Task Scheduler.

base/task_scheduler/thread_pool.cc

+// Copyright 2016 The Chromium Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+#include "base/task_scheduler/thread_pool.h"
+
+#include <utility>
+
+#include "base/bind.h"
+#include "base/logging.h"
+#include "base/task_scheduler/utils.h"
+
+namespace base {
+namespace task_scheduler {
+
+namespace {
+
+// A task runner that runs tasks with the PARALLEL strategy.
+class SchedulerParallelTaskRunner : public TaskRunner {
+ public:
+  // Tasks posted through this task runner have |traits| and are inserted in
+  // |shared_priority_queue|. |delayed_task_manager| is used to post delayed
+  // tasks. |shutdown_manager| is notified when a task is posted.
+  SchedulerParallelTaskRunner(const TaskTraits& traits,
+                              PriorityQueue* priority_queue,
+                              DelayedTaskManager* delayed_task_manager,
+                              ShutdownManager* shutdown_manager);
+
+  // TaskRunner:
+  bool PostDelayedTask(const tracked_objects::Location& from_here,
+                       const Closure& closure,
+                       TimeDelta delay) override;
+  bool RunsTasksOnCurrentThread() const override;
+
+ private:
+  ~SchedulerParallelTaskRunner() override;
+
+  TaskTraits traits_;
+  PriorityQueue* priority_queue_;
+  DelayedTaskManager* delayed_task_manager_;
+  ShutdownManager* shutdown_manager_;
+
+  DISALLOW_COPY_AND_ASSIGN(SchedulerParallelTaskRunner);
+};
+
+SchedulerParallelTaskRunner::SchedulerParallelTaskRunner(
+    const TaskTraits& traits,
+    PriorityQueue* priority_queue,
+    DelayedTaskManager* delayed_task_manager,
+    ShutdownManager* shutdown_manager)
+    : traits_(traits),
+      priority_queue_(priority_queue),
+      delayed_task_manager_(delayed_task_manager),
+      shutdown_manager_(shutdown_manager) {}
+
+bool SchedulerParallelTaskRunner::PostDelayedTask(
+    const tracked_objects::Location& from_here,
+    const Closure& closure,
+    TimeDelta delay) {
+  Task task(from_here, closure, traits_, TimeTicks::Now());
+  if (!delay.is_zero())
+    task.delayed_run_time = task.post_time + delay;
+  PostTaskHelper(task, make_scoped_refptr(new Sequence), priority_queue_,
+                 shutdown_manager_, delayed_task_manager_);
+  return true;
+}
+
+bool SchedulerParallelTaskRunner::RunsTasksOnCurrentThread() const {
+  // TODO(fdoray): Return true only if tasks posted may actually run on the
+  // current thread. It is valid, but not ideal, to always return true.
+  return true;
+}
+
+SchedulerParallelTaskRunner::~SchedulerParallelTaskRunner() = default;
+
+// A task runner that runs tasks in with the SEQUENCED strategy.
+class SchedulerSequencedTaskRunner : public SequencedTaskRunner {
+ public:
+  // Tasks posted through this task runner have |traits| and are inserted in
+  // |sequence|. When appropriate, |sequence| is inserted in |priority_queue|.
+  // |delayed_task_manager| is used to post delayed tasks. |shutdown_manager| is
+  // notified when a task is posted.
+  SchedulerSequencedTaskRunner(const TaskTraits& traits,
+                               scoped_refptr<Sequence> sequence,
+                               PriorityQueue* priority_queue,
+                               DelayedTaskManager* delayed_task_manager,
+                               ShutdownManager* shutdown_manager);
+
+  // SequencedTaskRunner:
+  bool PostNonNestableDelayedTask(const tracked_objects::Location& from_here,
+                                  const Closure& task,
+                                  TimeDelta delay) override;
+  bool PostDelayedTask(const tracked_objects::Location& from_here,
+                       const Closure& closure,
+                       TimeDelta delay) override;
+  bool RunsTasksOnCurrentThread() const override;
+
+ private:
+  ~SchedulerSequencedTaskRunner() override;
+
+  TaskTraits traits_;
+  scoped_refptr<Sequence> sequence_;
+  PriorityQueue* priority_queue_;
+  DelayedTaskManager* delayed_task_manager_;
+  ShutdownManager* shutdown_manager_;
+
+  DISALLOW_COPY_AND_ASSIGN(SchedulerSequencedTaskRunner);
+};
+
+SchedulerSequencedTaskRunner::SchedulerSequencedTaskRunner(
+    const TaskTraits& traits,
+    scoped_refptr<Sequence> sequence,
+    PriorityQueue* priority_queue,
+    DelayedTaskManager* delayed_task_manager,
+    ShutdownManager* shutdown_manager)
+    : traits_(traits),
+      sequence_(sequence),
+      priority_queue_(priority_queue),
+      delayed_task_manager_(delayed_task_manager),
+      shutdown_manager_(shutdown_manager) {}
+
+bool SchedulerSequencedTaskRunner::PostDelayedTask(
+    const tracked_objects::Location& from_here,
+    const Closure& closure,
+    TimeDelta delay) {
+  Task task(from_here, closure, traits_, TimeTicks::Now());
+  if (!delay.is_zero())
+    task.delayed_run_time = task.post_time + delay;
+  PostTaskHelper(task, sequence_, priority_queue_, shutdown_manager_,
