content/child/scheduler/task_queue_manager.cc

   1 // Copyright 2014 The Chromium Authors. All rights reserved.
   2 // Use of this source code is governed by a BSD-style license that can be
   3 // found in the LICENSE file.
   4
   5 #include "content/child/scheduler/task_queue_manager.h"
   6
   7 #include <queue>
   8 #include <set>
   9
  10 #include "base/bind.h"
  11 #include "base/trace_event/trace_event.h"
  12 #include "base/trace_event/trace_event_argument.h"
  13 #include "cc/test/test_now_source.h"
  14 #include "content/child/scheduler/nestable_single_thread_task_runner.h"
  15 #include "content/child/scheduler/task_queue_selector.h"
  16
  17 namespace {
  18 const int64_t kMaxTimeTicks = std::numeric_limits<int64>::max();
  19 }
  20
  21 namespace content {
  22 namespace internal {
  23
  24 // Now() is somewhat expensive so it makes sense not to call Now() unless we
  25 // really need to.
  26 class LazyNow {
  27  public:
  28   explicit LazyNow(base::TimeTicks now)
  29       : task_queue_manager_(nullptr), now_(now) {
  30     DCHECK(!now.is_null());
  31   }
  32
  33   explicit LazyNow(TaskQueueManager* task_queue_manager)
  34       : task_queue_manager_(task_queue_manager) {}
  35
  36   base::TimeTicks Now() {
  37     if (now_.is_null())
  38       now_ = task_queue_manager_->Now();
  39     return now_;
  40   }
  41
  42  private:
  43   TaskQueueManager* task_queue_manager_;  // NOT OWNED
  44   base::TimeTicks now_;
  45 };
  46
  47 class TaskQueue : public base::SingleThreadTaskRunner {
  48  public:
  49   TaskQueue(TaskQueueManager* task_queue_manager,
  50             const char* disabled_by_default_tracing_category);
  51
  52   // base::SingleThreadTaskRunner implementation.
  53   bool RunsTasksOnCurrentThread() const override;
  54   bool PostDelayedTask(const tracked_objects::Location& from_here,
  55                        const base::Closure& task,
  56                        base::TimeDelta delay) override {
  57     return PostDelayedTaskImpl(from_here, task, delay, TaskType::NORMAL);
  58   }
  59
  60   bool PostNonNestableDelayedTask(const tracked_objects::Location& from_here,
  61                                   const base::Closure& task,
  62                                   base::TimeDelta delay) override {
  63     return PostDelayedTaskImpl(from_here, task, delay, TaskType::NON_NESTABLE);
  64   }
  65
  66   bool IsQueueEmpty() const;
  67
  68   void SetPumpPolicy(TaskQueueManager::PumpPolicy pump_policy);
  69   void PumpQueue();
  70
  71   bool NextPendingDelayedTaskRunTime(
  72       base::TimeTicks* next_pending_delayed_task);
  73
  74   bool UpdateWorkQueue(LazyNow* lazy_now,
  75                        const base::PendingTask* previous_task);
  76   base::PendingTask TakeTaskFromWorkQueue();
  77
  78   void WillDeleteTaskQueueManager();
  79
  80   base::TaskQueue& work_queue() { return work_queue_; }
  81
  82   void set_name(const char* name) { name_ = name; }
  83
  84   void AsValueInto(base::trace_event::TracedValue* state) const;
  85
  86  private:
  87   enum class TaskType {
  88     NORMAL,
  89     NON_NESTABLE,
  90   };
  91
  92   ~TaskQueue() override;
  93
  94   bool PostDelayedTaskImpl(const tracked_objects::Location& from_here,
  95                            const base::Closure& task,
  96                            base::TimeDelta delay,
  97                            TaskType task_type);
  98
  99   // Delayed task posted to the underlying run loop, which locks |lock_| and
 100   // calls MoveReadyDelayedTasksToIncomingQueueLocked to process dealyed tasks
 101   // that need to be run now.
