| <!DOCTYPE html><html lang="en"><head><meta charset="utf-8"><meta name="viewport" content="width=device-width, initial-scale=1.0"><meta name="generator" content="rustdoc"><meta name="description" content="Source of the Rust file `/root/.cargo/registry/src/github.com-1ecc6299db9ec823/tokio-1.28.2/src/runtime/scheduler/multi_thread/queue.rs`."><meta name="keywords" content="rust, rustlang, rust-lang"><title>queue.rs - source</title><link rel="preload" as="font" type="font/woff2" crossorigin href="../../../../../SourceSerif4-Regular.ttf.woff2"><link rel="preload" as="font" type="font/woff2" crossorigin href="../../../../../FiraSans-Regular.woff2"><link rel="preload" as="font" type="font/woff2" crossorigin href="../../../../../FiraSans-Medium.woff2"><link rel="preload" as="font" type="font/woff2" crossorigin href="../../../../../SourceCodePro-Regular.ttf.woff2"><link rel="preload" as="font" type="font/woff2" crossorigin href="../../../../../SourceSerif4-Bold.ttf.woff2"><link rel="preload" as="font" type="font/woff2" crossorigin href="../../../../../SourceCodePro-Semibold.ttf.woff2"><link rel="stylesheet" href="../../../../../normalize.css"><link rel="stylesheet" href="../../../../../rustdoc.css" id="mainThemeStyle"><link rel="stylesheet" href="../../../../../ayu.css" disabled><link rel="stylesheet" href="../../../../../dark.css" disabled><link rel="stylesheet" href="../../../../../light.css" id="themeStyle"><script id="default-settings" ></script><script src="../../../../../storage.js"></script><script defer src="../../../../../source-script.js"></script><script defer src="../../../../../source-files.js"></script><script defer src="../../../../../main.js"></script><noscript><link rel="stylesheet" href="../../../../../noscript.css"></noscript><link rel="alternate icon" type="image/png" href="../../../../../favicon-16x16.png"><link rel="alternate icon" type="image/png" href="../../../../../favicon-32x32.png"><link rel="icon" type="image/svg+xml" href="../../../../../favicon.svg"></head><body class="rustdoc source"><!--[if lte IE 11]><div class="warning">This old browser is unsupported and will most likely display funky things.</div><![endif]--><nav class="sidebar"><a class="sidebar-logo" href="../../../../../tokio/index.html"><div class="logo-container"><img class="rust-logo" src="../../../../../rust-logo.svg" alt="logo"></div></a></nav><main><div class="width-limiter"><nav class="sub"><a class="sub-logo-container" href="../../../../../tokio/index.html"><img class="rust-logo" src="../../../../../rust-logo.svg" alt="logo"></a><form class="search-form"><div class="search-container"><span></span><input class="search-input" name="search" autocomplete="off" spellcheck="false" placeholder="Click or press ‘S’ to search, ‘?’ for more options…" type="search"><div id="help-button" title="help" tabindex="-1"><a href="../../../../../help.html">?</a></div><div id="settings-menu" tabindex="-1"><a href="../../../../../settings.html" title="settings"><img width="22" height="22" alt="Change settings" src="../../../../../wheel.svg"></a></div></div></form></nav><section id="main-content" class="content"><div class="example-wrap"><pre class="src-line-numbers"><span id="1">1</span> |
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| <span id="522">522</span> |
| <span id="523">523</span> |
| <span id="524">524</span> |
| <span id="525">525</span> |
| <span id="526">526</span> |
| <span id="527">527</span> |
| <span id="528">528</span> |
| <span id="529">529</span> |
| <span id="530">530</span> |
| <span id="531">531</span> |
| <span id="532">532</span> |
| <span id="533">533</span> |
| </pre><pre class="rust"><code><span class="doccomment">//! Run-queue structures to support a work-stealing scheduler |
| |
| </span><span class="kw">use </span><span class="kw">crate</span>::loom::cell::UnsafeCell; |
| <span class="kw">use </span><span class="kw">crate</span>::loom::sync::Arc; |
