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//! Thread pool for blocking operations
use crate::loom::sync::{Arc, Condvar, Mutex};
use crate::loom::thread;
use crate::runtime::blocking::schedule::BlockingSchedule;
use crate::runtime::blocking::{shutdown, BlockingTask};
use crate::runtime::builder::ThreadNameFn;
use crate::runtime::task::{self, JoinHandle};
use crate::runtime::{Builder, Callback, Handle};
use std::collections::{HashMap, VecDeque};
use std::fmt;
use std::io;
use std::sync::atomic::{AtomicUsize, Ordering};
use std::time::Duration;
pub(crate) struct BlockingPool {
spawner: Spawner,
shutdown_rx: shutdown::Receiver,
}
#[derive(Clone)]
pub(crate) struct Spawner {
inner: Arc<Inner>,
}
#[derive(Default)]
pub(crate) struct SpawnerMetrics {
num_threads: AtomicUsize,
num_idle_threads: AtomicUsize,
queue_depth: AtomicUsize,
}
impl SpawnerMetrics {
fn num_threads(&self) -> usize {
self.num_threads.load(Ordering::Relaxed)
}
fn num_idle_threads(&self) -> usize {
self.num_idle_threads.load(Ordering::Relaxed)
}
cfg_metrics! {
fn queue_depth(&self) -> usize {
self.queue_depth.load(Ordering::Relaxed)
}
}
fn inc_num_threads(&self) {
self.num_threads.fetch_add(1, Ordering::Relaxed);
}
fn dec_num_threads(&self) {
self.num_threads.fetch_sub(1, Ordering::Relaxed);
}
fn inc_num_idle_threads(&self) {
self.num_idle_threads.fetch_add(1, Ordering::Relaxed);
}
fn dec_num_idle_threads(&self) -> usize {
self.num_idle_threads.fetch_sub(1, Ordering::Relaxed)
}
fn inc_queue_depth(&self) {
self.queue_depth.fetch_add(1, Ordering::Relaxed);
}
fn dec_queue_depth(&self) {
self.queue_depth.fetch_sub(1, Ordering::Relaxed);
}
}
struct Inner {
/// State shared between worker threads.
shared: Mutex<Shared>,
/// Pool threads wait on this.
condvar: Condvar,
/// Spawned threads use this name.
thread_name: ThreadNameFn,
/// Spawned thread stack size.
stack_size: Option<usize>,
/// Call after a thread starts.
after_start: Option<Callback>,
/// Call before a thread stops.
before_stop: Option<Callback>,
// Maximum number of threads.
thread_cap: usize,
// Customizable wait timeout.
keep_alive: Duration,
// Metrics about the pool.
metrics: SpawnerMetrics,
}
struct Shared {
queue: VecDeque<Task>,
num_notify: u32,
shutdown: bool,
shutdown_tx: Option<shutdown::Sender>,
/// Prior to shutdown, we clean up JoinHandles by having each timed-out
/// thread join on the previous timed-out thread. This is not strictly
/// necessary but helps avoid Valgrind false positives, see
/// for more information.
last_exiting_thread: Option<thread::JoinHandle<()>>,
/// This holds the JoinHandles for all running threads; on shutdown, the thread
/// calling shutdown handles joining on these.
worker_threads: HashMap<usize, thread::JoinHandle<()>>,
/// This is a counter used to iterate worker_threads in a consistent order (for loom's
/// benefit).
worker_thread_index: usize,
}
pub(crate) struct Task {
task: task::UnownedTask<BlockingSchedule>,
mandatory: Mandatory,
}
#[derive(PartialEq, Eq)]
pub(crate) enum Mandatory {
#[cfg_attr(not(fs), allow(dead_code))]
Mandatory,
NonMandatory,
}
pub(crate) enum SpawnError {
/// Pool is shutting down and the task was not scheduled
ShuttingDown,
/// There are no worker threads available to take the task
/// and the OS failed to spawn a new one
NoThreads(io::Error),
}
impl From<SpawnError> for io::Error {
fn from(e: SpawnError) -> Self {
match e {
SpawnError::ShuttingDown => {
io::Error::new(io::ErrorKind::Other, "blocking pool shutting down")
}
SpawnError::NoThreads(e) => e,
}
}
}
impl Task {
pub(crate) fn new(task: task::UnownedTask<BlockingSchedule>, mandatory: Mandatory) -> Task {
Task { task, mandatory }
}
fn run(self) {
self.task.run();
}
fn shutdown_or_run_if_mandatory(self) {
match self.mandatory {
Mandatory::NonMandatory => self.task.shutdown(),
Mandatory::Mandatory => self.task.run(),
}
}
}
const KEEP_ALIVE: Duration = Duration::from_secs(10);
/// Runs the provided function on an executor dedicated to blocking operations.