 102   void MoveReadyDelayedTasksToIncomingQueue();
 103
 104   // Enqueues any delayed tasks which should be run now on the incoming_queue_
 105   // and calls ScheduleDelayedWorkLocked to ensure future tasks are scheduled.
 106   // Must be called with |lock_| locked.
 107   void MoveReadyDelayedTasksToIncomingQueueLocked(LazyNow* lazy_now);
 108
 109   // Posts MoveReadyDelayedTasksToIncomingQueue if there isn't already a task
 110   // posted on the underlying runloop for the next task's scheduled run time.
 111   void ScheduleDelayedWorkLocked(LazyNow* lazy_now);
 112
 113   void PumpQueueLocked();
 114   bool TaskIsOlderThanQueuedTasks(const base::PendingTask* task);
 115   bool ShouldAutoPumpQueueLocked(const base::PendingTask* previous_task);
 116   void EnqueueTaskLocked(const base::PendingTask& pending_task);
 117
 118   void TraceQueueSize(bool is_locked) const;
 119   static const char* PumpPolicyToString(
 120       TaskQueueManager::PumpPolicy pump_policy);
 121   static void QueueAsValueInto(const base::TaskQueue& queue,
 122                                base::trace_event::TracedValue* state);
 123   static void QueueAsValueInto(const base::DelayedTaskQueue& queue,
 124                                base::trace_event::TracedValue* state);
 125   static void TaskAsValueInto(const base::PendingTask& task,
 126                               base::trace_event::TracedValue* state);
 127
 128   // This lock protects all members except the work queue and the
 129   // main_thread_checker_.
 130   mutable base::Lock lock_;
 131   base::PlatformThreadId thread_id_;
 132   TaskQueueManager* task_queue_manager_;
 133   base::TaskQueue incoming_queue_;
 134   TaskQueueManager::PumpPolicy pump_policy_;
 135   const char* name_;
 136   const char* disabled_by_default_tracing_category_;
 137   base::DelayedTaskQueue delayed_task_queue_;
 138   std::set<base::TimeTicks> in_flight_kick_delayed_tasks_;
 139
 140   base::ThreadChecker main_thread_checker_;
 141   base::TaskQueue work_queue_;
 142
 143   DISALLOW_COPY_AND_ASSIGN(TaskQueue);
 144 };
 145
 146 TaskQueue::TaskQueue(TaskQueueManager* task_queue_manager,
 147                      const char* disabled_by_default_tracing_category)
 148     : thread_id_(base::PlatformThread::CurrentId()),
 149       task_queue_manager_(task_queue_manager),
 150       pump_policy_(TaskQueueManager::PumpPolicy::AUTO),
 151       name_(nullptr),
 152       disabled_by_default_tracing_category_(
 153           disabled_by_default_tracing_category) {
 154 }
 155
 156 TaskQueue::~TaskQueue() {
 157 }
 158
 159 void TaskQueue::WillDeleteTaskQueueManager() {
 160   base::AutoLock lock(lock_);
 161   task_queue_manager_ = nullptr;
 162   delayed_task_queue_ = base::DelayedTaskQueue();
 163   incoming_queue_ = base::TaskQueue();
 164   work_queue_ = base::TaskQueue();
 165 }
 166
 167 bool TaskQueue::RunsTasksOnCurrentThread() const {
 168   base::AutoLock lock(lock_);
 169   return base::PlatformThread::CurrentId() == thread_id_;
 170 }
 171
 172 bool TaskQueue::PostDelayedTaskImpl(const tracked_objects::Location& from_here,
 173                                     const base::Closure& task,
 174                                     base::TimeDelta delay,
 175                                     TaskType task_type) {
 176   base::AutoLock lock(lock_);
 177   if (!task_queue_manager_)
 178     return false;
 179
 180   base::PendingTask pending_task(from_here, task, base::TimeTicks(),
 181                                  task_type != TaskType::NON_NESTABLE);
 182   task_queue_manager_->DidQueueTask(&pending_task);
 183
 184   if (delay > base::TimeDelta()) {
 185     base::TimeTicks now = task_queue_manager_->Now();
 186     pending_task.delayed_run_time = now + delay;
 187     delayed_task_queue_.push(pending_task);
 188     TraceQueueSize(true);
 189     // If we changed the topmost task, then it is time to reschedule.