| <span class="kw">use </span><span class="kw">crate</span>::runtime::task::{<span class="self">self</span>, Inject}; |
| <span class="kw">use </span><span class="kw">crate</span>::runtime::MetricsBatch; |
| |
| <span class="kw">use </span>std::mem::{<span class="self">self</span>, MaybeUninit}; |
| <span class="kw">use </span>std::ptr; |
| <span class="kw">use </span>std::sync::atomic::Ordering::{AcqRel, Acquire, Relaxed, Release}; |
| |
| <span class="comment">// Use wider integers when possible to increase ABA resilience. |
| // |
| // See issue #5041: <https://github.com/tokio-rs/tokio/issues/5041>. |
| </span><span class="macro">cfg_has_atomic_u64! </span>{ |
| <span class="kw">type </span>UnsignedShort = u32; |
| <span class="kw">type </span>UnsignedLong = u64; |
| <span class="kw">type </span>AtomicUnsignedShort = <span class="kw">crate</span>::loom::sync::atomic::AtomicU32; |
| <span class="kw">type </span>AtomicUnsignedLong = <span class="kw">crate</span>::loom::sync::atomic::AtomicU64; |
| } |
| <span class="macro">cfg_not_has_atomic_u64! </span>{ |
| <span class="kw">type </span>UnsignedShort = u16; |
| <span class="kw">type </span>UnsignedLong = u32; |
| <span class="kw">type </span>AtomicUnsignedShort = <span class="kw">crate</span>::loom::sync::atomic::AtomicU16; |
| <span class="kw">type </span>AtomicUnsignedLong = <span class="kw">crate</span>::loom::sync::atomic::AtomicU32; |
| } |
| |
| <span class="doccomment">/// Producer handle. May only be used from a single thread. |
| </span><span class="kw">pub</span>(<span class="kw">crate</span>) <span class="kw">struct </span>Local<T: <span class="lifetime">'static</span>> { |
| inner: Arc<Inner<T>>, |
| } |
| |
| <span class="doccomment">/// Consumer handle. May be used from many threads. |
| </span><span class="kw">pub</span>(<span class="kw">crate</span>) <span class="kw">struct </span>Steal<T: <span class="lifetime">'static</span>>(Arc<Inner<T>>); |
| |
| <span class="kw">pub</span>(<span class="kw">crate</span>) <span class="kw">struct </span>Inner<T: <span class="lifetime">'static</span>> { |
| <span class="doccomment">/// Concurrently updated by many threads. |
| /// |
| /// Contains two `UnsignedShort` values. The LSB byte is the "real" head of |
| /// the queue. The `UnsignedShort` in the MSB is set by a stealer in process |
| /// of stealing values. It represents the first value being stolen in the |
| /// batch. The `UnsignedShort` indices are intentionally wider than strictly |
| /// required for buffer indexing in order to provide ABA mitigation and make |
| /// it possible to distinguish between full and empty buffers. |
| /// |
| /// When both `UnsignedShort` values are the same, there is no active |
| /// stealer. |
| /// |
| /// Tracking an in-progress stealer prevents a wrapping scenario. |
| </span>head: AtomicUnsignedLong, |
| |
| <span class="doccomment">/// Only updated by producer thread but read by many threads. |
| </span>tail: AtomicUnsignedShort, |
| |
| <span class="doccomment">/// Elements |
| </span>buffer: Box<[UnsafeCell<MaybeUninit<task::Notified<T>>>; LOCAL_QUEUE_CAPACITY]>, |
| } |
| |
| <span class="kw">unsafe impl</span><T> Send <span class="kw">for </span>Inner<T> {} |
| <span class="kw">unsafe impl</span><T> Sync <span class="kw">for </span>Inner<T> {} |
| |
| <span class="attribute">#[cfg(not(loom))] |
| </span><span class="kw">const </span>LOCAL_QUEUE_CAPACITY: usize = <span class="number">256</span>; |
| |
| <span class="comment">// Shrink the size of the local queue when using loom. This shouldn't impact |
| // logic, but allows loom to test more edge cases in a reasonable a mount of |
| // time. |
| </span><span class="attribute">#[cfg(loom)] |
| </span><span class="kw">const </span>LOCAL_QUEUE_CAPACITY: usize = <span class="number">4</span>; |
| |
| <span class="kw">const </span>MASK: usize = LOCAL_QUEUE_CAPACITY - <span class="number">1</span>; |
| |
