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//! A scheduler is initialized with a fixed number of workers. Each worker is //! driven by a thread. Each worker has a "core" which contains data such as the //! run queue and other state. When `block_in_place` is called, the worker's //! "core" is handed off to a new thread allowing the scheduler to continue to //! make progress while the originating thread blocks. use crate::coop; use crate::loom::rand::seed; use crate::loom::sync::{Arc, Mutex}; use crate::park::{Park, Unpark}; use crate::runtime; use crate::runtime::enter::EnterContext; use crate::runtime::park::{Parker, Unparker}; use crate::runtime::thread_pool::{AtomicCell, Idle}; use crate::runtime::{queue, task}; use crate::util::linked_list::{Link, LinkedList}; use crate::util::FastRand; use std::cell::RefCell; use std::time::Duration; /// A scheduler worker pub(super) struct Worker { /// Reference to shared state shared: Arc<Shared>, /// Index holding this worker's remote state index: usize, /// Used to hand-off a worker's core to another thread. core: AtomicCell<Core>, } /// Core data struct Core { /// Used to schedule bookkeeping tasks every so often. tick: u8, /// 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. lifo_slot: Option<Notified>, /// The worker-local run queue. run_queue: queue::Local<Arc<Worker>>, /// True if the worker is currently searching for more work. Searching /// involves attempting to steal from other workers. is_searching: bool, /// True if the scheduler is being shutdown is_shutdown: bool, /// Tasks owned by the core tasks: LinkedList<Task, <Task as Link>::Target>, /// Parker /// /// Stored in an `Option` as the parker is added / removed to make the /// borrow checker happy. park: Option<Parker>, /// Fast random number generator. rand: FastRand, } /// State shared across all workers pub(super) struct Shared { /// Per-worker remote state. All other workers have access to this and is /// how they communicate between each other. remotes: Box<[Remote]>, /// Submit work to the scheduler while **not** currently on a worker thread. inject: queue::Inject<Arc<Worker>>, /// Coordinates idle workers idle: Idle, /// Cores that have observed the shutdown signal /// /// The core is **not** placed back in the worker to avoid it from being /// stolen by a thread that was spawned as part of `block_in_place`. #[allow(clippy::vec_box)] // we're moving an already-boxed value shutdown_cores: Mutex<Vec<Box<Core>>>, } /// Used to communicate with a worker from other threads. struct Remote { /// Steal tasks from this worker. steal: queue::Steal<Arc<Worker>>, /// Transfers tasks to be released. Any worker pushes tasks, only the owning /// worker pops. pending_drop: task::TransferStack<Arc<Worker>>, /// Unparks the associated worker thread unpark: Unparker, } /// Thread-local context struct Context { /// Worker worker: Arc<Worker>, /// Core data core: RefCell<Option<Box<Core>>>, } /// Starts the workers pub(crate) struct Launch(Vec<Arc<Worker>>); /// Running a task may consume the core. If the core is still available when /// running the task completes, it is returned. Otherwise, the worker will need /// to stop processing. type RunResult = Result<Box<Core>, ()>; /// A task handle type Task = task::Task<Arc<Worker>>; /// A notified task handle type Notified = task::Notified<Arc<Worker>>; // Tracks thread-local state scoped_thread_local!(static CURRENT: Context); pub(super) fn create(size: usize, park: Parker) -> (Arc<Shared>, Launch) { let mut cores = vec![]; let mut remotes = vec![]; // Create the local queues for _ in 0..size { let (steal, run_queue) = queue::local(); let park = park.clone(); let unpark = park.unpark(); cores.push(Box::new(Core { tick: 0, lifo_slot: None, run_queue, is_searching: false, is_shutdown: false, tasks: LinkedList::new(), park: Some(park), rand: FastRand::new(seed()), })); remotes.push(Remote { steal, pending_drop: task::TransferStack::new(), unpark, }); } let shared = Arc::new(Shared { remotes: remotes.into_boxed_slice(), inject: queue::Inject::new(), idle: Idle::new(size), shutdown_cores: Mutex::new(vec![]), }); let mut launch = Launch(vec![]); for (index, core) in cores.drain(..).enumerate() { launch.0.push(Arc::new(Worker { shared: shared.clone(), index, core: AtomicCell::new(Some(core)), })); } (shared, launch) } pub(crate) fn block_in_place<F, R>(f: F) -> R where F: FnOnce() -> R, { // Try to steal the worker core back struct Reset(coop::Budget); impl Drop for Reset { fn drop(&mut self) { CURRENT.with(|maybe_cx| { if let Some(cx) = maybe_cx { let core = cx.worker.core.take(); let mut cx_core = cx.core.borrow_mut(); assert!