 190     if (delayed_task_queue_.top().task.Equals(pending_task.task)) {
 191       LazyNow lazy_now(now);
 192       ScheduleDelayedWorkLocked(&lazy_now);
 193     }
 194     return true;
 195   }
 196   EnqueueTaskLocked(pending_task);
 197   return true;
 198 }
 199
 200 void TaskQueue::MoveReadyDelayedTasksToIncomingQueue() {
 201   DCHECK(main_thread_checker_.CalledOnValidThread());
 202   base::AutoLock lock(lock_);
 203   if (!task_queue_manager_)
 204     return;
 205
 206   LazyNow lazy_now(task_queue_manager_);
 207   MoveReadyDelayedTasksToIncomingQueueLocked(&lazy_now);
 208 }
 209
 210 void TaskQueue::MoveReadyDelayedTasksToIncomingQueueLocked(LazyNow* lazy_now) {
 211   lock_.AssertAcquired();
 212   // Enqueue all delayed tasks that should be running now.
 213   while (!delayed_task_queue_.empty() &&
 214          delayed_task_queue_.top().delayed_run_time <= lazy_now->Now()) {
 215     in_flight_kick_delayed_tasks_.erase(
 216         delayed_task_queue_.top().delayed_run_time);
 217     EnqueueTaskLocked(delayed_task_queue_.top());
 218     delayed_task_queue_.pop();
 219   }
 220   TraceQueueSize(true);
 221   ScheduleDelayedWorkLocked(lazy_now);
 222 }
 223
 224 void TaskQueue::ScheduleDelayedWorkLocked(LazyNow* lazy_now) {
 225   lock_.AssertAcquired();
 226   // Any remaining tasks are in the future, so queue a task to kick them.
 227   if (!delayed_task_queue_.empty()) {
 228     base::TimeTicks next_run_time = delayed_task_queue_.top().delayed_run_time;
 229     DCHECK_GT(next_run_time, lazy_now->Now());
 230     // Make sure we don't have more than one
 231     // MoveReadyDelayedTasksToIncomingQueue posted for a particular scheduled
 232     // run time (note it's fine to have multiple ones in flight for distinct
 233     // run times).
 234     if (in_flight_kick_delayed_tasks_.find(next_run_time) ==
 235         in_flight_kick_delayed_tasks_.end()) {
 236       in_flight_kick_delayed_tasks_.insert(next_run_time);
 237       base::TimeDelta delay = next_run_time - lazy_now->Now();
 238       task_queue_manager_->PostDelayedTask(
 239           FROM_HERE,
 240           Bind(&TaskQueue::MoveReadyDelayedTasksToIncomingQueue, this), delay);
 241     }
 242   }
 243 }
 244
 245 bool TaskQueue::IsQueueEmpty() const {
 246   if (!work_queue_.empty())
 247     return false;
 248
 249   {
 250     base::AutoLock lock(lock_);
 251     return incoming_queue_.empty();
 252   }
 253 }
 254
 255 bool TaskQueue::TaskIsOlderThanQueuedTasks(const base::PendingTask* task) {
 256   lock_.AssertAcquired();
 257   // A null task is passed when UpdateQueue is called before any task is run.
 258   // In this case we don't want to pump an after_wakeup queue, so return true
 259   // here.
 260   if (!task)
 261     return true;
 262
 263   // Return false if there are no task in the incoming queue.
 264   if (incoming_queue_.empty())
 265     return false;
 266
 267   base::PendingTask oldest_queued_task = incoming_queue_.front();
 268   DCHECK(oldest_queued_task.delayed_run_time.is_null());
 269   DCHECK(task->delayed_run_time.is_null());
 270
 271   // Note: the comparison is correct due to the fact that the PendingTask
 272   // operator inverts its comparison operation in order to work well in a heap
 273   // based priority queue.