/// Tasks will be scheduled as non-mandatory, meaning they may not get executed
/// in case of runtime shutdown.
#[track_caller]
#[cfg_attr(tokio_wasi, allow(dead_code))]
pub(crate) fn spawn_blocking<F, R>(func: F) -> JoinHandle<R>
where
F: FnOnce() -> R + Send + 'static,
R: Send + 'static,
{
let rt = Handle::current();
rt.spawn_blocking(func)
}
cfg_fs! {
#[cfg_attr(any(
all(loom, not(test)), // the function is covered by loom tests
test
), allow(dead_code))]
/// Runs the provided function on an executor dedicated to blocking
/// operations. Tasks will be scheduled as mandatory, meaning they are
/// guaranteed to run unless a shutdown is already taking place. In case a
/// shutdown is already taking place, `None` will be returned.
pub(crate) fn spawn_mandatory_blocking<F, R>(func: F) -> Option<JoinHandle<R>>
where
F: FnOnce() -> R + Send + 'static,
R: Send + 'static,
{
let rt = Handle::current();
rt.inner.blocking_spawner().spawn_mandatory_blocking(&rt, func)
}
}
// ===== impl BlockingPool =====
impl BlockingPool {
pub(crate) fn new(builder: &Builder, thread_cap: usize) -> BlockingPool {
let (shutdown_tx, shutdown_rx) = shutdown::channel();
let keep_alive = builder.keep_alive.unwrap_or(KEEP_ALIVE);
BlockingPool {
spawner: Spawner {
inner: Arc::new(Inner {
shared: Mutex::new(Shared {
queue: VecDeque::new(),
num_notify: 0,
shutdown: false,
shutdown_tx: Some(shutdown_tx),
last_exiting_thread: None,
worker_threads: HashMap::new(),
worker_thread_index: 0,
}),
condvar: Condvar::new(),
thread_name: builder.thread_name.clone(),
stack_size: builder.thread_stack_size,
after_start: builder.after_start.clone(),
before_stop: builder.before_stop.clone(),
thread_cap,
keep_alive,
metrics: Default::default(),
}),
},
shutdown_rx,
}
}
pub(crate) fn spawner(&self) -> &Spawner {
&self.spawner
}
pub(crate) fn shutdown(&mut self, timeout: Option<Duration>) {
let mut shared = self.spawner.inner.shared.lock();
// The function can be called multiple times. First, by explicitly
// calling `shutdown` then by the drop handler calling `shutdown`. This
// prevents shutting down twice.
if shared.shutdown {
return;
}
shared.shutdown = true;
shared.shutdown_tx = None;
self.spawner.inner.condvar.notify_all();
let last_exited_thread = std::mem::take(&mut shared.last_exiting_thread);
let workers = std::mem::take(&mut shared.worker_threads);
drop(shared);
if self.shutdown_rx.wait(timeout) {
let _ = last_exited_thread.map(|th| th.join());
// Loom requires that execution be deterministic, so sort by thread ID before joining.
