1#![cfg_attr(not(feature = "full"), allow(dead_code))]
2
3//! Yield points for improved cooperative scheduling.
4//!
5//! Documentation for this can be found in the [`tokio::task`] module.
6//!
7//! [`tokio::task`]: crate::task.
8
9// ```ignore
10// # use tokio_stream::{Stream, StreamExt};
11// async fn drop_all<I: Stream + Unpin>(mut input: I) {
12// while let Some(_) = input.next().await {
13// tokio::coop::proceed().await;
14// }
15// }
16// ```
17//
18// The `proceed` future will coordinate with the executor to make sure that
19// every so often control is yielded back to the executor so it can run other
20// tasks.
21//
22// # Placing yield points
23//
24// Voluntary yield points should be placed _after_ at least some work has been
25// done. If they are not, a future sufficiently deep in the task hierarchy may
26// end up _never_ getting to run because of the number of yield points that
27// inevitably appear before it is reached. In general, you will want yield
28// points to only appear in "leaf" futures -- those that do not themselves poll
29// other futures. By doing this, you avoid double-counting each iteration of
30// the outer future against the cooperating budget.
31
32use crate::runtime::context;
33
34/// Opaque type tracking the amount of "work" a task may still do before
35/// yielding back to the scheduler.
36#[derive(Debug, Copy, Clone)]
37pub(crate) struct Budget(Option<u8>);
38
39pub(crate) struct BudgetDecrement {
40 success: bool,
41 hit_zero: bool,
42}
43
44impl Budget {
45 /// Budget assigned to a task on each poll.
46 ///
47 /// The value itself is chosen somewhat arbitrarily. It needs to be high
48 /// enough to amortize wakeup and scheduling costs, but low enough that we
49 /// do not starve other tasks for too long. The value also needs to be high
50 /// enough that particularly deep tasks are able to do at least some useful
51 /// work at all.
52 ///
53 /// Note that as more yield points are added in the ecosystem, this value
54 /// will probably also have to be raised.
55 const fn initial() -> Budget {
56 Budget(Some(128))
57 }
58
59 /// Returns an unconstrained budget. Operations will not be limited.
60 pub(super) const fn unconstrained() -> Budget {
61 Budget(None)
62 }
63
64 fn has_remaining(self) -> bool {
65 self.0.map_or(default:true, |budget: u8| budget > 0)
66 }
67}
68
69/// Runs the given closure with a cooperative task budget. When the function
70/// returns, the budget is reset to the value prior to calling the function.
71#[inline(always)]
72pub(crate) fn budget<R>(f: impl FnOnce() -> R) -> R {
73 with_budget(Budget::initial(), f)
74}
75
76/// Runs the given closure with an unconstrained task budget. When the function returns, the budget
77/// is reset to the value prior to calling the function.
78#[inline(always)]
79pub(crate) fn with_unconstrained<R>(f: impl FnOnce() -> R) -> R {
80 with_budget(Budget::unconstrained(), f)
81}
82
83#[inline(always)]
84fn with_budget<R>(budget: Budget, f: impl FnOnce() -> R) -> R {
85 struct ResetGuard {
86 prev: Budget,
87 }
88
89 impl Drop for ResetGuard {
90 fn drop(&mut self) {
91 let _ = context::budget(|cell| {
92 cell.set(self.prev);
93 });
94 }
95 }
96
97 #[allow(unused_variables)]
98 let maybe_guard = context::budget(|cell| {
99 let prev = cell.get();
100 cell.set(budget);
101
102 ResetGuard { prev }
103 });
104
105 // The function is called regardless even if the budget is not successfully
106 // set due to the thread-local being destroyed.
107 f()
108}
109
110#[inline(always)]
111pub(crate) fn has_budget_remaining() -> bool {
112 // If the current budget cannot be accessed due to the thread-local being
113 // shutdown, then we assume there is budget remaining.
114 context::budget(|cell| cell.get().has_remaining()).unwrap_or(default:true)
115}
116
117cfg_rt_multi_thread! {
118 /// Sets the current task's budget.
119 pub(crate) fn set(budget: Budget) {
120 let _ = context::budget(|cell| cell.set(budget));
121 }
122}
123
124cfg_rt! {
125 /// Forcibly removes the budgeting constraints early.
126 ///
127 /// Returns the remaining budget
128 pub(crate) fn stop() -> Budget {
129 context::budget(|cell| {
130 let prev = cell.get();
131 cell.set(Budget::unconstrained());
132 prev
133 }).unwrap_or(Budget::unconstrained())
134 }
135}
136
137cfg_coop! {
138 use std::cell::Cell;
139 use std::task::{Context, Poll};
140
141 #[must_use]
142 pub(crate) struct RestoreOnPending(Cell<Budget>);
143
144 impl RestoreOnPending {
145 pub(crate) fn made_progress(&self) {
146 self.0.set(Budget::unconstrained());
147 }
148 }
149
150 impl Drop for RestoreOnPending {
151 fn drop(&mut self) {
152 // Don't reset if budget was unconstrained or if we made progress.
153 // They are both represented as the remembered budget being unconstrained.
154 let budget = self.0.get();
155 if !budget.is_unconstrained() {
156 let _ = context::budget(|cell| {
157 cell.set(budget);
158 });
159 }
160 }
161 }
162
163 /// Returns `Poll::Pending` if the current task has exceeded its budget and should yield.