 274   return oldest_queued_task < *task;
 275 }
 276
 277 bool TaskQueue::ShouldAutoPumpQueueLocked(
 278     const base::PendingTask* previous_task) {
 279   lock_.AssertAcquired();
 280   if (pump_policy_ == TaskQueueManager::PumpPolicy::MANUAL)
 281     return false;
 282   if (pump_policy_ == TaskQueueManager::PumpPolicy::AFTER_WAKEUP &&
 283       TaskIsOlderThanQueuedTasks(previous_task))
 284     return false;
 285   if (incoming_queue_.empty())
 286     return false;
 287   return true;
 288 }
 289
 290 bool TaskQueue::NextPendingDelayedTaskRunTime(
 291     base::TimeTicks* next_pending_delayed_task) {
 292   base::AutoLock lock(lock_);
 293   if (delayed_task_queue_.empty())
 294     return false;
 295   *next_pending_delayed_task = delayed_task_queue_.top().delayed_run_time;
 296   return true;
 297 }
 298
 299 bool TaskQueue::UpdateWorkQueue(LazyNow* lazy_now,
 300                                 const base::PendingTask* previous_task) {
 301   if (!work_queue_.empty())
 302     return true;
 303
 304   {
 305     base::AutoLock lock(lock_);
 306     MoveReadyDelayedTasksToIncomingQueueLocked(lazy_now);
 307     if (!ShouldAutoPumpQueueLocked(previous_task))
 308       return false;
 309     MoveReadyDelayedTasksToIncomingQueueLocked(lazy_now);
 310     work_queue_.Swap(&incoming_queue_);
 311     TraceQueueSize(true);
 312     return true;
 313   }
 314 }
 315
 316 base::PendingTask TaskQueue::TakeTaskFromWorkQueue() {
 317   base::PendingTask pending_task = work_queue_.front();
 318   work_queue_.pop();
 319   TraceQueueSize(false);
 320   return pending_task;
 321 }
 322
 323 void TaskQueue::TraceQueueSize(bool is_locked) const {
 324   bool is_tracing;
 325   TRACE_EVENT_CATEGORY_GROUP_ENABLED(disabled_by_default_tracing_category_,
 326                                      &is_tracing);
 327   if (!is_tracing || !name_)
 328     return;
 329   if (!is_locked)
 330     lock_.Acquire();
 331   else
 332     lock_.AssertAcquired();
 333   TRACE_COUNTER1(
 334       disabled_by_default_tracing_category_, name_,
 335       incoming_queue_.size() + work_queue_.size() + delayed_task_queue_.size());
 336   if (!is_locked)
 337     lock_.Release();
 338 }
 339
 340 void TaskQueue::EnqueueTaskLocked(const base::PendingTask& pending_task) {
 341   lock_.AssertAcquired();
 342   if (!task_queue_manager_)
 343     return;
 344   if (pump_policy_ == TaskQueueManager::PumpPolicy::AUTO &&
 345       incoming_queue_.empty())
 346     task_queue_manager_->MaybePostDoWorkOnMainRunner();
 347   incoming_queue_.push(pending_task);
 348
 349   if (!pending_task.delayed_run_time.is_null()) {
 350     // Clear the delayed run time because we've already applied the delay
 351     // before getting here.