// (HashMaps use a randomly-seeded hash function, so the order is nondeterministic)
let mut workers: Vec<(usize, thread::JoinHandle<()>)> = workers.into_iter().collect();
workers.sort_by_key(|(id, _)| *id);
for (_id, handle) in workers.into_iter() {
let _ = handle.join();
}
}
}
}
impl Drop for BlockingPool {
fn drop(&mut self) {
self.shutdown(None);
}
}
impl fmt::Debug for BlockingPool {
fn fmt(&self, fmt: &mut fmt::Formatter<'_>) -> fmt::Result {
fmt.debug_struct("BlockingPool").finish()
}
}
// ===== impl Spawner =====
impl Spawner {
#[track_caller]
pub(crate) fn spawn_blocking<F, R>(&self, rt: &Handle, func: F) -> JoinHandle<R>
where
F: FnOnce() -> R + Send + 'static,
R: Send + 'static,
{
let (join_handle, spawn_result) =
if cfg!(debug_assertions) && std::mem::size_of::<F>() > 2048 {
self.spawn_blocking_inner(Box::new(func), Mandatory::NonMandatory, None, rt)
} else {
self.spawn_blocking_inner(func, Mandatory::NonMandatory, None, rt)
};
match spawn_result {
Ok(()) => join_handle,
// Compat: do not panic here, return the join_handle even though it will never resolve
Err(SpawnError::ShuttingDown) => join_handle,
Err(SpawnError::NoThreads(e)) => {
panic!("OS can't spawn worker thread: {}", e)
}
}
}
cfg_fs! {
#[track_caller]
#[cfg_attr(any(
all(loom, not(test)), // the function is covered by loom tests
test
), allow(dead_code))]
pub(crate) fn spawn_mandatory_blocking<F, R>(&self, rt: &Handle, func: F) -> Option<JoinHandle<R>>
where
F: FnOnce() -> R + Send + 'static,
R: Send + 'static,
{
let (join_handle, spawn_result) = if cfg!(debug_assertions) && std::mem::size_of::<F>() > 2048 {
self.spawn_blocking_inner(
Box::new(func),
Mandatory::Mandatory,
None,
rt,
)
} else {
self.spawn_blocking_inner(
func,
Mandatory::Mandatory,
None,
rt,
)
};
if spawn_result.is_ok() {
Some(join_handle)
} else {
None
}
}
}
#[track_caller]
pub(crate) fn spawn_blocking_inner<F, R>(
&self,
func: F,
is_mandatory: Mandatory,
name: Option<&str>,
rt: &Handle,
) -> (JoinHandle<R>, Result<(), SpawnError>)
where
F: FnOnce() -> R + Send + 'static,
R: Send + 'static,
{
let fut = BlockingTask::new(func);
let id = task::Id::next();
#[cfg(all(tokio_unstable, feature = "tracing"))]
let fut = {
use tracing::Instrument;
let location = std::panic::Location::caller();
let span = tracing::trace_span!(
target: "tokio::task::blocking",
"runtime.spawn",
kind = %"blocking",
task.name = %name.unwrap_or_default(),
task.id = id.as_u64(),
"fn" = %std::any::type_name::<F>(),
loc.file = location.file(),
loc.line = location.line(),
loc.col = location.column(),
);
fut.instrument(span)
};
#[cfg(not(all(tokio_unstable, feature = "tracing")))]
let _ = name;
let (task, handle) = task::unowned(fut, BlockingSchedule::new(rt), id);
let spawned = self.spawn_task(Task::new(task, is_mandatory), rt);
(handle, spawned)
}
fn spawn_task(&self, task: Task, rt: &Handle) -> Result<(), SpawnError> {
let mut shared = self.inner.shared.lock();
if shared.shutdown {
// Shutdown the task: it's fine to shutdown this task (even if
// mandatory) because it was scheduled after the shutdown of the
// runtime began.
task.task.shutdown();
// no need to even push this task; it would never get picked up
return Err(SpawnError::ShuttingDown);
}
shared.queue.push_back(task);
self.inner.metrics.inc_queue_depth();
if self.inner.metrics.num_idle_threads() == 0 {
// No threads are able to process the task.
if self.inner.metrics.num_threads() == self.inner.thread_cap {
// At max number of threads
} else {
assert!(shared.shutdown_tx.is_some());
let shutdown_tx = shared.shutdown_tx.clone();
if let Some(shutdown_tx) = shutdown_tx {
let id = shared.worker_thread_index;
match self.spawn_thread(shutdown_tx, rt, id) {
Ok(handle) => {
self.inner.metrics.inc_num_threads();
shared.worker_thread_index += 1;
shared.worker_threads.insert(id, handle);
}
Err(ref e)
if is_temporary_os_thread_error(e)
&& self.inner.metrics.num_threads() > 0 =>
{
// OS temporarily failed to spawn a new thread.
// The task will be picked up eventually by a currently
// busy thread.