| <span class="comment">// Constructing the fixed size array directly is very awkward. The only way to |
| // do it is to repeat `UnsafeCell::new(MaybeUninit::uninit())` 256 times, as |
| // the contents are not Copy. The trick with defining a const doesn't work for |
| // generic types. |
| </span><span class="kw">fn </span>make_fixed_size<T>(buffer: Box<[T]>) -> Box<[T; LOCAL_QUEUE_CAPACITY]> { |
| <span class="macro">assert_eq!</span>(buffer.len(), LOCAL_QUEUE_CAPACITY); |
| |
| <span class="comment">// safety: We check that the length is correct. |
| </span><span class="kw">unsafe </span>{ Box::from_raw(Box::into_raw(buffer).cast()) } |
| } |
| |
| <span class="doccomment">/// Create a new local run-queue |
| </span><span class="kw">pub</span>(<span class="kw">crate</span>) <span class="kw">fn </span>local<T: <span class="lifetime">'static</span>>() -> (Steal<T>, Local<T>) { |
| <span class="kw">let </span><span class="kw-2">mut </span>buffer = Vec::with_capacity(LOCAL_QUEUE_CAPACITY); |
| |
| <span class="kw">for _ in </span><span class="number">0</span>..LOCAL_QUEUE_CAPACITY { |
| buffer.push(UnsafeCell::new(MaybeUninit::uninit())); |
| } |
| |
| <span class="kw">let </span>inner = Arc::new(Inner { |
| head: AtomicUnsignedLong::new(<span class="number">0</span>), |
| tail: AtomicUnsignedShort::new(<span class="number">0</span>), |
| buffer: make_fixed_size(buffer.into_boxed_slice()), |
| }); |
| |
| <span class="kw">let </span>local = Local { |
| inner: inner.clone(), |
| }; |
| |
| <span class="kw">let </span>remote = Steal(inner); |
| |
| (remote, local) |
| } |
| |
| <span class="kw">impl</span><T> Local<T> { |
| <span class="doccomment">/// Returns true if the queue has entries that can be stolen. |
| </span><span class="kw">pub</span>(<span class="kw">crate</span>) <span class="kw">fn </span>is_stealable(<span class="kw-2">&</span><span class="self">self</span>) -> bool { |
| !<span class="self">self</span>.inner.is_empty() |
| } |
| |
| <span class="doccomment">/// Returns false if there are any entries in the queue |
| /// |
| /// Separate to is_stealable so that refactors of is_stealable to "protect" |
| /// some tasks from stealing won't affect this |
| </span><span class="kw">pub</span>(<span class="kw">crate</span>) <span class="kw">fn </span>has_tasks(<span class="kw-2">&</span><span class="self">self</span>) -> bool { |
| !<span class="self">self</span>.inner.is_empty() |
| } |
| |
| <span class="doccomment">/// Pushes a task to the back of the local queue, skipping the LIFO slot. |
| </span><span class="kw">pub</span>(<span class="kw">crate</span>) <span class="kw">fn </span>push_back( |
| <span class="kw-2">&mut </span><span class="self">self</span>, |
| <span class="kw-2">mut </span>task: task::Notified<T>, |
| inject: <span class="kw-2">&</span>Inject<T>, |
| metrics: <span class="kw-2">&mut </span>MetricsBatch, |
| ) { |
| <span class="kw">let </span>tail = <span class="kw">loop </span>{ |
| <span class="kw">let </span>head = <span class="self">self</span>.inner.head.load(Acquire); |
| <span class="kw">let </span>(steal, real) = unpack(head); |
| |
| <span class="comment">// safety: this is the **only** thread that updates this cell. |
| </span><span class="kw">let </span>tail = <span class="kw">unsafe </span>{ <span class="self">self</span>.inner.tail.unsync_load() }; |
| |
| <span class="kw">if </span>tail.wrapping_sub(steal) < LOCAL_QUEUE_CAPACITY <span class="kw">as </span>UnsignedShort { |
| <span class="comment">// There is capacity for the task |
| </span><span class="kw">break </span>tail; |
| } <span class="kw">else if </span>steal != real { |
| <span class="comment">// Concurrently stealing, this will free up capacity, so only |
| // push the task onto the inject queue |
| </span>inject.push(task); |
| <span class="kw">return</span>; |
| } <span class="kw">else </span>{ |
| <span class="comment">// Push the current task and half of the queue into the |
