TaskScheduler [11] Support ExecutionMode::SINGLE_THREADED.

base/task_scheduler/scheduler_thread_pool.cc

Index: base/task_scheduler/scheduler_thread_pool.cc
diff --git a/base/task_scheduler/scheduler_thread_pool.cc b/base/task_scheduler/scheduler_thread_pool.cc
index 8cd9620e26041965dbf9cb2779bbfe52b54944b1..9f2174ba45f8470656bbc7d4fa26262c81b46f35 100644
--- a/base/task_scheduler/scheduler_thread_pool.cc
+++ b/base/task_scheduler/scheduler_thread_pool.cc
@@ -12,6 +12,7 @@
 #include "base/logging.h"
 #include "base/memory/ptr_util.h"
 #include "base/sequenced_task_runner.h"
+#include "base/single_thread_task_runner.h"
 #include "base/task_scheduler/delayed_task_manager.h"
 #include "base/task_scheduler/task_tracker.h"
 #include "base/threading/thread_local.h"
@@ -25,6 +26,10 @@ namespace {
 LazyInstance<ThreadLocalPointer<const SchedulerThreadPool>>::Leaky
     tls_current_thread_pool = LAZY_INSTANCE_INITIALIZER;
 
+// SchedulerWorkerThread that owns the current thread, if any.
+LazyInstance<ThreadLocalPointer<const SchedulerWorkerThread>>::Leaky
+    tls_current_worker_thread = LAZY_INSTANCE_INITIALIZER;
+
 // A task runner that runs tasks with the PARALLEL ExecutionMode.
 class SchedulerParallelTaskRunner : public TaskRunner {
  public:
@@ -42,7 +47,7 @@ class SchedulerParallelTaskRunner : public TaskRunner {
     // Post the task as part of a one-off single-task Sequence.
     return thread_pool_->PostTaskWithSequence(
         WrapUnique(new Task(from_here, closure, traits_, delay)),
-        make_scoped_refptr(new Sequence));
+        make_scoped_refptr(new Sequence), nullptr);
   }
 
   bool RunsTasksOnCurrentThread() const override {
@@ -72,9 +77,10 @@ class SchedulerSequencedTaskRunner : public SequencedTaskRunner {
   bool PostDelayedTask(const tracked_objects::Location& from_here,
                        const Closure& closure,
                        TimeDelta delay) override {
-    // Post the task as part of |sequence|.
+    // Post the task as part of |sequence_|.
     return thread_pool_->PostTaskWithSequence(
-        WrapUnique(new Task(from_here, closure, traits_, delay)), sequence_);
+        WrapUnique(new Task(from_here, closure, traits_, delay)), sequence_,
+        nullptr);
   }
 
   bool PostNonNestableDelayedTask(const tracked_objects::Location& from_here,
@@ -100,6 +106,54 @@ class SchedulerSequencedTaskRunner : public SequencedTaskRunner {
   DISALLOW_COPY_AND_ASSIGN(SchedulerSequencedTaskRunner);
 };
 
+// A task runner that runs tasks with the SINGLE_THREADED ExecutionMode.
+class SchedulerSingleThreadTaskRunner : public SingleThreadTaskRunner {
+ public:
+  // Constructs a SchedulerSingleThreadTaskRunner which can be used to post
+  // tasks so long as |thread_pool| and |worker_thread| are alive.
+  // TODO(robliao): Find a concrete way to manage the memory of |thread_pool|
+  // and |worker_thread|.
+  SchedulerSingleThreadTaskRunner(const TaskTraits& traits,
+                                  SchedulerThreadPool* thread_pool,
+                                  SchedulerWorkerThread* worker_thread)
+      : traits_(traits),
+        thread_pool_(thread_pool),
+        worker_thread_(worker_thread) {}
+
+  // SingleThreadTaskRunner:
+  bool PostDelayedTask(const tracked_objects::Location& from_here,
+                       const Closure& closure,
+                       TimeDelta delay) override {
+    // Post the task to be executed by |worker_thread_| as part of |sequence_|.
+    return thread_pool_->PostTaskWithSequence(
+        WrapUnique(new Task(from_here, closure, traits_, delay)), sequence_,
+        worker_thread_);
+  }
+
+  bool PostNonNestableDelayedTask(const tracked_objects::Location& from_here,
+                                  const Closure& closure,
+                                  base::TimeDelta delay) override {
+    // Tasks are never nested within the task scheduler.
+    return PostDelayedTask(from_here, closure, delay);
+  }
+
+  bool RunsTasksOnCurrentThread() const override {
+    return tls_current_worker_thread.Get().Get() == worker_thread_;
+  }
+
+ private:
+  ~SchedulerSingleThreadTaskRunner() override = default;
+
+  // Sequence for all Tasks posted through this TaskRunner.
+  const scoped_refptr<Sequence> sequence_ = new Sequence;
+
+  const TaskTraits traits_;
+  SchedulerThreadPool* const thread_pool_;
+  SchedulerWorkerThread* const worker_thread_;
+
+  DISALLOW_COPY_AND_ASSIGN(SchedulerSingleThreadTaskRunner);
+};
+
 }  // namespace
 
