1//==-- llvm/Support/ThreadPool.cpp - A ThreadPool implementation -*- C++ -*-==//
2//
3// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
4// See https://llvm.org/LICENSE.txt for license information.
5// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
6//
7//===----------------------------------------------------------------------===//
8//
9// This file implements a crude C++11 based thread pool.
10//
11//===----------------------------------------------------------------------===//
12
13#include "llvm/Support/ThreadPool.h"
14
15#include "llvm/Config/llvm-config.h"
16
17#include "llvm/Support/FormatVariadic.h"
18#include "llvm/Support/Threading.h"
19#include "llvm/Support/raw_ostream.h"
20
21using namespace llvm;
22
23ThreadPoolInterface::~ThreadPoolInterface() = default;
24
25// A note on thread groups: Tasks are by default in no group (represented
26// by nullptr ThreadPoolTaskGroup pointer in the Tasks queue) and functionality
27// here normally works on all tasks regardless of their group (functions
28// in that case receive nullptr ThreadPoolTaskGroup pointer as argument).
29// A task in a group has a pointer to that ThreadPoolTaskGroup in the Tasks
30// queue, and functions called to work only on tasks from one group take that
31// pointer.
32
33#if LLVM_ENABLE_THREADS
34
35StdThreadPool::StdThreadPool(ThreadPoolStrategy S)
36 : Strategy(S), MaxThreadCount(S.compute_thread_count()) {}
37
38void StdThreadPool::grow(int requested) {
39 llvm::sys::ScopedWriter LockGuard(ThreadsLock);
40 if (Threads.size() >= MaxThreadCount)
41 return; // Already hit the max thread pool size.
42 int newThreadCount = std::min<int>(a: requested, b: MaxThreadCount);
43 while (static_cast<int>(Threads.size()) < newThreadCount) {
44 int ThreadID = Threads.size();
45 Threads.emplace_back(args: [this, ThreadID] {
46 set_thread_name(formatv(Fmt: "llvm-worker-{0}", Vals: ThreadID));
47 Strategy.apply_thread_strategy(ThreadPoolNum: ThreadID);
48 processTasks(WaitingForGroup: nullptr);
49 });
50 }
51}
52
53#ifndef NDEBUG
54// The group of the tasks run by the current thread.
55static LLVM_THREAD_LOCAL std::vector<ThreadPoolTaskGroup *>
56 *CurrentThreadTaskGroups = nullptr;
57#endif
58
59// WaitingForGroup == nullptr means all tasks regardless of their group.
60void StdThreadPool::processTasks(ThreadPoolTaskGroup *WaitingForGroup) {
61 while (true) {
62 std::function<void()> Task;
63 ThreadPoolTaskGroup *GroupOfTask;
64 {
65 std::unique_lock<std::mutex> LockGuard(QueueLock);
66 bool workCompletedForGroup = false; // Result of workCompletedUnlocked()
67 // Wait for tasks to be pushed in the queue
68 QueueCondition.wait(lock&: LockGuard, p: [&] {
69 return !EnableFlag || !Tasks.empty() ||
70 (WaitingForGroup != nullptr &&
71 (workCompletedForGroup =
72 workCompletedUnlocked(Group: WaitingForGroup)));
73 });
74 // Exit condition
75 if (!EnableFlag && Tasks.empty())
76 return;
77 if (WaitingForGroup != nullptr && workCompletedForGroup)
78 return;
79 // Yeah, we have a task, grab it and release the lock on the queue
80
81 // We first need to signal that we are active before popping the queue
82 // in order for wait() to properly detect that even if the queue is
83 // empty, there is still a task in flight.
84 ++ActiveThreads;
85 Task = std::move(Tasks.front().first);
86 GroupOfTask = Tasks.front().second;
87 // Need to count active threads in each group separately, ActiveThreads
88 // would never be 0 if waiting for another group inside a wait.