| // inject queue. |
| </span><span class="kw">match </span><span class="self">self</span>.push_overflow(task, real, tail, inject, metrics) { |
| <span class="prelude-val">Ok</span>(<span class="kw">_</span>) => <span class="kw">return</span>, |
| <span class="comment">// Lost the race, try again |
| </span><span class="prelude-val">Err</span>(v) => { |
| task = v; |
| } |
| } |
| } |
| }; |
| |
| <span class="comment">// Map the position to a slot index. |
| </span><span class="kw">let </span>idx = tail <span class="kw">as </span>usize & MASK; |
| |
| <span class="self">self</span>.inner.buffer[idx].with_mut(|ptr| { |
| <span class="comment">// Write the task to the slot |
| // |
| // Safety: There is only one producer and the above `if` |
| // condition ensures we don't touch a cell if there is a |
| // value, thus no consumer. |
| </span><span class="kw">unsafe </span>{ |
| ptr::write((<span class="kw-2">*</span>ptr).as_mut_ptr(), task); |
| } |
| }); |
| |
| <span class="comment">// Make the task available. Synchronizes with a load in |
| // `steal_into2`. |
| </span><span class="self">self</span>.inner.tail.store(tail.wrapping_add(<span class="number">1</span>), Release); |
| } |
| |
| <span class="doccomment">/// Moves a batch of tasks into the inject queue. |
| /// |
| /// This will temporarily make some of the tasks unavailable to stealers. |
| /// Once `push_overflow` is done, a notification is sent out, so if other |
| /// workers "missed" some of the tasks during a steal, they will get |
| /// another opportunity. |
| </span><span class="attribute">#[inline(never)] |
| </span><span class="kw">fn </span>push_overflow( |
| <span class="kw-2">&mut </span><span class="self">self</span>, |
| task: task::Notified<T>, |
| head: UnsignedShort, |
| tail: UnsignedShort, |
| inject: <span class="kw-2">&</span>Inject<T>, |
| metrics: <span class="kw-2">&mut </span>MetricsBatch, |
| ) -> <span class="prelude-ty">Result</span><(), task::Notified<T>> { |
| <span class="doccomment">/// How many elements are we taking from the local queue. |
| /// |
| /// This is one less than the number of tasks pushed to the inject |
| /// queue as we are also inserting the `task` argument. |
| </span><span class="kw">const </span>NUM_TASKS_TAKEN: UnsignedShort = (LOCAL_QUEUE_CAPACITY / <span class="number">2</span>) <span class="kw">as </span>UnsignedShort; |
| |
| <span class="macro">assert_eq!</span>( |
| tail.wrapping_sub(head) <span class="kw">as </span>usize, |
| LOCAL_QUEUE_CAPACITY, |
| <span class="string">"queue is not full; tail = {}; head = {}"</span>, |
| tail, |
| head |
| ); |
| |
| <span class="kw">let </span>prev = pack(head, head); |
| |
| <span class="comment">// Claim a bunch of tasks |
| // |
| // We are claiming the tasks **before** reading them out of the buffer. |
| // This is safe because only the **current** thread is able to push new |
| // tasks. |
| // |
| // There isn't really any need for memory ordering... Relaxed would |
| // work. This is because all tasks are pushed into the queue from the |
| // current thread (or memory has been acquired if the local queue handle |
| // moved). |
| </span><span class="kw">if </span><span class="self">self |
| </span>.inner |
| .head |
| .compare_exchange( |
| prev, |
| pack( |
| head.wrapping_add(NUM_TASKS_TAKEN), |
| head.wrapping_add(NUM_TASKS_TAKEN), |
| ), |
| Release, |
| Relaxed, |
| ) |
| .is_err() |
| { |
| <span class="comment">// We failed to claim the tasks, losing the race. Return out of |
| // this function and try the full `push` routine again. The queue |
| // may not be full anymore. |
| </span><span class="kw">return </span><span class="prelude-val">Err</span>(task); |
| } |
| |
| <span class="doccomment">/// An iterator that takes elements out of the run queue. |
| </span><span class="kw">struct </span>BatchTaskIter<<span class="lifetime">'a</span>, T: <span class="lifetime">'static</span>> { |