 352     incoming_queue_.back().delayed_run_time = base::TimeTicks();
 353   }
 354   TraceQueueSize(true);
 355 }
 356
 357 void TaskQueue::SetPumpPolicy(TaskQueueManager::PumpPolicy pump_policy) {
 358   base::AutoLock lock(lock_);
 359   if (pump_policy == TaskQueueManager::PumpPolicy::AUTO &&
 360       pump_policy_ != TaskQueueManager::PumpPolicy::AUTO) {
 361     PumpQueueLocked();
 362   }
 363   pump_policy_ = pump_policy;
 364 }
 365
 366 void TaskQueue::PumpQueueLocked() {
 367   lock_.AssertAcquired();
 368   if (task_queue_manager_) {
 369     LazyNow lazy_now(task_queue_manager_);
 370     MoveReadyDelayedTasksToIncomingQueueLocked(&lazy_now);
 371   }
 372   while (!incoming_queue_.empty()) {
 373     work_queue_.push(incoming_queue_.front());
 374     incoming_queue_.pop();
 375   }
 376   if (!work_queue_.empty())
 377     task_queue_manager_->MaybePostDoWorkOnMainRunner();
 378 }
 379
 380 void TaskQueue::PumpQueue() {
 381   base::AutoLock lock(lock_);
 382   PumpQueueLocked();
 383 }
 384
 385 void TaskQueue::AsValueInto(base::trace_event::TracedValue* state) const {
 386   base::AutoLock lock(lock_);
 387   state->BeginDictionary();
 388   if (name_)
 389     state->SetString("name", name_);
 390   state->SetString("pump_policy", PumpPolicyToString(pump_policy_));
 391   state->BeginArray("incoming_queue");
 392   QueueAsValueInto(incoming_queue_, state);
 393   state->EndArray();
 394   state->BeginArray("work_queue");
 395   QueueAsValueInto(work_queue_, state);
 396   state->EndArray();
 397   state->BeginArray("delayed_task_queue");
 398   QueueAsValueInto(delayed_task_queue_, state);
 399   state->EndArray();
 400   state->EndDictionary();
 401 }
 402
 403 // static
 404 const char* TaskQueue::PumpPolicyToString(
 405     TaskQueueManager::PumpPolicy pump_policy) {
 406   switch (pump_policy) {
 407     case TaskQueueManager::PumpPolicy::AUTO:
 408       return "auto";
 409     case TaskQueueManager::PumpPolicy::AFTER_WAKEUP:
 410       return "after_wakeup";
 411     case TaskQueueManager::PumpPolicy::MANUAL:
 412       return "manual";
 413     default:
 414       NOTREACHED();
 415       return nullptr;
 416   }
 417 }
 418
 419 // static
 420 void TaskQueue::QueueAsValueInto(const base::TaskQueue& queue,
 421                                  base::trace_event::TracedValue* state) {
 422   base::TaskQueue queue_copy(queue);
 423   while (!queue_copy.empty()) {
 424     TaskAsValueInto(queue_copy.front(), state);
 425     queue_copy.pop();
 426   }
 427 }
 428
 429 // static
 430 void TaskQueue::QueueAsValueInto(const base::DelayedTaskQueue& queue,
 431                                  base::trace_event::TracedValue* state) {
 432   base::DelayedTaskQueue queue_copy(queue);
 433   while (!queue_copy.empty()) {
 434     TaskAsValueInto(queue_copy.top(), state);
 435     queue_copy.pop();
 436   }
 437 }
 438
 439 // static
 440 void TaskQueue::TaskAsValueInto(const base::PendingTask& task,
 441                                 base::trace_event::TracedValue* state) {
 442   state->BeginDictionary();
 443   state->SetString("posted_from", task.posted_from.ToString());
 444   state->SetInteger("sequence_num", task.sequence_num);
 445   state->SetBoolean("nestable", task.nestable);
 446   state->SetBoolean("is_high_res", task.is_high_res);
 447   state->SetDouble(
 448       "delayed_run_time",
 449       (task.delayed_run_time - base::TimeTicks()).InMicroseconds() / 1000.0L);
 450   state->EndDictionary();
 451 }
 452
 453 }  // namespace internal
 454
 455 TaskQueueManager::TaskQueueManager(
 456     size_t task_queue_count,
 457     scoped_refptr<NestableSingleThreadTaskRunner> main_task_runner,
