Handle delayed tasks more natively in the scheduler

content/renderer/scheduler/task_queue_manager.h

 // Copyright 2014 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 CONTENT_RENDERER_SCHEDULER_TASK_QUEUE_MANAGER_H_
 #define CONTENT_RENDERER_SCHEDULER_TASK_QUEUE_MANAGER_H_
 
 #include "base/atomic_sequence_num.h"
 #include "base/debug/task_annotator.h"
 #include "base/macros.h"
@@ skipping 11 matching lines @@
 class TracedValue;
 }
 }
 
 namespace cc {
 class TestNowSource;
 }
 
 namespace content {
 namespace internal {
+class LazyNow;
 class TaskQueue;
 }
 class TaskQueueSelector;
 class NestableSingleThreadTaskRunner;
 
 // The task queue manager provides N task queues and a selector interface for
 // choosing which task queue to service next. Each task queue consists of two
 // sub queues:
 //
 // 1. Incoming task queue. Tasks that are posted get immediately appended here.
@@ skipping 49 matching lines @@
   // This function only needs to be called if automatic pumping is disabled
   // for |queue_index|. See |SetQueueAutoPumpPolicy|. By default automatic
   // pumping is enabled for all queues.
   void PumpQueue(size_t queue_index);
 
   // Returns true if there no tasks in either the work or incoming task queue
   // identified by |queue_index|. Note that this function involves taking a
   // lock, so calling it has some overhead.
   bool IsQueueEmpty(size_t queue_index) const;
 
-  // Returns the time of the next pending delayed task in any queue. Returns
-  // a null TimeTicks object if no tasks are pending.
+  // Returns the time of the next pending delayed task in any queue.  Ignores
+  // any delayed tasks whose delay has expired. Returns a null TimeTicks object
+  // if no tasks are pending.  NOTE this is somewhat expensive since every queue
+  // will get locked.
   base::TimeTicks NextPendingDelayedTaskRunTime();
 
   // Set the name |queue_index| for tracing purposes. |name| must be a pointer
   // to a static string.
   void SetQueueName(size_t queue_index, const char* name);
 
   // Set the number of tasks executed in a single invocation of the task queue
   // manager. Increasing the batch size can reduce the overhead of yielding
   // back to the main message loop -- at the cost of potentially delaying other
   // tasks posted to the main loop. The batch size is 1 by default.
   void SetWorkBatchSize(int work_batch_size);
 
   // These functions can only be called on the same thread that the task queue
   // manager executes its tasks on.
   void AddTaskObserver(base::MessageLoop::TaskObserver* task_observer);
   void RemoveTaskObserver(base::MessageLoop::TaskObserver* task_observer);
 
   void SetTimeSourceForTesting(scoped_refptr<cc::TestNowSource> time_source);
 
  private:
+  friend class internal::LazyNow;
   friend class internal::TaskQueue;
 
   // Called by the task queue to register a new pending task and allocate a
   // sequence number for it.
   void DidQueueTask(base::PendingTask* pending_task);
 
   // Post a task to call DoWork() on the main task runner.  Only one pending
   // DoWork is allowed from the main thread, to prevent an explosion of pending
   // DoWorks.
   void MaybePostDoWorkOnMainRunner();
 
   // Use the selector to choose a pending task and run it.
   void DoWork(bool posted_from_main_thread);
 
+  // Delayed Tasks with run_times <= Now() are enqueued onto the work queue.
   // Reloads any empty work queues which have automatic pumping enabled and
   // which are eligible to be auto pumped based on the |previous_task| which was
   // run. Call with an empty |previous_task| if no task was just run. Returns
   // true if any work queue has tasks after doing this.
   // |next_pending_delayed_task| should be the time of the next known delayed
   // task. It is updated if any task is found which should run earlier.
-  bool UpdateWorkQueues(base::TimeTicks* next_pending_delayed_task,
-                        const base::PendingTask* previous_task);
+  bool UpdateWorkQueues(const base::PendingTask* previous_task);
 
   // Chooses the next work queue to service. Returns true if |out_queue_index|
   // indicates the queue from which the next task should be run, false to
   // avoid running any tasks.
   bool SelectWorkQueueToService(size_t* out_queue_index);
 
   // Runs a single nestable task from the work queue designated by
   // |queue_index|. If |has_previous_task| is true, |previous_task| should
   // contain the previous task in this work batch. Non-nestable task are
   // reposted on the run loop. The queue must not be empty.
@@ skipping 36 matching lines @@
   ObserverList<base::MessageLoop::TaskObserver> task_observers_;
 
   base::WeakPtrFactory<TaskQueueManager> weak_factory_;
 
   DISALLOW_COPY_AND_ASSIGN(TaskQueueManager);
 };
 
 }  // namespace content
 
 #endif  // CONTENT_RENDERER_SCHEDULER_TASK_QUEUE_MANAGER_H_