| buffer: <span class="kw-2">&</span><span class="lifetime">'a </span>[UnsafeCell<MaybeUninit<task::Notified<T>>>; LOCAL_QUEUE_CAPACITY], |
| head: UnsignedLong, |
| i: UnsignedLong, |
| } |
| <span class="kw">impl</span><<span class="lifetime">'a</span>, T: <span class="lifetime">'static</span>> Iterator <span class="kw">for </span>BatchTaskIter<<span class="lifetime">'a</span>, T> { |
| <span class="kw">type </span>Item = task::Notified<T>; |
| |
| <span class="attribute">#[inline] |
| </span><span class="kw">fn </span>next(<span class="kw-2">&mut </span><span class="self">self</span>) -> <span class="prelude-ty">Option</span><task::Notified<T>> { |
| <span class="kw">if </span><span class="self">self</span>.i == UnsignedLong::from(NUM_TASKS_TAKEN) { |
| <span class="prelude-val">None |
| </span>} <span class="kw">else </span>{ |
| <span class="kw">let </span>i_idx = <span class="self">self</span>.i.wrapping_add(<span class="self">self</span>.head) <span class="kw">as </span>usize & MASK; |
| <span class="kw">let </span>slot = <span class="kw-2">&</span><span class="self">self</span>.buffer[i_idx]; |
| |
| <span class="comment">// safety: Our CAS from before has assumed exclusive ownership |
| // of the task pointers in this range. |
| </span><span class="kw">let </span>task = slot.with(|ptr| <span class="kw">unsafe </span>{ ptr::read((<span class="kw-2">*</span>ptr).as_ptr()) }); |
| |
| <span class="self">self</span>.i += <span class="number">1</span>; |
| <span class="prelude-val">Some</span>(task) |
| } |
| } |
| } |
| |
| <span class="comment">// safety: The CAS above ensures that no consumer will look at these |
| // values again, and we are the only producer. |
| </span><span class="kw">let </span>batch_iter = BatchTaskIter { |
| buffer: <span class="kw-2">&</span><span class="self">self</span>.inner.buffer, |
| head: head <span class="kw">as </span>UnsignedLong, |
| i: <span class="number">0</span>, |
| }; |
| inject.push_batch(batch_iter.chain(std::iter::once(task))); |
| |
| <span class="comment">// Add 1 to factor in the task currently being scheduled. |
| </span>metrics.incr_overflow_count(); |
| |
| <span class="prelude-val">Ok</span>(()) |
| } |
| |
| <span class="doccomment">/// Pops a task from the local queue. |
| </span><span class="kw">pub</span>(<span class="kw">crate</span>) <span class="kw">fn </span>pop(<span class="kw-2">&mut </span><span class="self">self</span>) -> <span class="prelude-ty">Option</span><task::Notified<T>> { |
| <span class="kw">let </span><span class="kw-2">mut </span>head = <span class="self">self</span>.inner.head.load(Acquire); |
| |
| <span class="kw">let </span>idx = <span class="kw">loop </span>{ |
| <span class="kw">let </span>(steal, real) = unpack(head); |
| |
| <span class="comment">// safety: this is the **only** thread that updates this cell. |
| </span><span class="kw">let </span>tail = <span class="kw">unsafe </span>{ <span class="self">self</span>.inner.tail.unsync_load() }; |
| |
| <span class="kw">if </span>real == tail { |
| <span class="comment">// queue is empty |
| </span><span class="kw">return </span><span class="prelude-val">None</span>; |
| } |
| |
| <span class="kw">let </span>next_real = real.wrapping_add(<span class="number">1</span>); |
| |
| <span class="comment">// If `steal == real` there are no concurrent stealers. Both `steal` |
| // and `real` are updated. |
| </span><span class="kw">let </span>next = <span class="kw">if </span>steal == real { |
| pack(next_real, next_real) |
| } <span class="kw">else </span>{ |
| <span class="macro">assert_ne!</span>(steal, next_real); |
| pack(steal, next_real) |
| }; |
| |
| <span class="comment">// Attempt to claim a task. |
| </span><span class="kw">let </span>res = <span class="self">self |
| </span>.inner |
| .head |
| .compare_exchange(head, next, AcqRel, Acquire); |
| |
| <span class="kw">match </span>res { |