 458     TaskQueueSelector* selector,
 459     const char* disabled_by_default_tracing_category)
 460     : main_task_runner_(main_task_runner),
 461       selector_(selector),
 462       pending_dowork_count_(0),
 463       work_batch_size_(1),
 464       time_source_(nullptr),
 465       disabled_by_default_tracing_category_(
 466           disabled_by_default_tracing_category),
 467       weak_factory_(this) {
 468   DCHECK(main_task_runner->RunsTasksOnCurrentThread());
 469   TRACE_EVENT_OBJECT_CREATED_WITH_ID(disabled_by_default_tracing_category,
 470                                      "TaskQueueManager", this);
 471
 472   task_queue_manager_weak_ptr_ = weak_factory_.GetWeakPtr();
 473   for (size_t i = 0; i < task_queue_count; i++) {
 474     scoped_refptr<internal::TaskQueue> queue(make_scoped_refptr(
 475         new internal::TaskQueue(this, disabled_by_default_tracing_category)));
 476     queues_.push_back(queue);
 477   }
 478
 479   std::vector<const base::TaskQueue*> work_queues;
 480   for (const auto& queue : queues_)
 481     work_queues.push_back(&queue->work_queue());
 482   selector_->RegisterWorkQueues(work_queues);
 483 }
 484
 485 TaskQueueManager::~TaskQueueManager() {
 486   TRACE_EVENT_OBJECT_DELETED_WITH_ID(disabled_by_default_tracing_category_,
 487                                      "TaskQueueManager", this);
 488   for (auto& queue : queues_)
 489     queue->WillDeleteTaskQueueManager();
 490 }
 491
 492 internal::TaskQueue* TaskQueueManager::Queue(size_t queue_index) const {
 493   DCHECK_LT(queue_index, queues_.size());
 494   return queues_[queue_index].get();
 495 }
 496
 497 scoped_refptr<base::SingleThreadTaskRunner>
 498 TaskQueueManager::TaskRunnerForQueue(size_t queue_index) const {
 499   return Queue(queue_index);
 500 }
 501
 502 bool TaskQueueManager::IsQueueEmpty(size_t queue_index) const {
 503   internal::TaskQueue* queue = Queue(queue_index);
 504   return queue->IsQueueEmpty();
 505 }
 506
 507 base::TimeTicks TaskQueueManager::NextPendingDelayedTaskRunTime() {
 508   DCHECK(main_thread_checker_.CalledOnValidThread());
 509   bool found_pending_task = false;
 510   base::TimeTicks next_pending_delayed_task(
 511       base::TimeTicks::FromInternalValue(kMaxTimeTicks));
 512   for (auto& queue : queues_) {
 513     base::TimeTicks queues_next_pending_delayed_task;
 514     if (queue->NextPendingDelayedTaskRunTime(
 515             &queues_next_pending_delayed_task)) {
 516       found_pending_task = true;
 517       next_pending_delayed_task =
 518           std::min(next_pending_delayed_task, queues_next_pending_delayed_task);
 519     }
 520   }
 521
 522   if (!found_pending_task)
 523     return base::TimeTicks();
 524
 525   DCHECK_NE(next_pending_delayed_task,
 526             base::TimeTicks::FromInternalValue(kMaxTimeTicks));
 527   return next_pending_delayed_task;
 528 }
 529
 530 void TaskQueueManager::SetPumpPolicy(size_t queue_index,
 531                                      PumpPolicy pump_policy) {
 532   DCHECK(main_thread_checker_.CalledOnValidThread());
 533   internal::TaskQueue* queue = Queue(queue_index);
 534   queue->SetPumpPolicy(pump_policy);
 535 }
 536
 537 void TaskQueueManager::PumpQueue(size_t queue_index) {
 538   DCHECK(main_thread_checker_.CalledOnValidThread());
 539   internal::TaskQueue* queue = Queue(queue_index);
 540   queue->PumpQueue();
 541 }
 542
 543 bool TaskQueueManager::UpdateWorkQueues(
 544     const base::PendingTask* previous_task) {
 545   // TODO(skyostil): This is not efficient when the number of queues grows very
 546   // large due to the number of locks taken. Consider optimizing when we get
 547   // there.
