path: root/tokio/src/runtime/basic_scheduler.rs

use crate::future::poll_fn;
use crate::loom::sync::Mutex;
use crate::park::{Park, Unpark};
use crate::runtime::task::{self, JoinHandle, Schedule, Task};
use crate::sync::notify::Notify;
use crate::util::linked_list::{Link, LinkedList};
use crate::util::{waker_ref, Wake, WakerRef};

use std::cell::RefCell;
use std::collections::VecDeque;
use std::fmt;
use std::future::Future;
use std::sync::Arc;
use std::task::Poll::{Pending, Ready};
use std::time::Duration;

/// Executes tasks on the current thread
pub(crate) struct BasicScheduler<P: Park> {
    /// Inner state guarded by a mutex that is shared
    /// between all `block_on` calls.
    inner: Mutex<Option<Inner<P>>>,

    /// Notifier for waking up other threads to steal the
    /// parker.
    notify: Notify,

    /// Sendable task spawner
    spawner: Spawner,
}

/// The inner scheduler that owns the task queue and the main parker P.
struct Inner<P: Park> {
    /// Scheduler run queue
    ///
    /// When the scheduler is executed, the queue is removed from `self` and
    /// moved into `Context`.
    ///
    /// This indirection is to allow `BasicScheduler` to be `Send`.
    tasks: Option<Tasks>,

    /// Sendable task spawner
    spawner: Spawner,

    /// Current tick
    tick: u8,

    /// Thread park handle
    park: P,
}

#[derive(Clone)]
pub(crate) struct Spawner {
    shared: Arc<Shared>,
}

struct Tasks {
    /// Collection of all active tasks spawned onto this executor.
    owned: LinkedList<Task<Arc<Shared>>, <Task<Arc<Shared>> as Link>::Target>,

    /// Local run queue.
    ///
    /// Tasks notified from the current thread are pushed into this queue.
    queue: VecDeque<task::Notified<Arc<Shared>>>,
}

/// Scheduler state shared between threads.
struct Shared {
    /// Remote run queue
    queue: Mutex<VecDeque<task::Notified<Arc<Shared>>>>,

    /// Unpark the blocked thread
    unpark: Box<dyn Unpark>,
}

/// Thread-local context.
struct Context {
    /// Shared scheduler state
    shared: Arc<Shared>,

    /// Local queue
    tasks: RefCell<Tasks>,
}

/// Initial queue capacity.
const INITIAL_CAPACITY: usize = 64;

/// Max number of tasks to poll per tick.
const MAX_TASKS_PER_TICK: usize = 61;

/// How often to check the remote queue first.
const REMOTE_FIRST_INTERVAL: u8 = 31;

// Tracks the current BasicScheduler.
scoped_thread_local!(static CURRENT: Context);

impl<P: Park> BasicScheduler<P> {
    pub(crate) fn new(park: P) -> BasicScheduler<P> {
        let unpark = Box::new(park.unpark());

        let spawner = Spawner {
            shared: Arc::new(Shared {
                queue: Mutex::new(VecDeque::with_capacity(INITIAL_CAPACITY)),
                unpark: unpark as Box<dyn Unpark>,
            }),
        };

        let inner = Mutex::new(Some(Inner {
            tasks: Some(Tasks {
                owned: LinkedList::new(),
                queue: VecDeque::with_capacity(INITIAL_CAPACITY),
            }),
            spawner: spawner.clone(),
            tick: 0,
            park,
        }));

        BasicScheduler {
            inner,
            notify: Notify::new(),
            spawner,
        }
    }

    pub(crate) fn spawner(&self) -> &Spawner {
        &self.spawner
    }

    /// Spawns a future onto the thread pool
    pub(crate) fn spawn<F>(&self, future: F) -> JoinHandle<F::Output>
    where
        F: Future + Send + 'static,
        F::Output: Send + 'static,
    {
        self.spawner.spawn(future)
    }

    pub(crate) fn block_on<F: Future>(&self, future: F) -> F::Output {
        pin!(future);

        // Attempt to steal the dedicated parker and block_on the future if we can there,
        // othwerwise, lets select on a notification that the parker is available
        // or the future is complete.
        loop {
            if let Some(inner) = &mut self.take_inner() {
                return inner.block_on(future);
            } else {
                let mut enter = crate::runtime::enter(false);

                let notified = self.notify.notified();
                pin!(notified);

                if let Some(out) = enter
                    .block_on(poll_fn(|cx| {
                        if notified.as_mut().poll(cx).is_ready() {
                            return Ready(None);
                        }

                        if let Ready(out) = future.as_mut().poll(cx) {
                            return Ready(Some(out));
                        }

                        Pending
                    }))
                    .expect("Failed to `Enter::block_on`")
                {
                    return out;
                }
            }
        }
    }

    fn take_inner(&self) -> Option<InnerGuard<'_, P>> {
        let inner = self.inner.lock().take()?;

        Some(InnerGuard {
            inner: Some(inner),
            basic_scheduler: &self,
        })
    }
}

impl<P: Park> Inner<P> {
    /// Block on the future provided and drive the runtime's driver.
    fn block_on<F: Future>(&mut self, future: F) -> F::Output {
        enter(self, |scheduler, context| {
            let _enter = crate::runtime::enter(false);
            let waker = scheduler.spawner.waker_ref();
            let mut cx = std::task::Context::from_waker(&waker);

            pin!(future);

            'outer: loop {
                if let Ready(v) = crate::coop::budget(|| future.as_mut().poll(&mut cx)) {
                    return v;
                }

                for _ in 0..MAX_TASKS_PER_TICK {
                    // Get and increment the current tick
                    let tick = scheduler.tick;
                    scheduler.tick = scheduler.tick.wrapping_add(1);

                    let next = if tick % REMOTE_FIRST_INTERVAL == 0 {
                        scheduler
                            .spawner
                            .pop()
                            .or_else(|| context.tasks.borrow_mut().queue.pop_front())
                    } else {
                        context
                            .tasks
                            .borrow_mut()
                            .queue
                            .pop_front()
                            .or_else(|| scheduler.spawner.pop())
                    };

                    match next {
                        Some(task) => crate::coop::budget(|| task.run()),
                        None => {
                            // Park until the thread is signaled
                            scheduler.park.park().ok().expect("failed to park");

                            // Try polling the `block_on` future next
                            continue 'outer;
                        }
                    }
                }

                // Yield to the park, this drives the timer and pulls any pending
                // I/O events.
                scheduler
                    .park
                    .park_timeout(Duration::from_millis(0))
                    .ok()
                    .expect("failed to park");
            }
        })
    }
}

/// Enter the scheduler context. This sets the queue and other necessary
/// scheduler state in the thread-local
fn enter<F, R, P>(scheduler: &mut Inner<P>, f: F) -> R
where
    F: FnOnce(&mut Inner<P>, &Context) -> R,
    P: Park,
{
    // Ensures the run queue is placed back in the `BasicScheduler` instance
    // once `block_on` returns.`
    struct Guard<'a, P: Park> {
        context: Option<Context>,
        scheduler: &'a mut Inner<P>,
    }

    impl<P: Park> Drop for Guard<'_, P> {
        fn drop(&mut self) {
            let Context { tasks, .. } = self.context.take().expect("context missing");
            self.scheduler.tasks = Some(tasks.into_inner());
        }
    }

    // Remove `tasks` from `self` and place it in a `Context`.
    let tasks = scheduler.tasks.take().expect("invalid state");

    let guard = Guard