| <span class="prelude-val">Ok</span>(<span class="kw">_</span>) => <span class="kw">break </span>real <span class="kw">as </span>usize & MASK, |
| <span class="prelude-val">Err</span>(actual) => head = actual, |
| } |
| }; |
| |
| <span class="prelude-val">Some</span>(<span class="self">self</span>.inner.buffer[idx].with(|ptr| <span class="kw">unsafe </span>{ ptr::read(ptr).assume_init() })) |
| } |
| } |
| |
| <span class="kw">impl</span><T> Steal<T> { |
| <span class="kw">pub</span>(<span class="kw">crate</span>) <span class="kw">fn </span>is_empty(<span class="kw-2">&</span><span class="self">self</span>) -> bool { |
| <span class="self">self</span>.<span class="number">0</span>.is_empty() |
| } |
| |
| <span class="doccomment">/// Steals half the tasks from self and place them into `dst`. |
| </span><span class="kw">pub</span>(<span class="kw">crate</span>) <span class="kw">fn </span>steal_into( |
| <span class="kw-2">&</span><span class="self">self</span>, |
| dst: <span class="kw-2">&mut </span>Local<T>, |
| dst_metrics: <span class="kw-2">&mut </span>MetricsBatch, |
| ) -> <span class="prelude-ty">Option</span><task::Notified<T>> { |
| <span class="comment">// Safety: the caller is the only thread that mutates `dst.tail` and |
| // holds a mutable reference. |
| </span><span class="kw">let </span>dst_tail = <span class="kw">unsafe </span>{ dst.inner.tail.unsync_load() }; |
| |
| <span class="comment">// To the caller, `dst` may **look** empty but still have values |
| // contained in the buffer. If another thread is concurrently stealing |
| // from `dst` there may not be enough capacity to steal. |
| </span><span class="kw">let </span>(steal, <span class="kw">_</span>) = unpack(dst.inner.head.load(Acquire)); |
| |
| <span class="kw">if </span>dst_tail.wrapping_sub(steal) > LOCAL_QUEUE_CAPACITY <span class="kw">as </span>UnsignedShort / <span class="number">2 </span>{ |
| <span class="comment">// we *could* try to steal less here, but for simplicity, we're just |
| // going to abort. |
| </span><span class="kw">return </span><span class="prelude-val">None</span>; |
| } |
| |
| <span class="comment">// Steal the tasks into `dst`'s buffer. This does not yet expose the |
| // tasks in `dst`. |
| </span><span class="kw">let </span><span class="kw-2">mut </span>n = <span class="self">self</span>.steal_into2(dst, dst_tail); |
| |
| <span class="kw">if </span>n == <span class="number">0 </span>{ |
| <span class="comment">// No tasks were stolen |
| </span><span class="kw">return </span><span class="prelude-val">None</span>; |
| } |
| |
| dst_metrics.incr_steal_count(n <span class="kw">as </span>u16); |
| dst_metrics.incr_steal_operations(); |
| |
| <span class="comment">// We are returning a task here |
| </span>n -= <span class="number">1</span>; |
| |
| <span class="kw">let </span>ret_pos = dst_tail.wrapping_add(n); |
| <span class="kw">let </span>ret_idx = ret_pos <span class="kw">as </span>usize & MASK; |
| |
| <span class="comment">// safety: the value was written as part of `steal_into2` and not |
| // exposed to stealers, so no other thread can access it. |
| </span><span class="kw">let </span>ret = dst.inner.buffer[ret_idx].with(|ptr| <span class="kw">unsafe </span>{ ptr::read((<span class="kw-2">*</span>ptr).as_ptr()) }); |
| |
| <span class="kw">if </span>n == <span class="number">0 </span>{ |
| <span class="comment">// The `dst` queue is empty, but a single task was stolen |
| </span><span class="kw">return </span><span class="prelude-val">Some</span>(ret); |
| } |
| |
| <span class="comment">// Make the stolen items available to consumers |
| </span>dst.inner.tail.store(dst_tail.wrapping_add(n), Release); |
| |
| <span class="prelude-val">Some</span>(ret) |
| } |
| |
| <span class="comment">// Steal tasks from `self`, placing them into `dst`. Returns the number of |
| // tasks that were stolen. |
| </span><span class="kw">fn </span>steal_into2(<span class="kw-2">&</span><span class="self">self</span>, dst: <span class="kw-2">&mut </span>Local<T>, dst_tail: UnsignedShort) -> UnsignedShort { |