 548   DCHECK(main_thread_checker_.CalledOnValidThread());
 549   internal::LazyNow lazy_now(this);
 550   bool has_work = false;
 551   for (auto& queue : queues_) {
 552     has_work |= queue->UpdateWorkQueue(&lazy_now, previous_task);
 553     if (!queue->work_queue().empty()) {
 554       // Currently we should not be getting tasks with delayed run times in any
 555       // of the work queues.
 556       DCHECK(queue->work_queue().front().delayed_run_time.is_null());
 557     }
 558   }
 559   return has_work;
 560 }
 561
 562 void TaskQueueManager::MaybePostDoWorkOnMainRunner() {
 563   bool on_main_thread = main_task_runner_->BelongsToCurrentThread();
 564   if (on_main_thread) {
 565     // We only want one pending DoWork posted from the main thread, or we risk
 566     // an explosion of pending DoWorks which could starve out everything else.
 567     if (pending_dowork_count_ > 0) {
 568       return;
 569     }
 570     pending_dowork_count_++;
 571   }
 572
 573   main_task_runner_->PostTask(
 574       FROM_HERE, Bind(&TaskQueueManager::DoWork, task_queue_manager_weak_ptr_,
 575                       on_main_thread));
 576 }
 577
 578 void TaskQueueManager::DoWork(bool posted_from_main_thread) {
 579   if (posted_from_main_thread) {
 580     pending_dowork_count_--;
 581     DCHECK_GE(pending_dowork_count_, 0);
 582   }
 583   DCHECK(main_thread_checker_.CalledOnValidThread());
 584
 585   // Pass nullptr to UpdateWorkQueues here to prevent waking up a
 586   // pump-after-wakeup queue.
 587   if (!UpdateWorkQueues(nullptr))
 588     return;
 589
 590   base::PendingTask previous_task((tracked_objects::Location()),
 591                                   (base::Closure()));
 592   for (int i = 0; i < work_batch_size_; i++) {
 593     size_t queue_index;
 594     if (!SelectWorkQueueToService(&queue_index))
 595       return;
 596     // Note that this function won't post another call to DoWork if one is
 597     // already pending, so it is safe to call it in a loop.
 598     MaybePostDoWorkOnMainRunner();
 599     ProcessTaskFromWorkQueue(queue_index, i > 0, &previous_task);
 600
 601     if (!UpdateWorkQueues(&previous_task))
 602       return;
 603   }
 604 }
 605
 606 bool TaskQueueManager::SelectWorkQueueToService(size_t* out_queue_index) {
 607   bool should_run = selector_->SelectWorkQueueToService(out_queue_index);
 608   TRACE_EVENT_OBJECT_SNAPSHOT_WITH_ID(
 609       disabled_by_default_tracing_category_, "TaskQueueManager", this,
 610       AsValueWithSelectorResult(should_run, *out_queue_index));
 611   return should_run;
 612 }
 613
 614 void TaskQueueManager::DidQueueTask(base::PendingTask* pending_task) {
 615   pending_task->sequence_num = task_sequence_num_.GetNext();
 616   task_annotator_.DidQueueTask("TaskQueueManager::PostTask", *pending_task);
 617 }
 618
 619 void TaskQueueManager::ProcessTaskFromWorkQueue(
 620     size_t queue_index,
 621     bool has_previous_task,
 622     base::PendingTask* previous_task) {
 623   DCHECK(main_thread_checker_.CalledOnValidThread());
 624   internal::TaskQueue* queue = Queue(queue_index);
 625   base::PendingTask pending_task = queue->TakeTaskFromWorkQueue();
 626   if (!pending_task.nestable && main_task_runner_->IsNested()) {
 627     // Defer non-nestable work to the main task runner.  NOTE these tasks can be
 628     // arbitrarily delayed so the additional delay should not be a problem.