}
Err(e) => {
// The OS refused to spawn the thread and there is no thread
// to pick up the task that has just been pushed to the queue.
return Err(SpawnError::NoThreads(e));
}
}
}
}
} else {
// Notify an idle worker thread. The notification counter
// is used to count the needed amount of notifications
// exactly. Thread libraries may generate spurious
// wakeups, this counter is used to keep us in a
// consistent state.
self.inner.metrics.dec_num_idle_threads();
shared.num_notify += 1;
self.inner.condvar.notify_one();
}
Ok(())
}
fn spawn_thread(
&self,
shutdown_tx: shutdown::Sender,
rt: &Handle,
id: usize,
) -> std::io::Result<thread::JoinHandle<()>> {
let mut builder = thread::Builder::new().name((self.inner.thread_name)());
if let Some(stack_size) = self.inner.stack_size {
builder = builder.stack_size(stack_size);
}
let rt = rt.clone();
builder.spawn(move || {
// Only the reference should be moved into the closure
let _enter = rt.enter();
rt.inner.blocking_spawner().inner.run(id);
drop(shutdown_tx);
})
}
}
cfg_metrics! {
impl Spawner {
pub(crate) fn num_threads(&self) -> usize {
self.inner.metrics.num_threads()
}
pub(crate) fn num_idle_threads(&self) -> usize {
self.inner.metrics.num_idle_threads()
}
pub(crate) fn queue_depth(&self) -> usize {
self.inner.metrics.queue_depth()
}
}
}
// Tells whether the error when spawning a thread is temporary.
#[inline]
fn is_temporary_os_thread_error(error: &std::io::Error) -> bool {
matches!(error.kind(), std::io::ErrorKind::WouldBlock)
}
impl Inner {
fn run(&self, worker_thread_id: usize) {
if let Some(f) = &self.after_start {
f()
}
let mut shared = self.shared.lock();
let mut join_on_thread = None;
'main: loop {
// BUSY
while let Some(task) = shared.queue.pop_front() {
self.metrics.dec_queue_depth();
drop(shared);
task.run();
shared = self.shared.lock();
}
// IDLE
self.metrics.inc_num_idle_threads();
while !shared.shutdown {
let lock_result = self.condvar.wait_timeout(shared, self.keep_alive).unwrap();
shared = lock_result.0;
let timeout_result = lock_result.1;
if shared.num_notify != 0 {
// We have received a legitimate wakeup,
// acknowledge it by decrementing the counter
// and transition to the BUSY state.
shared.num_notify -= 1;
break;
}
// Even if the condvar "timed out", if the pool is entering the
// shutdown phase, we want to perform the cleanup logic.
if !shared.shutdown && timeout_result.timed_out() {
// We'll join the prior timed-out thread's JoinHandle after dropping the lock.
// This isn't done when shutting down, because the thread calling shutdown will
// handle joining everything.
let my_handle = shared.worker_threads.remove(&worker_thread_id);
join_on_thread = std::mem::replace(&mut shared.last_exiting_thread, my_handle);
break 'main;
}
// Spurious wakeup detected, go back to sleep.
}
if shared.shutdown {
// Drain the queue
while let Some(task) = shared.queue.pop_front() {
self.metrics.dec_queue_depth();
drop(shared);
task.shutdown_or_run_if_mandatory();
shared = self.shared.lock();
}
// Work was produced, and we "took" it (by decrementing num_notify).
// This means that num_idle was decremented once for our wakeup.
// But, since we are exiting, we need to "undo" that, as we'll stay idle.
self.metrics.inc_num_idle_threads();
// NOTE: Technically we should also do num_notify++ and notify again,
// but since we're shutting down anyway, that won't be necessary.
break;
}
}
// Thread exit
self.metrics.dec_num_threads();
// num_idle should now be tracked exactly, panic
// with a descriptive message if it is not the
// case.
let prev_idle = self.metrics.dec_num_idle_threads();
if prev_idle < self.metrics.num_idle_threads() {
panic!("num_idle_threads underflowed on thread exit")
}
if shared.shutdown && self.metrics.num_threads() == 0 {
self.condvar.notify_one();
}
drop(shared);
if let Some(f) = &self.before_stop {
f()
}
if let Some(handle) = join_on_thread {
let _ = handle.join();
}
}
}
impl fmt::Debug for Spawner {
fn fmt(&self, fmt: &mut fmt::Formatter<'_>) -> fmt::Result {
fmt.debug_struct("blocking::Spawner").finish()
}
}