| <span class="kw">let </span><span class="kw-2">mut </span>prev_packed = <span class="self">self</span>.<span class="number">0</span>.head.load(Acquire); |
| <span class="kw">let </span><span class="kw-2">mut </span>next_packed; |
| |
| <span class="kw">let </span>n = <span class="kw">loop </span>{ |
| <span class="kw">let </span>(src_head_steal, src_head_real) = unpack(prev_packed); |
| <span class="kw">let </span>src_tail = <span class="self">self</span>.<span class="number">0</span>.tail.load(Acquire); |
| |
| <span class="comment">// If these two do not match, another thread is concurrently |
| // stealing from the queue. |
| </span><span class="kw">if </span>src_head_steal != src_head_real { |
| <span class="kw">return </span><span class="number">0</span>; |
| } |
| |
| <span class="comment">// Number of available tasks to steal |
| </span><span class="kw">let </span>n = src_tail.wrapping_sub(src_head_real); |
| <span class="kw">let </span>n = n - n / <span class="number">2</span>; |
| |
| <span class="kw">if </span>n == <span class="number">0 </span>{ |
| <span class="comment">// No tasks available to steal |
| </span><span class="kw">return </span><span class="number">0</span>; |
| } |
| |
| <span class="comment">// Update the real head index to acquire the tasks. |
| </span><span class="kw">let </span>steal_to = src_head_real.wrapping_add(n); |
| <span class="macro">assert_ne!</span>(src_head_steal, steal_to); |
| next_packed = pack(src_head_steal, steal_to); |
| |
| <span class="comment">// Claim all those tasks. This is done by incrementing the "real" |
| // head but not the steal. By doing this, no other thread is able to |
| // steal from this queue until the current thread completes. |
| </span><span class="kw">let </span>res = <span class="self">self |
| </span>.<span class="number">0 |
| </span>.head |
| .compare_exchange(prev_packed, next_packed, AcqRel, Acquire); |
| |
| <span class="kw">match </span>res { |
| <span class="prelude-val">Ok</span>(<span class="kw">_</span>) => <span class="kw">break </span>n, |
| <span class="prelude-val">Err</span>(actual) => prev_packed = actual, |
| } |
| }; |
| |
| <span class="macro">assert!</span>( |
| n <= LOCAL_QUEUE_CAPACITY <span class="kw">as </span>UnsignedShort / <span class="number">2</span>, |
| <span class="string">"actual = {}"</span>, |
| n |
| ); |
| |
| <span class="kw">let </span>(first, <span class="kw">_</span>) = unpack(next_packed); |
| |
| <span class="comment">// Take all the tasks |
| </span><span class="kw">for </span>i <span class="kw">in </span><span class="number">0</span>..n { |
| <span class="comment">// Compute the positions |
| </span><span class="kw">let </span>src_pos = first.wrapping_add(i); |
| <span class="kw">let </span>dst_pos = dst_tail.wrapping_add(i); |
| |
| <span class="comment">// Map to slots |
| </span><span class="kw">let </span>src_idx = src_pos <span class="kw">as </span>usize & MASK; |
| <span class="kw">let </span>dst_idx = dst_pos <span class="kw">as </span>usize & MASK; |
| |
| <span class="comment">// Read the task |
| // |
| // safety: We acquired the task with the atomic exchange above. |
| </span><span class="kw">let </span>task = <span class="self">self</span>.<span class="number">0</span>.buffer[src_idx].with(|ptr| <span class="kw">unsafe </span>{ ptr::read((<span class="kw-2">*</span>ptr).as_ptr()) }); |
| |
| <span class="comment">// Write the task to the new slot |
| // |
| // safety: `dst` queue is empty and we are the only producer to |
| // this queue. |
| </span>dst.inner.buffer[dst_idx] |
| .with_mut(|ptr| <span class="kw">unsafe </span>{ ptr::write((<span class="kw-2">*</span>ptr).as_mut_ptr(), task) }); |
| } |
| |
| <span class="kw">let </span><span class="kw-2">mut </span>prev_packed = next_packed; |
| |
| <span class="comment">// Update `src_head_steal` to match `src_head_real` signalling that the |