 class SchedulerThreadPool::SchedulerWorkerThreadDelegateImpl
@@ -107,19 +161,25 @@ class SchedulerThreadPool::SchedulerWorkerThreadDelegateImpl
  public:
   SchedulerWorkerThreadDelegateImpl(
       SchedulerThreadPool* outer,
+      PriorityQueue* single_threaded_priority_queue,
       const EnqueueSequenceCallback& enqueue_sequence_callback);
   ~SchedulerWorkerThreadDelegateImpl() override;
 
   // SchedulerWorkerThread::Delegate:
-  void OnMainEntry() override;
+  void OnMainEntry(SchedulerWorkerThread* worker_thread) override;
   scoped_refptr<Sequence> GetWork(
       SchedulerWorkerThread* worker_thread) override;
   void EnqueueSequence(scoped_refptr<Sequence> sequence) override;
 
  private:
   SchedulerThreadPool* outer_;
+  PriorityQueue* const single_threaded_priority_queue_;
   const EnqueueSequenceCallback enqueue_sequence_callback_;
 
+  // True if the last Sequence returned by GetWork() was extracted from
+  // |single_threaded_priority_queue_|.
+  bool last_sequence_is_single_threaded_ = false;
+
   DISALLOW_COPY_AND_ASSIGN(SchedulerWorkerThreadDelegateImpl);
 };
 
@@ -153,10 +213,19 @@ scoped_refptr<TaskRunner> SchedulerThreadPool::CreateTaskRunnerWithTraits(
     case ExecutionMode::SEQUENCED:
       return make_scoped_refptr(new SchedulerSequencedTaskRunner(traits, this));
 
-    case ExecutionMode::SINGLE_THREADED:
-      // TODO(fdoray): Support SINGLE_THREADED TaskRunners.
-      NOTREACHED();
-      return nullptr;
+    case ExecutionMode::SINGLE_THREADED: {
+      // TODO(fdoray): Find a better way to assign a worker thread to a
+      // SingleThreadTaskRunner.
+      size_t worker_thread_index;
+      {
+        AutoSchedulerLock auto_lock(next_worker_thread_index_lock_);
+        worker_thread_index = next_worker_thread_index_;
+        next_worker_thread_index_ =
+            (next_worker_thread_index_ + 1) % worker_threads_.size();
+      }
+      return make_scoped_refptr(new SchedulerSingleThreadTaskRunner(
+          traits, this, worker_threads_[worker_thread_index].get()));
+    }
   }
 
   NOTREACHED();
@@ -198,7 +267,8 @@ void SchedulerThreadPool::JoinForTesting() {
 
 bool SchedulerThreadPool::PostTaskWithSequence(
     std::unique_ptr<Task> task,
-    scoped_refptr<Sequence> sequence) {
+    scoped_refptr<Sequence> sequence,
+    SchedulerWorkerThread* worker_thread) {
   DCHECK(task);
   DCHECK(sequence);
 
