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 #if LLVM_ENABLE_THREADS
18 #include "llvm/Support/Threading.h"
19 #else
20 #include "llvm/Support/raw_ostream.h"
21 #endif
22
23 using namespace llvm;
24
25 #if LLVM_ENABLE_THREADS
26
27 // A note on thread groups: Tasks are by default in no group (represented
28 // by nullptr ThreadPoolTaskGroup pointer in the Tasks queue) and functionality
29 // here normally works on all tasks regardless of their group (functions
30 // in that case receive nullptr ThreadPoolTaskGroup pointer as argument).
31 // A task in a group has a pointer to that ThreadPoolTaskGroup in the Tasks
32 // queue, and functions called to work only on tasks from one group take that
33 // pointer.
34
ThreadPool(ThreadPoolStrategy S)35 ThreadPool::ThreadPool(ThreadPoolStrategy S)
36 : Strategy(S), MaxThreadCount(S.compute_thread_count()) {}
37
grow(int requested)38 void ThreadPool::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>(requested, MaxThreadCount);
43 while (static_cast<int>(Threads.size()) < newThreadCount) {
44 int ThreadID = Threads.size();
45 Threads.emplace_back([this, ThreadID] {
46 Strategy.apply_thread_strategy(ThreadID);
47 processTasks(nullptr);
48 });
49 }
50 }
51
52 #ifndef NDEBUG
53 // The group of the tasks run by the current thread.
54 static LLVM_THREAD_LOCAL std::vector<ThreadPoolTaskGroup *>
55 *CurrentThreadTaskGroups = nullptr;
56 #endif
57
58 // WaitingForGroup == nullptr means all tasks regardless of their group.
processTasks(ThreadPoolTaskGroup * WaitingForGroup)59 void ThreadPool::processTasks(ThreadPoolTaskGroup *WaitingForGroup) {
60 while (true) {
61 std::function<void()> Task;
62 ThreadPoolTaskGroup *GroupOfTask;
63 {
64 std::unique_lock<std::mutex> LockGuard(QueueLock);
65 bool workCompletedForGroup = false; // Result of workCompletedUnlocked()
66 // Wait for tasks to be pushed in the queue
67 QueueCondition.wait(LockGuard, [&] {
68 return !EnableFlag || !Tasks.empty() ||
69 (WaitingForGroup != nullptr &&
70 (workCompletedForGroup =
71 workCompletedUnlocked(WaitingForGroup)));
72 });
73 // Exit condition
74 if (!EnableFlag && Tasks.empty())
75 return;
76 if (WaitingForGroup != nullptr && workCompletedForGroup)
77 return;
78 // Yeah, we have a task, grab it and release the lock on the queue
79
80 // We first need to signal that we are active before popping the queue
81 // in order for wait() to properly detect that even if the queue is
82 // empty, there is still a task in flight.
83 ++ActiveThreads;
84 Task = std::move(Tasks.front().first);
85 GroupOfTask = Tasks.front().second;
86 // Need to count active threads in each group separately, ActiveThreads
87 // would never be 0 if waiting for another group inside a wait.
88 if (GroupOfTask != nullptr)
89 ++ActiveGroups[GroupOfTask]; // Increment or set to 1 if new item
90 Tasks.pop_front();
91 }
92 #ifndef NDEBUG
93 if (CurrentThreadTaskGroups == nullptr)
94 CurrentThreadTaskGroups = new std::vector<ThreadPoolTaskGroup *>;
95 CurrentThreadTaskGroups->push_back(GroupOfTask);
96 #endif
97
98 // Run the task we just grabbed
99 Task();
100
101 #ifndef NDEBUG
102 CurrentThreadTaskGroups->pop_back();
103 if (CurrentThreadTaskGroups->empty()) {
104 delete CurrentThreadTaskGroups;
105 CurrentThreadTaskGroups = nullptr;
106 }
107 #endif
108
109 bool Notify;
110 bool NotifyGroup;
111 {
112 // Adjust `ActiveThreads`, in case someone waits on ThreadPool::wait()
113 std::lock_guard<std::mutex> LockGuard(QueueLock);
114 --ActiveThreads;
115 if (GroupOfTask != nullptr) {
116 auto A = ActiveGroups.find(GroupOfTask);
117 if (--(A->second) == 0)
118 ActiveGroups.erase(A);
119 }
120 Notify = workCompletedUnlocked(GroupOfTask);
121 NotifyGroup = GroupOfTask != nullptr && Notify;
122 }
123 // Notify task completion if this is the last active thread, in case
124 // someone waits on ThreadPool::wait().
125 if (Notify)
126 CompletionCondition.notify_all();
127 // If this was a task in a group, notify also threads waiting for tasks
128 // in this function on QueueCondition, to make a recursive wait() return
129 // after the group it's been waiting for has finished.
130 if (NotifyGroup)
131 QueueCondition.notify_all();
132 }
133 }
134
workCompletedUnlocked(ThreadPoolTaskGroup * Group) const135 bool ThreadPool::workCompletedUnlocked(ThreadPoolTaskGroup *Group) const {
136 if (Group == nullptr)
137 return !ActiveThreads && Tasks.empty();
138 return ActiveGroups.count(Group) == 0 &&
139 !llvm::any_of(Tasks,
140 [Group](const auto &T) { return T.second == Group; });
141 }
142
wait()143 void ThreadPool::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(LockGuard,
148 [&] { return workCompletedUnlocked(nullptr); });
149 }
150
wait(ThreadPoolTaskGroup & Group)151 void ThreadPool::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(LockGuard,
156 [&] { return workCompletedUnlocked(&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(&Group);
166 }
167
isWorkerThread() const168 bool ThreadPool::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.
~ThreadPool()178 ThreadPool::~ThreadPool() {
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 #else // LLVM_ENABLE_THREADS Disabled
190
191 // No threads are launched, issue a warning if ThreadCount is not 0
ThreadPool(ThreadPoolStrategy S)192 ThreadPool::ThreadPool(ThreadPoolStrategy S) : MaxThreadCount(1) {
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
wait()200 void ThreadPool::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
wait(ThreadPoolTaskGroup &)209 void ThreadPool::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
isWorkerThread() const215 bool ThreadPool::isWorkerThread() const {
216 report_fatal_error("LLVM compiled without multithreading");
217 }
218
~ThreadPool()219 ThreadPool::~ThreadPool() { wait(); }
220
221 #endif
222