+                 delayed_task_manager_);
+  return true;
+}
+
+bool SchedulerSequencedTaskRunner::RunsTasksOnCurrentThread() const {
+  // TODO(fdoray): Return true only if tasks posted may actually run on the
+  // current thread. It is valid, but not ideal, to always return true.
+  return true;
+}
+
+bool SchedulerSequencedTaskRunner::PostNonNestableDelayedTask(
+    const tracked_objects::Location& from_here,
+    const Closure& task,
+    TimeDelta delay) {
+  return PostDelayedTask(from_here, task, delay);
+}
+
+SchedulerSequencedTaskRunner::~SchedulerSequencedTaskRunner() = default;
+
+}  // namespace
+
+
+ThreadPool::~ThreadPool() = default;
+
+scoped_ptr<ThreadPool> ThreadPool::CreateThreadPool(
+    ThreadPriority thread_priority,
+    size_t num_threads,
+    const WorkerThread::ReinsertSequenceCallback& reinsert_sequence_callback,
+    ShutdownManager* shutdown_manager) {
+  scoped_ptr<ThreadPool> thread_pool(
+      new ThreadPool(thread_priority, num_threads, reinsert_sequence_callback,
+                     shutdown_manager));
+  return (thread_pool->GetNumThreads() > 0) ? std::move(thread_pool)
+                                            : scoped_ptr<ThreadPool>();
+}
+
+size_t ThreadPool::GetNumThreads() const {
+  return worker_threads_.size();
+}
+
+scoped_refptr<TaskRunner> ThreadPool::CreateTaskRunnerWithTraits(
+    const TaskTraits& traits,
+    ExecutionMode execution_mode) {
+  switch (execution_mode) {
+    case ExecutionMode::PARALLEL: {
+      return scoped_refptr<TaskRunner>(new SchedulerParallelTaskRunner(
+          traits, &priority_queue_, &delayed_task_manager_, shutdown_manager_));
+    }
+
+    case ExecutionMode::SEQUENCED: {
+      // TODO(fdoray): Support TaskTraits().WithSequenceToken().
+      return scoped_refptr<TaskRunner>(new SchedulerSequencedTaskRunner(
+          traits, scoped_refptr<Sequence>(new Sequence), &priority_queue_,
+          &delayed_task_manager_, shutdown_manager_));
+    }
+
+#if defined(OS_WIN)
+    case ExecutionMode::SINGLE_THREADED_COM_STA:
+#endif
+    case ExecutionMode::SINGLE_THREADED: {
+      DCHECK(!worker_threads_.empty());
+      // TODO(fdoray): Better thread assignment.
+      return scoped_refptr<TaskRunner>(
+          worker_threads_.front()
+              ->CreateTaskRunnerWithTraits(traits, execution_mode)
+              .get());
+    }
+
+    default: {
+      NOTREACHED();
+      return scoped_refptr<TaskRunner>();
+    }
+  }
+}
+
+void ThreadPool::ReinsertSequence(scoped_refptr<Sequence> sequence,
+                                  const SequenceSortKey& sequence_sort_key,
+                                  const WorkerThread* worker_thread) {
+  DCHECK(!disable_wake_up_thread_on_sequence_insertion_.Get());
+
+  // Set a flag to avoid waking up a thread when reinserting |sequence| in
+  // |priority_queue_| if the thread doing the reinsertion:
+  // - Can run tasks from |priority_queue_|, and,
+  // - Doesn't have pending single-threaded tasks.
+  // If these conditions are met, the thread doing the reinsertion will soon
+  // pop a sequence from |priority_queue_|. There is no need to wake up a new
+  // thread to do it.
+  if (worker_thread->shared_priority_queue() == &priority_queue_ &&
+      !worker_thread->HasSingleThreadedTasks()) {
+    disable_wake_up_thread_on_sequence_insertion_.Set(true);
+  }
+
+  // Insert the sequence in the priority queue.
+  priority_queue_.BeginTransaction()->PushSequence(sequence, sequence_sort_key);
+
+  disable_wake_up_thread_on_sequence_insertion_.Set(false);
+}
+
+void ThreadPool::JoinAllThreadsForTesting() {
+  for (const auto& worker_thread : worker_threads_) {
+    worker_thread->WakeUp();
+    worker_thread->JoinForTesting();
+  }
+}
+
+ThreadPool::ThreadPool(
+    ThreadPriority thread_priority,
+    size_t num_threads,
+    const WorkerThread::ReinsertSequenceCallback& reinsert_sequence_callback,
+    ShutdownManager* shutdown_manager)
+    : priority_queue_(Bind(&ThreadPool::OnSequenceInsertedInPriorityQueue,
+                           Unretained(this))),
+      shutdown_manager_(shutdown_manager),
+      delayed_task_manager_(
+          Bind(&ThreadPool::WakeUpOneThread, Unretained(this)),
+          shutdown_manager_) {
+  DCHECK_GT(num_threads, 0u);
+  DCHECK(shutdown_manager);
+
+  const WorkerThread::BecomesIdleCallback becomes_idle_callback =
+      Bind(&ThreadPool::WorkerThreadBecomesIdleCallback, Unretained(this));
+  worker_threads_.reserve(num_threads);
+
+  for (size_t i = 0; i < num_threads; ++i) {
+    scoped_ptr<WorkerThread> worker_thread = WorkerThread::CreateWorkerThread(
+        thread_priority, &priority_queue_, reinsert_sequence_callback,
+        becomes_idle_callback, &delayed_task_manager_, shutdown_manager_);
+    if (worker_thread.get() != nullptr)
+      worker_threads_.push_back(std::move(worker_thread));
+  }
+}
+
+void ThreadPool::WorkerThreadBecomesIdleCallback(WorkerThread* worker_thread) {

