TaskScheduler [7] SchedulerThreadPool

base/task_scheduler/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/scheduler_thread_pool.h"
+
+#include <utility>
+
+#include "base/bind.h"
+#include "base/bind_helpers.h"
+#include "base/lazy_instance.h"
+#include "base/logging.h"
+#include "base/memory/ptr_util.h"
+#include "base/task_scheduler/utils.h"
+#include "base/threading/thread_local.h"
+
+namespace base {
+namespace internal {
+
+namespace {
+
+// SchedulerThreadPool that owns the current thread. Not set for threads that
+// don't belong to a SchedulerThreadPool.

[gab, 2016/04/14 16:11:34]

s/. Not set for threads that.../, if any./

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

Done.

+LazyInstance<ThreadLocalPointer<const SchedulerThreadPool>>::Leaky
+    tls_current_thread_pool = LAZY_INSTANCE_INITIALIZER;
+
+// A task runner that runs tasks with the PARALLEL ExecutionMode.
+class SchedulerParallelTaskRunner : public TaskRunner {
+ public:
+  SchedulerParallelTaskRunner(const TaskTraits& traits,
+                              SchedulerTaskExecutor* executor,
+                              TaskTracker* task_tracker)
+      : traits_(traits), executor_(executor), task_tracker_(task_tracker) {}
+
+  // TaskRunner:
+  bool PostDelayedTask(const tracked_objects::Location& from_here,
+                       const Closure& closure,
+                       TimeDelta delay) override {
+    // Post the task as part of a one-off single-task Sequence.
+    return PostTaskHelper(from_here, closure, traits_, delay,
+                          make_scoped_refptr(new Sequence), executor_,
+                          task_tracker_);
+  }
+
+  bool RunsTasksOnCurrentThread() const override {
+    return tls_current_thread_pool.Get().Get() == executor_;

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

I think this requires a static_cast<SchedulerTaskExecutor> on the TLS pointer
otherwise the pointer may be an offset to the subclass' virtual table and may
not match even when it's the same underlying STP (I'm not an expert here but I
think this is the case).

Or we could make the TLS pointer a SchedulerTaskExecutor (and then I guess you
need to cast on the comparison on line 137..).

[robliao, 2016/04/14 17:06:28]

C++ operator== requires that the types of both sides be equivalent before
performing the compare (operator== const X& lhs, const X& rhs)

This means both the left and right sides must be convertable to the same type to
make the comparison, so the cast is already there, otherwise we can't see it.

Consider the following:

class Root {};
class Root2 {};
class Multiple : public Root, public Root2 {};

Multiple* m = new Multiple;
Root* r = m;
Root2* r2 = m;

r == r2 // Compile Error: Root* and Root2* are not convertable between each
other.

m == r  // true - Multiple* is castable to Root*

m == r2 // true - Multiple* is castable to Root2*.

static_cast<void*>(m) == static_cast<void*>(r2) // false - In my compiler's
implementation, m and r2 point to different portions of the object.

[gab, 2016/04/14 18:28:21]

Ah ok, then all is good :-).

Shall we make the TLS state only a SchedulerTaskExecutor pointer though then
since we don't need it to be a full STP?

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

Done. Made the TLS a SchedulerTaskExecutor*.

+  }
+
+ private:
+  ~SchedulerParallelTaskRunner() override = default;
+
+  const TaskTraits traits_;
+  SchedulerTaskExecutor* const executor_;

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

Ah, here it is, I knew we had already made another assumption about lifetime.
This code (and the one in previous PatchSets on this CL) assume STP is alive
forever.

Should document that where necessary.

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

Done. Added comment.