(cx_core.is_none()); *cx_core = core; // Reset the task budget as we are re-entering the // runtime. coop::set(self.0); } }); } } let mut had_entered = false; CURRENT.with(|maybe_cx| { match (crate::runtime::enter::context(), maybe_cx.is_some()) { (EnterContext::Entered { .. }, true) => { // We are on a thread pool runtime thread, so we just need to set up blocking. had_entered = true; } (EnterContext::Entered { allow_blocking }, false) => { // We are on an executor, but _not_ on the thread pool. // That is _only_ okay if we are in a thread pool runtime's block_on method: if allow_blocking { had_entered = true; return; } else { // This probably means we are on the basic_scheduler or in a LocalSet, // where it is _not_ okay to block. panic!("can call blocking only when running on the multi-threaded runtime"); } } (EnterContext::NotEntered, true) => { // This is a nested call to block_in_place (we already exited). // All the necessary setup has already been done. return; } (EnterContext::NotEntered, false) => { // We are outside of the tokio runtime, so blocking is fine. // We can also skip all of the thread pool blocking setup steps. return; } } let cx = maybe_cx.expect("no .is_some() == false cases above should lead here"); // Get the worker core. If none is set, then blocking is fine! let core = match cx.core.borrow_mut().take() { Some(core) => core, None => return, }; // The parker should be set here assert!(core.park.is_some()); // In order to block, the core must be sent to another thread for // execution. // // First, move the core back into the worker's shared core slot. cx.worker.core.set(core); // Next, clone the worker handle and send it to a new thread for // processing. // // Once the blocking task is done executing, we will attempt to // steal the core back. let worker = cx.worker.clone(); runtime::spawn_blocking(move || run(worker)); }); if had_entered { // Unset the current task's budget. Blocking sections are not // constrained by task budgets. let _reset = Reset(coop::stop()); crate::runtime::enter::exit(f) } else { f() } } /// After how many ticks is the global queue polled. This helps to ensure /// fairness. /// /// The number is fairly arbitrary. I believe this value was copied from golang. const GLOBAL_POLL_INTERVAL: u8 = 61; impl Launch { pub(crate) fn launch(mut self) { for worker in self.0.drain(..) { runtime::spawn_blocking(move || run(worker)); } } } fn run(worker: Arc<Worker>) { // Acquire a core. If this fails, then another thread is running this // worker and there is nothing further to do. let core = match worker.core.take() { Some(core) => core, None => return, }; // Set the worker context. let cx = Context { worker, core: RefCell::new(None), }; let _enter = crate::runtime::enter(true); CURRENT.set(&cx, || { // This should always be an error. It only returns a `Result` to support // using `?` to short circuit. assert!(cx.run(core).is_err()); }); } impl Context { fn run(&self, mut core: Box<Core>) -> RunResult { while !core.is_shutdown { // Increment the tick core.tick(); // Run maintenance, if needed core = self.maintenance(core); // First, check work available to the current worker. if let Some(task) = core.next_task(&self.worker) { core = self.run_task(task, core)?; continue; } // There is no more **local** work to process, try to steal work // from other workers. if let Some(task) = core.steal_work(&self.worker) { core = self.run_task(task, core)?; } else { // Wait for work core = self.park(core); } } core.pre_shutdown(&self.worker); // Signal shutdown self.worker.shared.shutdown(core); Err(()) } fn run_task(&self, task: Notified, mut core: Box<Core>) -> RunResult { // Make sure thew orker is not in the **searching** state. This enables // another idle worker to try to steal work. core.transition_from_searching(&self.worker); // Make the core available to the runtime context *self.core.borrow_mut() = Some(core); // Run the task coop::budget(|| { task.run(); // As long as there is budget remaining and a task exists in the // `lifo_slot`, then keep running. loop { // Check if we still have the core. If not, the core was stolen // by another worker. let mut core = match self.core.borrow_mut().take() { Some(core) => core, None => return Err(()), }; // Check for a task in the LIFO slot let task = match core.lifo_slot.take() { Some(task) => task, None => return Ok(core), }; if coop::has_budget_remaining() { // Run the LIFO task, then loop *self.core.borrow_mut() = Some(core); task.run(); } else { // Not enough budget left to run the LIFO task, push it to // the back of the queue and return. core.run_queue.push_back(task, self.worker.inject()); return Ok(core); } } }) } fn maintenance(&self, mut core: Box<Core>) -> Box<Core> { if core.tick % GLOBAL_POLL_INTERVAL == 0 { // Call `park` with a 0 timeout. This enables the I/O driver, timer, ... // to run without actually putting the thread to sleep. core = self.park_timeout(core, Some(Duration::from_millis(0))); // Run regularly scheduled maintenance core.maintenance(&self.worker); } core } fn park(&self, mut core: Box<Core>) -> Box<Core> { core.transition_to_parked(&self.worker); while !core.is_shutdown { core = self.park_timeout(core, None); // Run