@@ -206,10 +276,11 @@ bool SchedulerThreadPool::PostTaskWithSequence(
     return false;
 
   if (task->delayed_run_time.is_null()) {
-    PostTaskWithSequenceNow(std::move(task), std::move(sequence));
+    PostTaskWithSequenceNow(std::move(task), std::move(sequence),
+                            worker_thread);
   } else {
     delayed_task_manager_->AddDelayedTask(std::move(task), std::move(sequence),
-                                          this);
+                                          worker_thread, this);
   }
 
   return true;
@@ -217,7 +288,8 @@ bool SchedulerThreadPool::PostTaskWithSequence(
 
 void SchedulerThreadPool::PostTaskWithSequenceNow(
     std::unique_ptr<Task> task,
-    scoped_refptr<Sequence> sequence) {
+    scoped_refptr<Sequence> sequence,
+    SchedulerWorkerThread* worker_thread) {
   DCHECK(task);
   DCHECK(sequence);
 
@@ -225,47 +297,69 @@ void SchedulerThreadPool::PostTaskWithSequenceNow(
   // in the past).
   DCHECK_LE(task->delayed_run_time, TimeTicks::Now());
 
+  PriorityQueue* const priority_queue =
+      worker_thread ? single_threaded_priority_queues_[worker_thread].get()
+                    : &shared_priority_queue_;
+  DCHECK(priority_queue);
+
   const bool sequence_was_empty = sequence->PushTask(std::move(task));
   if (sequence_was_empty) {
-    // Insert |sequence| in |shared_priority_queue_| if it was empty before
-    // |task| was inserted into it. Otherwise, one of these must be true:
+    // Insert |sequence| in |priority_queue| if it was empty before |task| was
+    // inserted into it. Otherwise, one of these must be true:
     // - |sequence| is already in a PriorityQueue (not necessarily
     //   |shared_priority_queue_|), or,
     // - A worker thread is running a Task from |sequence|. It will insert
     //   |sequence| in a PriorityQueue once it's done running the Task.
     const auto sequence_sort_key = sequence->GetSortKey();
-    shared_priority_queue_.BeginTransaction()->Push(
+    priority_queue->BeginTransaction()->Push(
         WrapUnique(new PriorityQueue::SequenceAndSortKey(std::move(sequence),
                                                          sequence_sort_key)));
 
     // Wake up a worker thread to process |sequence|.
-    WakeUpOneThread();
+    if (worker_thread)
+      worker_thread->WakeUp();
+    else
+      WakeUpOneThread();
   }
 }
 
 SchedulerThreadPool::SchedulerWorkerThreadDelegateImpl::
     SchedulerWorkerThreadDelegateImpl(
         SchedulerThreadPool* outer,
+        PriorityQueue* single_threaded_priority_queue,
         const EnqueueSequenceCallback& enqueue_sequence_callback)
-    : outer_(outer), enqueue_sequence_callback_(enqueue_sequence_callback) {}
+    : outer_(outer),
+      single_threaded_priority_queue_(single_threaded_priority_queue),
+      enqueue_sequence_callback_(enqueue_sequence_callback) {}
 
 SchedulerThreadPool::SchedulerWorkerThreadDelegateImpl::
     ~SchedulerWorkerThreadDelegateImpl() = default;
 
-void SchedulerThreadPool::SchedulerWorkerThreadDelegateImpl::OnMainEntry() {
+void SchedulerThreadPool::SchedulerWorkerThreadDelegateImpl::OnMainEntry(
+    SchedulerWorkerThread* worker_thread) {
+  DCHECK(!tls_current_worker_thread.Get().Get());
   DCHECK(!tls_current_thread_pool.Get().Get());
+  tls_current_worker_thread.Get().Set(worker_thread);
   tls_current_thread_pool.Get().Set(outer_);
 }
 
 scoped_refptr<Sequence>
 SchedulerThreadPool::SchedulerWorkerThreadDelegateImpl::GetWork(
     SchedulerWorkerThread* worker_thread) {
-  std::unique_ptr<PriorityQueue::Transaction> transaction(
+  std::unique_ptr<PriorityQueue::Transaction> shared_transaction(
       outer_->shared_priority_queue_.BeginTransaction());
-  const auto& sequence_and_sort_key = transaction->Peek();
+  const auto& shared_sequence_and_sort_key = shared_transaction->Peek();
+
+  std::unique_ptr<PriorityQueue::Transaction> single_threaded_transaction(
+      single_threaded_priority_queue_->BeginTransaction());
+  const auto& single_threaded_sequence_and_sort_key =
+      single_threaded_transaction->Peek();
+
+  if (shared_sequence_and_sort_key.is_null() &&
+      single_threaded_sequence_and_sort_key.is_null()) {
+    single_threaded_transaction.reset();
 