+  TaskTracker* const task_tracker_;
+
+  DISALLOW_COPY_AND_ASSIGN(SchedulerParallelTaskRunner);
+};
+
+}  // namespace
+
+class SchedulerThreadPool::SchedulerWorkerThreadDelegateImpl
+    : public SchedulerWorkerThread::Delegate {
+ public:
+  SchedulerWorkerThreadDelegateImpl(
+      SchedulerThreadPool* outer,
+      const EnqueueSequenceCallback enqueue_sequence_callback);
+  ~SchedulerWorkerThreadDelegateImpl() override;
+
+  // SchedulerWorkerThread::Delegate:
+  void OnMainEntry() override;
+  scoped_refptr<Sequence> GetWork(
+      SchedulerWorkerThread* worker_thread) override;
+  void EnqueueSequence(scoped_refptr<Sequence> sequence) override;
+
+ private:
+  SchedulerThreadPool* outer_;
+  const EnqueueSequenceCallback enqueue_sequence_callback_;
+
+  DISALLOW_COPY_AND_ASSIGN(SchedulerWorkerThreadDelegateImpl);
+};
+
+SchedulerThreadPool::~SchedulerThreadPool() {
+#if DCHECK_IS_ON()
+  // SchedulerThreadPool should never be deleted in production unless its
+  // initialization failed.
+  AutoSchedulerLock auto_lock(idle_worker_threads_stack_lock_);
+  DCHECK(join_for_testing_returned_.IsSignaled() || worker_threads_.empty());
+#endif  // DCHECK_IS_ON()
+}
+
+std::unique_ptr<SchedulerThreadPool> SchedulerThreadPool::CreateThreadPool(
+    ThreadPriority thread_priority,
+    size_t max_threads,
+    const EnqueueSequenceCallback& enqueue_sequence_callback,
+    TaskTracker* task_tracker) {
+  std::unique_ptr<SchedulerThreadPool> thread_pool(
+      new SchedulerThreadPool(enqueue_sequence_callback, task_tracker));
+  if (thread_pool->Initialize(thread_priority, max_threads))
+    return thread_pool;
+  return nullptr;
+}
+
+scoped_refptr<TaskRunner> SchedulerThreadPool::CreateTaskRunnerWithTraits(
+    const TaskTraits& traits,
+    ExecutionMode execution_mode) {
+  switch (execution_mode) {
+    case ExecutionMode::PARALLEL:
+      return make_scoped_refptr(
+          new SchedulerParallelTaskRunner(traits, this, task_tracker_));
+
+    case ExecutionMode::SEQUENCED:
+    case ExecutionMode::SINGLE_THREADED:
+      // TODO(fdoray): Support SEQUENCED and SINGLE_THREADED TaskRunners.
+      NOTREACHED();
+      return nullptr;
+  }
+
+  NOTREACHED();
+  return nullptr;
+}
+
+void SchedulerThreadPool::EnqueueSequence(
+    scoped_refptr<Sequence> sequence,
+    const SequenceSortKey& sequence_sort_key) {
+  shared_priority_queue_.BeginTransaction()->Push(
+      WrapUnique(new PriorityQueue::SequenceAndSortKey(std::move(sequence),
+                                                       sequence_sort_key)));
+
+  // The thread calling this method just ran a Task from |sequence| and will
+  // soon try to get another Sequence from which to run a Task. If the thread
+  // belongs to this pool, it will get that Sequence from
+  // |shared_priority_queue_|. When that's the case, there is no need to wake up
+  // another thread after |sequence| is inserted in |shared_priority_queue_|. If
+  // we did wake up another thread, we would waste resources by having more
+  // threads trying to get a Sequence from |shared_priority_queue_| than the
+  // number of Sequences in it.
+  if (tls_current_thread_pool.Get().Get() != this)
+    WakeUpOneThread();
+}
+
+void SchedulerThreadPool::WaitForAllWorkerThreadsIdleForTesting() {
+  AutoSchedulerLock auto_lock(idle_worker_threads_stack_lock_);
+  while (idle_worker_threads_stack_.size() < worker_threads_.size())
+    idle_worker_threads_stack_cv_for_testing_->Wait();
+}
+
+void SchedulerThreadPool::JoinForTesting() {
+  for (const auto& worker_thread : worker_threads_)
+    worker_thread->JoinForTesting();
+
+  DCHECK(!join_for_testing_returned_.IsSignaled());
+  join_for_testing_returned_.Signal();
+}
+
+SchedulerThreadPool::SchedulerWorkerThreadDelegateImpl::
+    SchedulerWorkerThreadDelegateImpl(
+        SchedulerThreadPool* outer,
+        const EnqueueSequenceCallback enqueue_sequence_callback)
+    : outer_(outer), enqueue_sequence_callback_(enqueue_sequence_callback) {}
+
+SchedulerThreadPool::SchedulerWorkerThreadDelegateImpl::
+    ~SchedulerWorkerThreadDelegateImpl() = default;
+
+void SchedulerThreadPool::SchedulerWorkerThreadDelegateImpl::OnMainEntry() {
+  DCHECK(!tls_current_thread_pool.Get().Get());
+  tls_current_thread_pool.Get().Set(outer_);
+}
+
+scoped_refptr<Sequence>
+SchedulerThreadPool::SchedulerWorkerThreadDelegateImpl::GetWork(
+    SchedulerWorkerThread* worker_thread) {
+  std::unique_ptr<PriorityQueue::Transaction> transaction(
