TaskScheduler [7] SchedulerThreadPool

base/task_scheduler/scheduler_thread_pool.h

+// 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.
+
+#ifndef BASE_TASK_SCHEDULER_SCHEDULER_THREAD_POOL_H_
+#define BASE_TASK_SCHEDULER_SCHEDULER_THREAD_POOL_H_
+
+#include <stddef.h>
+
+#include <memory>
+#include <stack>
+#include <vector>
+
+#include "base/base_export.h"
+#include "base/callback.h"
+#include "base/macros.h"
+#include "base/memory/ref_counted.h"
+#include "base/synchronization/condition_variable.h"
+#include "base/task_runner.h"
+#include "base/task_scheduler/priority_queue.h"
+#include "base/task_scheduler/scheduler_lock.h"
+#include "base/task_scheduler/scheduler_task_executor.h"
+#include "base/task_scheduler/scheduler_worker_thread.h"
+#include "base/task_scheduler/sequence.h"
+#include "base/task_scheduler/task.h"
+#include "base/task_scheduler/task_traits.h"
+#include "base/threading/platform_thread.h"
+
+namespace base {
+namespace internal {
+
+struct SequenceSortKey;
+class TaskTracker;
+
+// A pool of threads that run Tasks. This class is thread-safe.
+class BASE_EXPORT SchedulerThreadPool : public SchedulerTaskExecutor {
+ public:
+  // Callback invoked when a Sequence isn't empty after a worker thread pops a
+  // Task from it.
+  using EnqueueSequenceCallback = Callback<void(scoped_refptr<Sequence>)>;
+
+  // Destroying a SchedulerThreadPool returned by CreateThreadPool() is not
+  // allowed in production; it is always leaked. In tests, it can only be
+  // destroyed after JoinForTesting() has returned.
+  ~SchedulerThreadPool() override;
+
+  // Creates a SchedulerThreadPool with up to |max_threads| threads of priority
+  // |thread_priority|. |enqueue_sequence_callback| will be invoked after a
+  // thread of this thread pool tries to run a Task. |task_tracker| is used to
+  // handle shutdown behavior of Tasks. Returns nullptr on failure to create a
+  // thread pool with at least one thread.
+  static std::unique_ptr<SchedulerThreadPool> CreateThreadPool(
+      ThreadPriority thread_priority,
+      size_t max_threads,
+      const EnqueueSequenceCallback& enqueue_sequence_callback,
+      TaskTracker* task_tracker);
+
+  // Returns a TaskRunner whose PostTask invocations will result in scheduling
+  // Tasks with |traits| and |execution_mode| in this thread pool.
+  scoped_refptr<TaskRunner> CreateTaskRunnerWithTraits(
+      const TaskTraits& traits,
+      ExecutionMode execution_mode);
+
+  // Inserts |sequence| into this thread pool's shared priority queue with
+  // |sequence_sort_key|. Must only be called from a worker thread to put
+  // |sequence| back into a PriorityQueue after running a Task from it. The
+  // worker thread doesn't have to belong to this thread pool.
+  void EnqueueSequence(scoped_refptr<Sequence> sequence,
+                       const SequenceSortKey& sequence_sort_key);
+
+  // Waits until all threads are idle.
+  void WaitForAllWorkerThreadsIdleForTesting();
+
+  // Joins all threads of this thread pool. Tasks that are already running are
+  // allowed to complete their execution. This can only be called once.
+  void JoinForTesting();
+
+ private:
+  class SchedulerWorkerThreadDelegateImpl;
+
+  SchedulerThreadPool(const EnqueueSequenceCallback& enqueue_sequence_callback,
+                      TaskTracker* task_tracker);
+
+  bool Initialize(ThreadPriority thread_priority, size_t max_threads);
+
+  // Wakes up the last thread from this thread pool to go idle, if any.
+  void WakeUpOneThread();
+
+  // Adds |worker_thread| to |idle_worker_threads_stack_|.
+  void AddToIdleWorkerThreadsStack(SchedulerWorkerThread* worker_thread);
+
+  // Pops one idle worker thread from |idle_worker_thread_stack_| and returns
+  // it. Returns nullptr if |idle_worker_thread_stack_| is empty.
+  SchedulerWorkerThread* PopOneIdleWorkerThread();
+
+  // SchedulerTaskExecutor:
+  void PostTaskWithSequence(std::unique_ptr<Task> task,

[gab, 2016/04/14 16:11:35]

Why is this private? I guess the caller is using the SchedulerTaskExecutor
interface and thus making a public call to it, but this being private may lead
the reader to think it's never called from outside this component which it
definitely is (this is not the same IMO as a private call which is bound and
given to a public caller, now this is trivially made public through a cast by
anyone).

[fdoray, 2016/04/14 18:41:12]

I made the method public because I agree with you that other components could
call it through a SchedulerTaskExecutor*. If the base class had been
ComponentADelegate, I would have preferred to keep the method private to
document that it shouldn't be called outside of ComponentA (something is clearly
wrong if ComponentB has to cast a SchedulerThreadPool* to ComponentADelegate* to
call one of its method).

Relevant discussion:
https://groups.google.com/a/chromium.org/forum/#!msg/chromium-dev/CwzjcWd9cYM...

[gab, 2016/04/14 19:34:16]

Right I agree for Delegates because they are single-purposed (i.e. typically
handed off to a few known classes, in that sense it's akin to Bind'ing a private
method a bit). In this case however many things will hold on to
SchedulerTaskExecutor and post to them so it feels right to make the method
public to make it clear the call can come from anywhere... (I think this is also
why you agree but just reply'ing for the purpose of discussion :-))

+                            scoped_refptr<Sequence> sequence) override;
+
+  // PriorityQueue from which all threads of this thread pool get work.
+  PriorityQueue shared_priority_queue_;
+
+  // All worker threads owned by this thread pool. Only modified during
+  // initialization of the thread pool.
+  std::vector<std::unique_ptr<SchedulerWorkerThread>> worker_threads_;
+
+  // Synchronizes access to |idle_worker_threads_stack_| and
+  // |idle_worker_threads_stack_cv_for_testing_|. Has |shared_priority_queue_|'s
+  // lock as its predecessor so that a thread can be pushed to
+  // |idle_worker_threads_stack_| within the scope of a Transaction (more
+  // details in GetWork()).
+  SchedulerLock idle_worker_threads_stack_lock_;
+
+  // Stack of idle worker threads.
+  std::stack<SchedulerWorkerThread*> idle_worker_threads_stack_;
+
+  // Signaled when all worker threads become idle.
+  std::unique_ptr<ConditionVariable> idle_worker_threads_stack_cv_for_testing_;
+
+  // Signaled once JoinForTesting() has returned.
+  WaitableEvent join_for_testing_returned_;
+
+  // Delegate for all worker threads in this pool.
+  std::unique_ptr<SchedulerWorkerThread::Delegate> worker_thread_delegate_;
+
+  TaskTracker* const task_tracker_;
+
+  DISALLOW_COPY_AND_ASSIGN(SchedulerThreadPool);
+};
+
+}  // namespace internal
+}  // namespace base
+
+#endif  // BASE_TASK_SCHEDULER_SCHEDULER_THREAD_POOL_H_