1//===--------------------- TaskPool.cpp -------------------------*- 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#include "lldb/Host/TaskPool.h"
10#include "lldb/Host/ThreadLauncher.h"
11#include "lldb/Utility/Log.h"
12
13#include <cstdint>
14#include <queue>
15#include <thread>
16
17namespace lldb_private {
18
19namespace {
20class TaskPoolImpl {
21public:
22  static TaskPoolImpl &GetInstance();
23
24  void AddTask(std::function<void()> &&task_fn);
25
26private:
27  TaskPoolImpl();
28
29  static lldb::thread_result_t WorkerPtr(void *pool);
30
31  static void Worker(TaskPoolImpl *pool);
32
33  std::queue<std::function<void()>> m_tasks;
34  std::mutex m_tasks_mutex;
35  uint32_t m_thread_count;
36};
37
38} // end of anonymous namespace
39
40TaskPoolImpl &TaskPoolImpl::GetInstance() {
41  static TaskPoolImpl g_task_pool_impl;
42  return g_task_pool_impl;
43}
44
45void TaskPool::AddTaskImpl(std::function<void()> &&task_fn) {
46  TaskPoolImpl::GetInstance().AddTask(std::move(task_fn));
47}
48
49TaskPoolImpl::TaskPoolImpl() : m_thread_count(0) {}
50
51unsigned GetHardwareConcurrencyHint() {
52  // std::thread::hardware_concurrency may return 0 if the value is not well
53  // defined or not computable.
54  static const unsigned g_hardware_concurrency =
55    std::max(1u, std::thread::hardware_concurrency());
56  return g_hardware_concurrency;
57}
58
59void TaskPoolImpl::AddTask(std::function<void()> &&task_fn) {
60  const size_t min_stack_size = 8 * 1024 * 1024;
61
62  std::unique_lock<std::mutex> lock(m_tasks_mutex);
63  m_tasks.emplace(std::move(task_fn));
64  if (m_thread_count < GetHardwareConcurrencyHint()) {
65    m_thread_count++;
66    // Note that this detach call needs to happen with the m_tasks_mutex held.
67    // This prevents the thread from exiting prematurely and triggering a linux
68    // libc bug (https://sourceware.org/bugzilla/show_bug.cgi?id=19951).
69    llvm::Expected<HostThread> host_thread =
70        lldb_private::ThreadLauncher::LaunchThread(
71            "task-pool.worker", WorkerPtr, this, min_stack_size);
72    if (host_thread) {
73      host_thread->Release();
74    } else {
75      LLDB_LOG(lldb_private::GetLogIfAllCategoriesSet(LIBLLDB_LOG_HOST),
76               "failed to launch host thread: {}",
77               llvm::toString(host_thread.takeError()));
78    }
79  }
80}
81
82lldb::thread_result_t TaskPoolImpl::WorkerPtr(void *pool) {
83  Worker((TaskPoolImpl *)pool);
84  return {};
85}
86
87void TaskPoolImpl::Worker(TaskPoolImpl *pool) {
88  while (true) {
89    std::unique_lock<std::mutex> lock(pool->m_tasks_mutex);
90    if (pool->m_tasks.empty()) {
91      pool->m_thread_count--;
92      break;
93    }
94
95    std::function<void()> f = std::move(pool->m_tasks.front());
96    pool->m_tasks.pop();
97    lock.unlock();
98
99    f();
100  }
101}
102
103void TaskMapOverInt(size_t begin, size_t end,
104                    const llvm::function_ref<void(size_t)> &func) {
105  const size_t num_workers = std::min<size_t>(end, GetHardwareConcurrencyHint());
106  std::atomic<size_t> idx{begin};
107
108  auto wrapper = [&idx, end, &func]() {
109    while (true) {
110      size_t i = idx.fetch_add(1);
111      if (i >= end)
112        break;
113      func(i);
114    }
115  };
116
117  std::vector<std::future<void>> futures;
118  futures.reserve(num_workers);
119  for (size_t i = 0; i < num_workers; i++)
120    futures.push_back(TaskPool::AddTask(wrapper));
121  for (size_t i = 0; i < num_workers; i++)
122    futures[i].wait();
123}
124
125} // namespace lldb_private
126
127