 629     main_task_runner_->PostNonNestableTask(pending_task.posted_from,
 630                                            pending_task.task);
 631   } else {
 632     // Suppress "will" task observer notifications for the first and "did"
 633     // notifications for the last task in the batch to avoid duplicate
 634     // notifications.
 635     if (has_previous_task) {
 636       FOR_EACH_OBSERVER(base::MessageLoop::TaskObserver, task_observers_,
 637                         DidProcessTask(*previous_task));
 638       FOR_EACH_OBSERVER(base::MessageLoop::TaskObserver, task_observers_,
 639                         WillProcessTask(pending_task));
 640     }
 641     task_annotator_.RunTask("TaskQueueManager::PostTask",
 642                             "TaskQueueManager::RunTask", pending_task);
 643     pending_task.task.Reset();
 644     *previous_task = pending_task;
 645   }
 646 }
 647
 648 bool TaskQueueManager::RunsTasksOnCurrentThread() const {
 649   return main_task_runner_->RunsTasksOnCurrentThread();
 650 }
 651
 652 bool TaskQueueManager::PostDelayedTask(
 653     const tracked_objects::Location& from_here,
 654     const base::Closure& task,
 655     base::TimeDelta delay) {
 656   DCHECK(delay > base::TimeDelta());
 657   return main_task_runner_->PostDelayedTask(from_here, task, delay);
 658 }
 659
 660 void TaskQueueManager::SetQueueName(size_t queue_index, const char* name) {
 661   DCHECK(main_thread_checker_.CalledOnValidThread());
 662   internal::TaskQueue* queue = Queue(queue_index);
 663   queue->set_name(name);
 664 }
 665
 666 void TaskQueueManager::SetWorkBatchSize(int work_batch_size) {
 667   DCHECK(main_thread_checker_.CalledOnValidThread());
 668   DCHECK_GE(work_batch_size, 1);
 669   work_batch_size_ = work_batch_size;
 670 }
 671
 672 void TaskQueueManager::AddTaskObserver(
 673     base::MessageLoop::TaskObserver* task_observer) {
 674   DCHECK(main_thread_checker_.CalledOnValidThread());
 675   base::MessageLoop::current()->AddTaskObserver(task_observer);
 676   task_observers_.AddObserver(task_observer);
 677 }
 678
 679 void TaskQueueManager::RemoveTaskObserver(
 680     base::MessageLoop::TaskObserver* task_observer) {
 681   DCHECK(main_thread_checker_.CalledOnValidThread());
 682   base::MessageLoop::current()->RemoveTaskObserver(task_observer);
 683   task_observers_.RemoveObserver(task_observer);
 684 }
 685
 686 void TaskQueueManager::SetTimeSourceForTesting(
 687     scoped_refptr<cc::TestNowSource> time_source) {
 688   DCHECK(main_thread_checker_.CalledOnValidThread());
 689   time_source_ = time_source;
 690 }
 691
 692 base::TimeTicks TaskQueueManager::Now() const {
 693   return UNLIKELY(time_source_) ? time_source_->Now() : base::TimeTicks::Now();
 694 }
 695
 696 scoped_refptr<base::trace_event::ConvertableToTraceFormat>
 697 TaskQueueManager::AsValueWithSelectorResult(bool should_run,
 698                                             size_t selected_queue) const {
 699   DCHECK(main_thread_checker_.CalledOnValidThread());
 700   scoped_refptr<base::trace_event::TracedValue> state =
 701       new base::trace_event::TracedValue();
 702   state->BeginArray("queues");
 703   for (auto& queue : queues_)
 704     queue->AsValueInto(state.get());
 705   state->EndArray();
 706   state->BeginDictionary("selector");
 707   selector_->AsValueInto(state.get());
 708   state->EndDictionary();
 709   if (should_run)
 710     state->SetInteger("selected_queue", selected_queue);
 711   return state;
 712 }
 713
 714 }  // namespace content