89 if (GroupOfTask != nullptr)
90 ++ActiveGroups[GroupOfTask]; // Increment or set to 1 if new item
91 Tasks.pop_front();
92 }
93#ifndef NDEBUG
94 if (CurrentThreadTaskGroups == nullptr)
95 CurrentThreadTaskGroups = new std::vector<ThreadPoolTaskGroup *>;
96 CurrentThreadTaskGroups->push_back(GroupOfTask);
97#endif
98
99 // Run the task we just grabbed
100 Task();
101
102#ifndef NDEBUG
103 CurrentThreadTaskGroups->pop_back();
104 if (CurrentThreadTaskGroups->empty()) {
105 delete CurrentThreadTaskGroups;
106 CurrentThreadTaskGroups = nullptr;
107 }
108#endif
109
110 bool Notify;
111 bool NotifyGroup;
112 {
113 // Adjust `ActiveThreads`, in case someone waits on StdThreadPool::wait()
114 std::lock_guard<std::mutex> LockGuard(QueueLock);
115 --ActiveThreads;
116 if (GroupOfTask != nullptr) {
117 auto A = ActiveGroups.find(Val: GroupOfTask);
118 if (--(A->second) == 0)
119 ActiveGroups.erase(I: A);
120 }
121 Notify = workCompletedUnlocked(Group: GroupOfTask);
122 NotifyGroup = GroupOfTask != nullptr && Notify;
123 }
124 // Notify task completion if this is the last active thread, in case
125 // someone waits on StdThreadPool::wait().
126 if (Notify)
127 CompletionCondition.notify_all();
128 // If this was a task in a group, notify also threads waiting for tasks
129 // in this function on QueueCondition, to make a recursive wait() return
130 // after the group it's been waiting for has finished.
131 if (NotifyGroup)
132 QueueCondition.notify_all();
133 }
134}
135
136bool StdThreadPool::workCompletedUnlocked(ThreadPoolTaskGroup *Group) const {
137 if (Group == nullptr)
138 return !ActiveThreads && Tasks.empty();
139 return ActiveGroups.count(Val: Group) == 0 &&
140 !llvm::is_contained(Range: llvm::make_second_range(c: Tasks), Element: Group);
141}
142
143void StdThreadPool::wait() {
144 assert(!isWorkerThread()); // Would deadlock waiting for itself.
145 // Wait for all threads to complete and the queue to be empty
146 std::unique_lock<std::mutex> LockGuard(QueueLock);
147 CompletionCondition.wait(lock&: LockGuard,
148 p: [&] { return workCompletedUnlocked(Group: nullptr); });
149}
150
151void StdThreadPool::wait(ThreadPoolTaskGroup &Group) {
152 // Wait for all threads in the group to complete.
153 if (!isWorkerThread()) {
154 std::unique_lock<std::mutex> LockGuard(QueueLock);
155 CompletionCondition.wait(lock&: LockGuard,
156 p: [&] { return workCompletedUnlocked(Group: &Group); });
157 return;
158 }
159 // Make sure to not deadlock waiting for oneself.
160 assert(CurrentThreadTaskGroups == nullptr ||
161 !llvm::is_contained(*CurrentThreadTaskGroups, &Group));
162 // Handle the case of recursive call from another task in a different group,
163 // in which case process tasks while waiting to keep the thread busy and avoid
164 // possible deadlock.
165 processTasks(WaitingForGroup: &Group);
166}
167
168bool StdThreadPool::isWorkerThread() const {
169 llvm::sys::ScopedReader LockGuard(ThreadsLock);
170 llvm::thread::id CurrentThreadId = llvm::this_thread::get_id();
171 for (const llvm::thread &Thread : Threads)
172 if (CurrentThreadId == Thread.get_id())
173 return true;
174 return false;
175}
176
177// The destructor joins all threads, waiting for completion.
178StdThreadPool::~StdThreadPool() {
179 {
180 std::unique_lock<std::mutex> LockGuard(QueueLock);
181 EnableFlag = false;
182 }
183 QueueCondition.notify_all();
184 llvm::sys::ScopedReader LockGuard(ThreadsLock);
185 for (auto &Worker : Threads)
186 Worker.join();
187}
188
189#endif // LLVM_ENABLE_THREADS Disabled
190
191// No threads are launched, issue a warning if ThreadCount is not 0
192SingleThreadExecutor::SingleThreadExecutor(ThreadPoolStrategy S) {
193 int ThreadCount = S.compute_thread_count();
194 if (ThreadCount != 1) {
195 errs() << "Warning: request a ThreadPool with " << ThreadCount
196 << " threads, but LLVM_ENABLE_THREADS has been turned off\n";
197 }
198}
199
200void SingleThreadExecutor::wait() {
201 // Sequential implementation running the tasks
202 while (!Tasks.empty()) {
203 auto Task = std::move(Tasks.front().first);
204 Tasks.pop_front();
205 Task();
206 }
207}
208
209void SingleThreadExecutor::wait(ThreadPoolTaskGroup &) {
210 // Simply wait for all, this works even if recursive (the running task
211 // is already removed from the queue).
212 wait();
213}
214
215bool SingleThreadExecutor::isWorkerThread() const {
216 report_fatal_error(reason: "LLVM compiled without multithreading");
217}
218
219SingleThreadExecutor::~SingleThreadExecutor() { wait(); }
220