regularly scheduled maintenance core.maintenance(&self.worker); if core.transition_from_parked(&self.worker) { return core; } } core } fn park_timeout(&self, mut core: Box<Core>, duration: Option<Duration>) -> Box<Core> { // Take the parker out of core let mut park = core.park.take().expect("park missing"); // Store `core` in context *self.core.borrow_mut() = Some(core); // Park thread if let Some(timeout) = duration { park.park_timeout(timeout).expect("park failed"); } else { park.park().expect("park failed"); } // Remove `core` from context core = self.core.borrow_mut().take().expect("core missing"); // 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() { self.worker.shared.notify_parked(); } core } } impl Core { /// Increment the tick fn tick(&mut self) { self.tick = self.tick.wrapping_add(1); } /// Return the next notified task available to this worker. fn next_task(&mut self, worker: &Worker) -> Option<Notified> { if self.tick % GLOBAL_POLL_INTERVAL == 0 { worker.inject().pop().or_else(|| self.next_local_task()) } else { self.next_local_task().or_else(|| worker.inject().pop()) } } fn next_local_task(&mut self) -> Option<Notified> { self.lifo_slot.take().or_else(|| self.run_queue.pop()) } fn steal_work(&mut self, worker: &Worker) -> Option<Notified> { if !self.transition_to_searching(worker) { return None; } let num = worker.shared.remotes.len(); // Start from a random worker let start = self.rand.fastrand_n(num as u32) as usize; for i in 0..num { let i = (start + i) % num; // Don't steal from ourself! We know we don't have work. if i == worker.index { continue; } let target = &worker.shared.remotes[i]; if let Some(task) = target.steal.steal_into(&mut self.run_queue) { return Some(task); } } // Fallback on checking the global queue worker.shared.inject.pop() } fn transition_to_searching(&mut self, worker: &Worker) -> bool { if !self.is_searching { self.is_searching = worker.shared.idle.transition_worker_to_searching(); } self.is_searching } fn transition_from_searching(&mut self, worker: &Worker) { if !self.is_searching { return; } self.is_searching = false; worker.shared.transition_worker_from_searching(); } /// Prepare the worker state for parking fn transition_to_parked(&mut self, worker: &Worker) { // When the final worker transitions **out** of searching to parked, it // must check all the queues one last time in case work materialized // between the last work scan and transitioning out of searching. let is_last_searcher = worker .shared .idle .transition_worker_to_parked(worker.index, self.is_searching); // The worker is no longer searching. Setting this is the local cache // only. self.is_searching = false; if is_last_searcher { worker.shared.notify_if_work_pending(); } } /// 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 // being notified if self.lifo_slot.is_some() { worker.shared.idle.unpark_worker_by_id(worker.index); self.is_searching = true; return true; } if worker.shared.idle.is_parked(worker.index) { return false; } // When unparked, the worker is in the searching state. self.is_searching = true; true } /// Runs maintenance work such as free pending tasks and check the pool's /// state. fn maintenance(&mut self, worker: &Worker) { self.drain_pending_drop(worker); if !self.is_shutdown { // Check if the scheduler has been shutdown self.is_shutdown = worker.inject().is_closed(); } } // Signals all tasks to shut down, and waits for them to complete. Must run // before we enter the single-threaded phase of shutdown processing. fn pre_shutdown(&mut self, worker: &Worker) { // Signal to all tasks to shut down. for header in self.tasks.iter() { header.shutdown(); } loop { self.drain_pending_drop(worker); if self.tasks.is_empty() { break; } // Wait until signalled let park = self.park.as_mut().expect("park missing"); park.park().expect("park failed"); } } // Shutdown the core fn shutdown(&mut self) { assert!(self.tasks.is_empty()); // Take the core let mut park = self.park.take().expect("park missing"); // Drain the queue while self.next_local_task().is_some() {} park.shutdown(); } fn drain_pending_drop(&mut self, worker: &Worker) { use std::mem::ManuallyDrop; for task in worker.remote().pending_drop.drain() { let task = ManuallyDrop::new(task); // safety: tasks are only pushed into the `pending_drop` stacks that // are associated with the list they are inserted into. When a task // is pushed into `pending_drop`, the ref-inc is skipped, so we must // not ref-dec here. // // See `bind` and `release` implementations. unsafe { self.tasks.remove(task.header().into()); } } } } impl Worker { /// Returns a reference to the scheduler's injection queue fn inject(&self) -> &queue::Inject<Arc<Worker>> { &self.shared.inject } /// Return a reference to this worker's remote data fn remote(&self) -> &Remote { &self.shared.remotes[self.index] } fn eq(&self, other: &Worker) -> bool { self.shared.ptr_eq(&other.shared) && self.index == other.index } } impl task::Schedule for Arc<Worker> { fn bind(task: Task) -> Arc<Worker> { CURRENT.with(|maybe_cx| { let cx = maybe_cx.expect("scheduler context missing"); // Track the task cx.core .borrow_mut() .as_mut() .expect("scheduler core missing") .tasks .push_front(task); // Return a clone of the worker cx.worker.clone() }) } fn release(&self, task: &Task) -> Option<Task> { use std::ptr::NonNull; enum Immediate { // Task has been synchronously removed from the Core owned by the // current thread Removed(Option<Task>), // Task is owned by another thread, so we need to notify it to clean // up the task later. MaybeRemote, } let immediate = CURRENT.with(|maybe_cx| { let cx = match maybe_cx { Some(cx) => cx, None => return Immediate::MaybeRemote, }; if !self.eq(&cx.worker) { // Task owned by another core, so we need to notify it. return Immediate::MaybeRemote; } let mut maybe_core = cx.core.borrow_mut(); if let Some(core) = &mut *maybe_core { // Directly remove the task // // safety: the task is inserted in the list in `bind`. unsafe { let ptr = NonNull::from(task.header()); return Immediate::Removed(core.tasks.remove(ptr)); } } Immediate::MaybeRemote }); // Checks if we were called from within a worker, allowing for immediate // removal of a scheduled task. Else we have to go through the slower // process below where we remotely mark a task as dropped. match immediate { Immediate::Removed(task) => return task, Immediate::MaybeRemote => (), }; // Track the task to be released by the worker that owns it // // Safety: We get a new handle without incrementing the ref-count. // A ref-count is held by the "owned" linked list and it is only // ever removed from that list as part of the release process: this // method or popping the task from `pending_drop`. Thus, we can rely // on the ref-count held by the linked-list to keep the memory // alive. // // When the task is removed from the stack, it is forgotten instead // of dropped. let task = unsafe { Task::from_raw(task.header().into()) }; self.remote().pending_drop.push(task); // The worker core has been handed off to another thread. In the // event that the scheduler is currently shutting down, the thread // that owns the task may be waiting on the release to complete // shutdown. if self.inject().is_closed() { self.remote().unpark.unpark(); } None } fn schedule(&self, task: Notified) { self.shared.schedule(task, false); } fn yield_now(&self, task: Notified) { self.shared.schedule(task, true); } } impl Shared { pub(super) fn schedule(&self, task: Notified, is_yield: bool) { CURRENT.with(|maybe_cx| { if let Some(cx) = maybe_cx { // Make sure the task is part of the **current** scheduler. if self.ptr_eq(&cx.worker.shared) { // And the current thread still holds a core if let Some(core) = cx.core.borrow_mut().as_mut() { self.schedule_local(core, task, is_yield); return; } } } // Otherwise, use the inject queue self.inject.push(task); self.notify_parked(); }); } fn schedule_local(&self, core: &mut Core, task: Notified, is_yield: bool) { // Spawning from the worker thread. If scheduling a "yield" then the // 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 { core.run_queue.push_back(task, &self.inject); true } else { // Push to the LIFO slot let prev = core.lifo_slot.take(); let ret = prev.is_some(); if let Some(prev) = prev { core.run_queue.push_back(prev, &self.inject); } core.lifo_slot = Some(task); ret }; // Only notify if not currently parked. If `park` is `None`, then the // scheduling is from a resource driver. As notifications often come in // batches, the notification is delayed until the park is complete. if should_notify && core.park.is_some() { self.notify_parked(); } } pub(super) fn close(&self) { if self.inject.close() { self.notify_all(); } } fn notify_parked(&self) { if let Some(index) = self.idle.worker_to_notify() { self.remotes[index].unpark.unpark(); } } fn notify_all(&self) { for remote in &self.remotes[..] { remote.unpark.unpark(); } } fn notify_if_work_pending(&self) { for remote in &self.remotes[..] { if !remote.steal.is_empty() { self.notify_parked(); return; } } if !self.inject.is_empty() { self.notify_parked(); } } fn transition_worker_from_searching(&self) { if self.idle.transition_worker_from_searching() { // We are the final searching worker. Because work was found, we // need to notify another worker. self.notify_parked(); } } /// Signals that a worker has observed the shutdown signal and has replaced /// its core back into its handle. /// /// If all workers have reached this point, the final cleanup is performed. fn shutdown(&self, core: Box<Core>) { let mut cores = self.shutdown_cores.lock(); cores.push(core); if cores.len() != self.remotes.len() { return; } for mut core in cores.drain(..) { core.shutdown(); } // Drain the injection queue while self.inject.pop().is_some() {} } fn ptr_eq(&self, other: &Shared) -> bool { self as *const _ == other as *const _ } }