164 ///
165 /// When you call this method, the current budget is decremented. However, to ensure that
166 /// progress is made every time a task is polled, the budget is automatically restored to its
167 /// former value if the returned `RestoreOnPending` is dropped. It is the caller's
168 /// responsibility to call `RestoreOnPending::made_progress` if it made progress, to ensure
169 /// that the budget empties appropriately.
170 ///
171 /// Note that `RestoreOnPending` restores the budget **as it was before `poll_proceed`**.
172 /// Therefore, if the budget is _further_ adjusted between when `poll_proceed` returns and
173 /// `RestRestoreOnPending` is dropped, those adjustments are erased unless the caller indicates
174 /// that progress was made.
175 #[inline]
176 pub(crate) fn poll_proceed(cx: &mut Context<'_>) -> Poll<RestoreOnPending> {
177 context::budget(|cell| {
178 let mut budget = cell.get();
179
180 let decrement = budget.decrement();
181
182 if decrement.success {
183 let restore = RestoreOnPending(Cell::new(cell.get()));
184 cell.set(budget);
185
186 // avoid double counting
187 if decrement.hit_zero {
188 inc_budget_forced_yield_count();
189 }
190
191 Poll::Ready(restore)
192 } else {
193 cx.waker().wake_by_ref();
194 Poll::Pending
195 }
196 }).unwrap_or(Poll::Ready(RestoreOnPending(Cell::new(Budget::unconstrained()))))
197 }
198
199 cfg_rt! {
200 cfg_metrics! {
201 #[inline(always)]
202 fn inc_budget_forced_yield_count() {
203 let _ = context::with_current(|handle| {
204 handle.scheduler_metrics().inc_budget_forced_yield_count();
205 });
206 }
207 }
208
209 cfg_not_metrics! {
210 #[inline(always)]
211 fn inc_budget_forced_yield_count() {}
212 }
213 }
214
215 cfg_not_rt! {
216 #[inline(always)]
217 fn inc_budget_forced_yield_count() {}
218 }
219
220 impl Budget {
221 /// Decrements the budget. Returns `true` if successful. Decrementing fails
222 /// when there is not enough remaining budget.
223 fn decrement(&mut self) -> BudgetDecrement {
224 if let Some(num) = &mut self.0 {
225 if *num > 0 {
226 *num -= 1;
227
228 let hit_zero = *num == 0;
229
230 BudgetDecrement { success: true, hit_zero }
231 } else {
232 BudgetDecrement { success: false, hit_zero: false }
233 }
234 } else {
235 BudgetDecrement { success: true, hit_zero: false }
236 }
237 }
238
239 fn is_unconstrained(self) -> bool {
240 self.0.is_none()
241 }
242 }
243}
244
245#[cfg(all(test, not(loom)))]
246mod test {
247 use super::*;
248
249 #[cfg(all(target_family = "wasm", not(target_os = "wasi")))]
250 use wasm_bindgen_test::wasm_bindgen_test as test;
251
252 fn get() -> Budget {
253 context::budget(|cell| cell.get()).unwrap_or(Budget::unconstrained())
254 }
255
256 #[test]
257 fn budgeting() {
258 use futures::future::poll_fn;
259 use tokio_test::*;
260
261 assert!(get().0.is_none());
262
263 let coop = assert_ready!(task::spawn(()).enter(|cx, _| poll_proceed(cx)));
264
265 assert!(get().0.is_none());
266 drop(coop);
267 assert!(get().0.is_none());
268
269 budget(|| {
270 assert_eq!(get().0, Budget::initial().0);
271
272 let coop = assert_ready!(task::spawn(()).enter(|cx, _| poll_proceed(cx)));
273 assert_eq!(get().0.unwrap(), Budget::initial().0.unwrap() - 1);
274 drop(coop);
275 // we didn't make progress
276 assert_eq!(get().0, Budget::initial().0);
277
278 let coop = assert_ready!(task::spawn(()).enter(|cx, _| poll_proceed(cx)));
279 assert_eq!(get().0.unwrap(), Budget::initial().0.unwrap() - 1);
280 coop.made_progress();
281 drop(coop);
282 // we _did_ make progress
283 assert_eq!(get().0.unwrap(), Budget::initial().0.unwrap() - 1);
284
285 let coop = assert_ready!(task::spawn(()).enter(|cx, _| poll_proceed(cx)));
286 assert_eq!(get().0.unwrap(), Budget::initial().0.unwrap() - 2);
287 coop.made_progress();
288 drop(coop);
289 assert_eq!(get().0.unwrap(), Budget::initial().0.unwrap() - 2);
290
291 budget(|| {
292 assert_eq!(get().0, Budget::initial().0);
293
294 let coop = assert_ready!(task::spawn(()).enter(|cx, _| poll_proceed(cx)));
295 assert_eq!(get().0.unwrap(), Budget::initial().0.unwrap() - 1);
296 coop.made_progress();
297 drop(coop);
298 assert_eq!(get().0.unwrap(), Budget::initial().0.unwrap() - 1);
299 });
300
301 assert_eq!(get().0.unwrap(), Budget::initial().0.unwrap() - 2);
302 });
303
304 assert!(get().0.is_none());
305
306 budget(|| {
307 let n = get().0.unwrap();
308
309 for _ in 0..n {
310 let coop = assert_ready!(task::spawn(()).enter(|cx, _| poll_proceed(cx)));
311 coop.made_progress();
312 }
313
314 let mut task = task::spawn(poll_fn(|cx| {
315 let coop = ready!(poll_proceed(cx));
316 coop.made_progress();
317 Poll::Ready(())
318 }));
319
320 assert_pending!(task.poll());
321 });
322 }
323}
324