| // stealing routine is complete. |
| </span><span class="kw">loop </span>{ |
| <span class="kw">let </span>head = unpack(prev_packed).<span class="number">1</span>; |
| next_packed = pack(head, head); |
| |
| <span class="kw">let </span>res = <span class="self">self |
| </span>.<span class="number">0 |
| </span>.head |
| .compare_exchange(prev_packed, next_packed, AcqRel, Acquire); |
| |
| <span class="kw">match </span>res { |
| <span class="prelude-val">Ok</span>(<span class="kw">_</span>) => <span class="kw">return </span>n, |
| <span class="prelude-val">Err</span>(actual) => { |
| <span class="kw">let </span>(actual_steal, actual_real) = unpack(actual); |
| |
| <span class="macro">assert_ne!</span>(actual_steal, actual_real); |
| |
| prev_packed = actual; |
| } |
| } |
| } |
| } |
| } |
| |
| <span class="macro">cfg_metrics! </span>{ |
| <span class="kw">impl</span><T> Steal<T> { |
| <span class="kw">pub</span>(<span class="kw">crate</span>) <span class="kw">fn </span>len(<span class="kw-2">&</span><span class="self">self</span>) -> usize { |
| <span class="self">self</span>.<span class="number">0</span>.len() <span class="kw">as _ |
| </span>} |
| } |
| } |
| |
| <span class="kw">impl</span><T> Clone <span class="kw">for </span>Steal<T> { |
| <span class="kw">fn </span>clone(<span class="kw-2">&</span><span class="self">self</span>) -> Steal<T> { |
| Steal(<span class="self">self</span>.<span class="number">0</span>.clone()) |
| } |
| } |
| |
| <span class="kw">impl</span><T> Drop <span class="kw">for </span>Local<T> { |
| <span class="kw">fn </span>drop(<span class="kw-2">&mut </span><span class="self">self</span>) { |
| <span class="kw">if </span>!std::thread::panicking() { |
| <span class="macro">assert!</span>(<span class="self">self</span>.pop().is_none(), <span class="string">"queue not empty"</span>); |
| } |
| } |
| } |
| |
| <span class="kw">impl</span><T> Inner<T> { |
| <span class="kw">fn </span>len(<span class="kw-2">&</span><span class="self">self</span>) -> UnsignedShort { |
| <span class="kw">let </span>(<span class="kw">_</span>, head) = unpack(<span class="self">self</span>.head.load(Acquire)); |
| <span class="kw">let </span>tail = <span class="self">self</span>.tail.load(Acquire); |
| |
| tail.wrapping_sub(head) |
| } |
| |
| <span class="kw">fn </span>is_empty(<span class="kw-2">&</span><span class="self">self</span>) -> bool { |
| <span class="self">self</span>.len() == <span class="number">0 |
| </span>} |
| } |
| |
| <span class="doccomment">/// Split the head value into the real head and the index a stealer is working |
| /// on. |
| </span><span class="kw">fn </span>unpack(n: UnsignedLong) -> (UnsignedShort, UnsignedShort) { |
| <span class="kw">let </span>real = n & UnsignedShort::MAX <span class="kw">as </span>UnsignedLong; |
| <span class="kw">let </span>steal = n >> (mem::size_of::<UnsignedShort>() * <span class="number">8</span>); |
| |
| (steal <span class="kw">as </span>UnsignedShort, real <span class="kw">as </span>UnsignedShort) |
| } |
| |
| <span class="doccomment">/// Join the two head values |
| </span><span class="kw">fn </span>pack(steal: UnsignedShort, real: UnsignedShort) -> UnsignedLong { |
| (real <span class="kw">as </span>UnsignedLong) | ((steal <span class="kw">as </span>UnsignedLong) << (mem::size_of::<UnsignedShort>() * <span class="number">8</span>)) |
| } |
| |
| <span class="attribute">#[test] |
| </span><span class="kw">fn </span>test_local_queue_capacity() { |
| <span class="macro">assert!</span>(LOCAL_QUEUE_CAPACITY - <span class="number">1 </span><= u8::MAX <span class="kw">as </span>usize); |
| } |
| </code></pre></div> |
| </section></div></main><div id="rustdoc-vars" data-root-path="../../../../../" data-current-crate="tokio" data-themes="ayu,dark,light" data-resource-suffix="" data-rustdoc-version="1.66.0-nightly (5c8bff74b 2022-10-21)" ></div></body></html> |