-  if (sequence_and_sort_key.is_null()) {
-    // |transaction| is kept alive while |worker_thread| is added to
+    // |shared_transaction| is kept alive while |worker_thread| is added to
     // |idle_worker_threads_stack_| to avoid this race:
     // 1. This thread creates a Transaction, finds |shared_priority_queue_|
     //    empty and ends the Transaction.
@@ -281,14 +375,43 @@ SchedulerThreadPool::SchedulerWorkerThreadDelegateImpl::GetWork(
     return nullptr;
   }
 
-  scoped_refptr<Sequence> sequence = sequence_and_sort_key.sequence;
-  transaction->Pop();
+  scoped_refptr<Sequence> sequence;
+
+  if (single_threaded_sequence_and_sort_key.is_null() ||
+      (!shared_sequence_and_sort_key.is_null() &&
+       single_threaded_sequence_and_sort_key.sort_key <
+           shared_sequence_and_sort_key.sort_key)) {
+    sequence = shared_sequence_and_sort_key.sequence;
+    shared_transaction->Pop();
+    last_sequence_is_single_threaded_ = false;
+  } else {
+    DCHECK(!single_threaded_sequence_and_sort_key.is_null());
+    sequence = single_threaded_sequence_and_sort_key.sequence;
+    single_threaded_transaction->Pop();
+    last_sequence_is_single_threaded_ = true;
+  }
+
+  shared_transaction.reset();
+  single_threaded_transaction.reset();
+  outer_->RemoveFromIdleWorkerThreadsStack(worker_thread);
+
   return sequence;
 }
 
 void SchedulerThreadPool::SchedulerWorkerThreadDelegateImpl::EnqueueSequence(
     scoped_refptr<Sequence> sequence) {
-  enqueue_sequence_callback_.Run(std::move(sequence));
+  if (last_sequence_is_single_threaded_) {
+    // A single-threaded Sequence is always re-enqueued in the single-threaded
+    // PriorityQueue from which it was extracted.
+    const SequenceSortKey sequence_sort_key = sequence->GetSortKey();
+    single_threaded_priority_queue_->BeginTransaction()->Push(
+        WrapUnique(new PriorityQueue::SequenceAndSortKey(std::move(sequence),
+                                                         sequence_sort_key)));
+  } else {
+    // |enqueue_sequence_callback_| will determine in which PriorityQueue
+    // |sequence| must be enqueued.
+    enqueue_sequence_callback_.Run(std::move(sequence));
+  }
 }
 
 SchedulerThreadPool::SchedulerThreadPool(
@@ -313,14 +436,19 @@ bool SchedulerThreadPool::Initialize(
   DCHECK(worker_threads_.empty());
 
   for (size_t i = 0; i < max_threads; ++i) {
+    std::unique_ptr<PriorityQueue> single_threaded_priority_queue(
+        new PriorityQueue(&shared_priority_queue_));
     std::unique_ptr<SchedulerWorkerThread> worker_thread =
         SchedulerWorkerThread::CreateWorkerThread(
             thread_priority, WrapUnique(new SchedulerWorkerThreadDelegateImpl(
-                                 this, enqueue_sequence_callback)),
+                                 this, single_threaded_priority_queue.get(),
+                                 enqueue_sequence_callback)),
             task_tracker_);
     if (!worker_thread)
       break;
     idle_worker_threads_stack_.Push(worker_thread.get());
+    single_threaded_priority_queues_[worker_thread.get()] =
+        std::move(single_threaded_priority_queue);
     worker_threads_.push_back(std::move(worker_thread));
   }
 
@@ -343,6 +471,12 @@ void SchedulerThreadPool::AddToIdleWorkerThreadsStack(
     idle_worker_threads_stack_cv_for_testing_->Broadcast();
 }
 
+void SchedulerThreadPool::RemoveFromIdleWorkerThreadsStack(
+    SchedulerWorkerThread* worker_thread) {
+  AutoSchedulerLock auto_lock(idle_worker_threads_stack_lock_);
+  idle_worker_threads_stack_.Remove(worker_thread);
+}
+
 SchedulerWorkerThread* SchedulerThreadPool::PopOneIdleWorkerThread() {
   AutoSchedulerLock auto_lock(idle_worker_threads_stack_lock_);