[fdoray, 2016/02/11 17:30:33]

DCHECK that |worker_thread| belongs to this thread pool?

[fdoray, 2016/02/12 04:16:20]

Done.

+  AutoSchedulerLock auto_lock_(idle_worker_threads_lock_);
+
+  if (idle_worker_threads_set_.find(worker_thread) !=
+      idle_worker_threads_set_.end()) {
+    // The worker thread is already on the stack of idle threads.
+    return;
+  }
+
+  // Add the worker thread to the stack of idle threads.
+  idle_worker_threads_stack_.push(worker_thread);
+  idle_worker_threads_set_.insert(worker_thread);
+}
+
+void ThreadPool::WakeUpOneThread() {
+  // Wake up the first thread found on |idle_worker_threads_stack_| that doesn't
+  // have pending or running single-threaded tasks.
+  AutoSchedulerLock auto_lock(idle_worker_threads_lock_);
+  while (!idle_worker_threads_stack_.empty()) {
+    WorkerThread* worker_thread = idle_worker_threads_stack_.top();
+
+    idle_worker_threads_stack_.pop();
+    idle_worker_threads_set_.erase(worker_thread);
+
+    // HasSingleThreadedTasks() can return stale results. However, when it
+    // returns true below, it is guaranteed that |worker_thread| is either awake
+    // or about to be woken up and that it will not enter
+    // WaitUntilWorkIsAvailable() before |priority_queue_| becomes empty. This
+    // is important because if all threads in |idle_worker_threads_stack_|
+    // report that they have single-threaded tasks, no thread is woken up by
+    // this method. If these threads don't check |priority_queue_| before
+    // entering WaitUntilWorkIsAvailable(), the work in |priority_queue_| could
+    // end up never being done. The guarantee works between the moment
+    // HasSingleThreadedTasks() goes from true to false and the moment
+    // |worker_thread| enters WaitUntilWorkIsAvailable(),
+    // WorkerThreadBecomesIdleCallback() has to be called on this ThreadPool.
+    // Both WorkerThreadBecomesIdleCallback() and the current method acquire
+    // |idle_worker_threads_lock_|, which synchronizes the value returned by
+    // HasSingleThreadedTasks().
+    //
+    // TODO(fdoray): A single-threaded task can be posted to |worker_thread|
+    // immediately after HasSingleThreadedTasks() has returned false. Ideally,
+    // when this happens, another worker thread should be woken up.
+    if (!worker_thread->HasSingleThreadedTasks()) {

[robliao, 2016/02/11 22:30:07]

Maybe a controller pattern might make reasoning over the global state of the
threadpool easier.

[fdoray, 2016/02/12 04:16:20]

I agree. I'll try to write this (tomorrow in the plane?) and we'll see if it
makes the code simpler. Since threads already have to lock
|shared_priority_queue_| each time that they get work, I don't think that having
an extra lock to protect a manager which knows about the state of each thread in
a pool will harm parallelism.

+      worker_thread->WakeUp();
+      break;
+    }
+  }
+}
+
+void ThreadPool::OnSequenceInsertedInPriorityQueue() {
+  if (disable_wake_up_thread_on_sequence_insertion_.Get())
+    return;
+
+  WakeUpOneThread();
+}
+
+}  // namespace task_scheduler
+}  // namespace base