+      outer_->shared_priority_queue_.BeginTransaction());
+  const auto sequence_and_sort_key = transaction->Peek();
+
+  if (sequence_and_sort_key.is_null()) {
+    // |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.
+    // 2. Other thread creates a Transaction, inserts a Sequence into
+    //    |shared_priority_queue_| and ends the Transaction. This can't happen
+    //    if the Transaction of step 1 is still active because because there can
+    //    only be one active Transaction per PriorityQueue at a time.
+    // 3. Other thread calls WakeUpOneThread(). No thread is woken up because
+    //    |idle_worker_threads_stack_| is empty.
+    // 4. This thread adds itself to |idle_worker_threads_stack_| and goes to
+    //    sleep. No thread runs the Sequence inserted in step 2.
+    outer_->AddToIdleWorkerThreadsStack(worker_thread);
+    return nullptr;
+  }
+
+  transaction->Pop();
+  return sequence_and_sort_key.sequence;
+}
+
+void SchedulerThreadPool::SchedulerWorkerThreadDelegateImpl::EnqueueSequence(
+    scoped_refptr<Sequence> sequence) {
+  enqueue_sequence_callback_.Run(std::move(sequence));
+}
+
+SchedulerThreadPool::SchedulerThreadPool(
+    const EnqueueSequenceCallback& enqueue_sequence_callback,
+    TaskTracker* task_tracker)
+    : idle_worker_threads_stack_lock_(shared_priority_queue_.container_lock()),
+      idle_worker_threads_stack_cv_for_testing_(
+          idle_worker_threads_stack_lock_.CreateConditionVariable()),
+      join_for_testing_returned_(true, false),
+      worker_thread_delegate_(
+          new SchedulerWorkerThreadDelegateImpl(this,
+                                                enqueue_sequence_callback)),
+      task_tracker_(task_tracker) {
+  DCHECK(task_tracker_);
+}
+
+bool SchedulerThreadPool::Initialize(ThreadPriority thread_priority,
+                                     size_t max_threads) {
+  AutoSchedulerLock auto_lock(idle_worker_threads_stack_lock_);
+
+  DCHECK(worker_threads_.empty());
+
+  for (size_t i = 0; i < max_threads; ++i) {
+    std::unique_ptr<SchedulerWorkerThread> worker_thread =
+        SchedulerWorkerThread::CreateSchedulerWorkerThread(
+            thread_priority, worker_thread_delegate_.get(), task_tracker_);
+    if (!worker_thread)
+      break;
+    idle_worker_threads_stack_.push(worker_thread.get());
+    worker_threads_.push_back(std::move(worker_thread));
+  }
+
+  return !worker_threads_.empty();
+}
+
+void SchedulerThreadPool::WakeUpOneThread() {
+  SchedulerWorkerThread* worker_thread = PopOneIdleWorkerThread();
+  if (worker_thread)
+    worker_thread->WakeUp();
+}
+
+void SchedulerThreadPool::AddToIdleWorkerThreadsStack(
+    SchedulerWorkerThread* worker_thread) {
+  AutoSchedulerLock auto_lock(idle_worker_threads_stack_lock_);
+  idle_worker_threads_stack_.push(worker_thread);
+  DCHECK_LE(idle_worker_threads_stack_.size(), worker_threads_.size());
+
+  if (idle_worker_threads_stack_.size() == worker_threads_.size())
+    idle_worker_threads_stack_cv_for_testing_->Broadcast();
+}
+
+SchedulerWorkerThread* SchedulerThreadPool::PopOneIdleWorkerThread() {
+  AutoSchedulerLock auto_lock(idle_worker_threads_stack_lock_);
+
+  if (idle_worker_threads_stack_.empty())
+    return nullptr;
+
+  auto worker_thread = idle_worker_threads_stack_.top();
+  idle_worker_threads_stack_.pop();
+  return worker_thread;
+}
+
+void SchedulerThreadPool::PostTaskWithSequence(
+    std::unique_ptr<Task> task,
+    scoped_refptr<Sequence> sequence) {
+  DCHECK(task);
+  DCHECK(sequence);
+  DCHECK_GE(TimeTicks::Now(), task->delayed_run_time);

[gab, 2016/04/14 16:11:34]

What is this verifying? From tracking_info.h, |delayed_run_time| is the time
when the task is scheduled to run, why does that have to be before Now()?

(also I find inequality checks read better when the "constant", i.e. Now(), is
on the RHS of the comparison)

Also, as discussed offline, right now task.cc initializes that to null so this
is always true, but you said this would change later but this DCHECK would stick
so that's okay.

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

Added comment to explain the purpose of this check.

+
+  const bool sequence_was_empty = PostTaskWithSequenceHelper(
+      std::move(task), std::move(sequence), &shared_priority_queue_);
+
+  // No thread has already been woken up to run Tasks from |sequence| if it was
+  // empty before |task| was inserted into it.
+  if (sequence_was_empty)
+    WakeUpOneThread();
+}
+
+}  // namespace internal
+}  // namespace base