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 
35 ThreadPool::ThreadPool(ThreadPoolStrategy S)
36     : Strategy(S), MaxThreadCount(S.compute_thread_count()) {}
37 
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.
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 
135 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 
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 
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 
168 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.
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
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 
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 
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 
215 bool ThreadPool::isWorkerThread() const {
216   report_fatal_error("LLVM compiled without multithreading");
217 }
218 
219 ThreadPool::~ThreadPool() { wait(); }
220 
221 #endif
222