ impl PoolRegistry {
             record: Record::new(pool_size),
         }
     }
-    /// Returns the state of the pool.
+
+    /// Returns whether the pool is idle, i.e. whether there are no active
+    /// workers.
     ///
-    /// This operation has Acquire semantic, which guarantees that if the pool
-    /// state returned is `PoolState::Idle`, then all operations performed by
-    /// the now-inactive workers are visible.
-    fn pool_state(&self) -> PoolState {
+    /// It is guaranteed that if `false` is returned, then all operations
+    /// performed by the now-inactive workers are visible.
+    fn is_idle(&self) -> bool {
         // Ordering: this Acquire operation synchronizes with all Release
         // RMWs in the `set_inactive` method via a release sequence.
-        let active_workers = self.active_workers.load(Ordering::Acquire);
-        if active_workers == 0 {
-            PoolState::Idle
-        } else {
-            PoolState::Busy
-        }
+        self.active_workers.load(Ordering::Acquire) == 0
     }
 
-    /// Marks the specified worker as inactive.
+    /// Marks the specified worker as inactive, unless this is the last worker.
     ///
-    /// The specified worker must currently be marked as active. Returns
-    /// `PoolState::Idle` if this was the last active thread.
-    ///
-    /// If this is the last active worker (i.e. `PoolState::Idle` is returned),
-    /// then it is guaranteed that all operations performed by the now-inactive
-    /// workers and by unsuccessful callers to `set_one_active` are now visible.
-    fn set_inactive(&self, worker_id: usize) -> PoolState {
-        // Ordering: this Release operation synchronizes with the Acquire
-        // fence in the below conditional when the pool becomes idle, and/or
+    /// The specified worker must currently be marked as active. This method
+    /// will always fail and return `false` if this was the last active worker,
+    /// because the worker is then expected to check again the global queue
+    /// before explicitly calling `set_all_inactive` to confirm that the pool is
+    /// indeed idle.
+    fn try_set_inactive(&self, worker_id: usize) -> bool {
+        // Ordering: this Release operation synchronizes with the Acquire fence
+        // in the below conditional if this is is the last active worker, and/or
         // with the Acquire state load in the `pool_state` method.
         let active_workers = self
             .active_workers
-            .fetch_and(!(1 << worker_id), Ordering::Release);
+            .fetch_update(Ordering::Release, Ordering::Relaxed, |active_workers| {
+                if active_workers == (1 << worker_id) {
+                    // It looks like this is the last worker, but the value
+                    // could be stale so it is necessary to make sure of this by
+                    // enforcing the CAS rather than returning `None`.
+                    Some(active_workers)
+                } else {
+                    Some(active_workers & !(1 << worker_id))
+                }
+            })
+            .unwrap();
 
-        if active_workers & !(1 << worker_id) == 0 {
-            // Ordering: this Acquire fence synchronizes with all Release
-            // RMWs in this and in the previous calls to `set_inactive` via a
-            // release sequence.
+        assert_ne!(active_workers & (1 << worker_id), 0);
+
+        if active_workers == (1 << worker_id) {
+            // This is the last worker so we need to ensures that after this
+            // call, all tasks pushed on the global queue before
+            // `set_one_active` was called unsuccessfully are visible.
+            //
+            // Ordering: this Acquire fence synchronizes with all Release RMWs
+            // in this and in the previous calls to `set_inactive` via a release
+            // sequence.
             atomic::fence(Ordering::Acquire);
-            PoolState::Idle
+
+            false
         } else {
-            PoolState::Busy
+            true
         }
     }
 
-    /// Marks the specified worker as active.
-    fn set_active(&self, worker_id: usize) {
-        self.active_workers
-            .fetch_or(1 << worker_id, Ordering::Relaxed);
+    /// Marks all workers as inactive.
+    fn set_all_inactive(&self) {
+        // Ordering: this Release store synchronizes with the Acquire load in
+        // `is_idle`.
+        self.active_workers.store(0, Ordering::Release);
     }
 
     /// Marks all workers as active.
@@ -359,8 +377,8 @@ impl PoolRegistry {
                 // There is apparently no free worker, so a dummy RMW with
                 // Release ordering is performed with the sole purpose of
                 // synchronizing with the Acquire fence in `set_inactive` so
-                // that the last worker to transition to idle can see the tasks
-                // that were queued prior to this call.
+                // that the last worker see the tasks that were queued prior to
+                // this call when calling (unsuccessfully) `set_inactive`..
                 let new_active_workers = self.active_workers.fetch_or(0, Ordering::Release);
                 if new_active_workers == active_workers {
                     return None;
@@ -385,12 +403,6 @@ impl PoolRegistry {
     }
 }
 
-#[derive(PartialEq)]
-pub(crate) enum PoolState {
-    Idle,
-    Busy,
-}
-
 #[cfg(feature = "dev-logs")]
 impl Drop for PoolRegistry {
     fn drop(&mut self) {