base/task_scheduler/task_tracker.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_TASK_TRACKER_H_
#define BASE_TASK_SCHEDULER_TASK_TRACKER_H_

#include <functional>
#include <memory>
#include <queue>

#include "base/atomicops.h"
#include "base/base_export.h"
#include "base/callback_forward.h"
#include "base/logging.h"
#include "base/macros.h"
#include "base/strings/string_piece.h"
#include "base/synchronization/waitable_event.h"
#include "base/task_scheduler/can_schedule_sequence_observer.h"
#include "base/task_scheduler/scheduler_lock.h"
#include "base/task_scheduler/sequence.h"
#include "base/task_scheduler/task.h"
#include "base/task_scheduler/task_traits.h"
#include "base/task_scheduler/tracked_ref.h"

namespace base {

class ConditionVariable;

namespace internal {

// TaskTracker enforces policies that determines whether:
// - A task can be added to a sequence (WillPostTask).
// - A sequence can be scheduled (WillScheduleSequence).
// - The next task in a scheduled sequence can run (RunAndPopNextTask).
// TaskTracker also sets up the environment to run a task (RunAndPopNextTask)
// and records metrics and trace events. This class is thread-safe.
//
// Life of a sequence:
// (possible states: IDLE, PREEMPTED, SCHEDULED, RUNNING)
//
//                            Create a sequence
//                                   |
//  ------------------------> Sequence is IDLE
//  |                                |
//  |                     Add a task to the sequence
//  |            (allowed by TaskTracker::WillPostTask)
//  |                                |
//  |               TaskTracker:WillScheduleSequence
//  |           _____________________|_____________________
//  |           |                                          |
//  |    Returns true                                Returns false
//  |           |                                          |
//  |           |                                Sequence is PREEMPTED <----
//  |           |                                          |               |
//  |           |                            Eventually,                   |
//  |           |                            CanScheduleSequenceObserver   |
//  |           |                            is notified that the          |
//  |           |                            sequence can be scheduled.    |
//  |           |__________________________________________|               |
//  |                               |                                      |
//  |                   (*) Sequence is SCHEDULED                          |
//  |                               |                                      |
//  |                A thread is ready to run the next                     |
//  |                      task in the sequence                            |
//  |                               |                                      |
//  |                TaskTracker::RunAndPopNextTask                        |
//  |                A task from the sequence is run                       |
//  |                      Sequence is RUNNING                             |
//  |                               |                                      |
//  |         ______________________|____                                  |
//  |         |                          |                                 |
//  |   Sequence is empty      Sequence has more tasks                     |
//  |_________|             _____________|_______________                  |
//                          |                            |                 |
//                   Sequence can be            Sequence cannot be         |
//                   scheduled                  scheduled at this          |
//                          |                   moment                     |
//                   Go back to (*)                      |_________________|
//
//
// Note: A background task is a task posted with TaskPriority::BACKGROUND. A
// foreground task is a task posted with TaskPriority::USER_VISIBLE or
// TaskPriority::USER_BLOCKING.
class BASE_EXPORT TaskTracker {
 public:
  // |histogram_label| is used as a suffix for histograms, it must not be empty.
  // The first constructor sets the maximum number of TaskPriority::BACKGROUND
  // sequences that can be scheduled concurrently to 0 if the
  // --disable-background-tasks flag is specified, max() otherwise. The second
  // constructor sets it to |max_num_scheduled_background_sequences|.
  TaskTracker(StringPiece histogram_label);
  TaskTracker(StringPiece histogram_label,
              int max_num_scheduled_background_sequences);

  virtual ~TaskTracker();

  // Synchronously shuts down the scheduler. Once this is called, only tasks
  // posted with the BLOCK_SHUTDOWN behavior will be run. Returns when:
  // - All SKIP_ON_SHUTDOWN tasks that were already running have completed their
  //   execution.
  // - All posted BLOCK_SHUTDOWN tasks have completed their execution.
  // CONTINUE_ON_SHUTDOWN tasks still may be running after Shutdown returns.
  // This can only be called once.
  void Shutdown();

  // Waits until there are no incomplete undelayed tasks. May be called in tests
  // to validate that a condition is met after all undelayed tasks have run.
  //
  // Does not wait for delayed tasks. Waits for undelayed tasks posted from
  // other threads during the call. Returns immediately when shutdown completes.
  void FlushForTesting();

  // Returns and calls |flush_callback| when there are no incomplete undelayed
  // tasks. |flush_callback| may be called back on any thread and should not
  // perform a lot of work. May be used when additional work on the current
  // thread needs to be performed during a flush. Only one
  // FlushAsyncForTesting() may be pending at any given time.
  void FlushAsyncForTesting(OnceClosure flush_callback);

  // Informs this TaskTracker that |task| is about to be posted. Returns true if
  // this operation is allowed (|task| should be posted if-and-only-if it is).
  bool WillPostTask(const Task& task);

  // Informs this TaskTracker that |sequence| is about to be scheduled. If this
  // returns |sequence|, it is expected that RunAndPopNextTask() will soon be
  // called with |sequence| as argument. Otherwise, RunAndPopNextTask() must not
  // be called with |sequence| as argument until |observer| is notified that
  // |sequence| can be scheduled (the caller doesn't need to keep a pointer to
  // |sequence|; it will be included in the notification to |observer|).
  // WillPostTask() must have allowed the task in front of |sequence| to be
  // posted before this is called. |observer| is only required if the priority
  // of |sequence| is TaskPriority::BACKGROUND
  scoped_refptr<Sequence> WillScheduleSequence(
      scoped_refptr<Sequence> sequence,
      CanScheduleSequenceObserver* observer);

  // Runs the next task in |sequence| unless the current shutdown state prevents
  // that. Then, pops the task from |sequence| (even if it didn't run). Returns
  // |sequence| if it can be rescheduled immediately. If |sequence| is non-empty
  // after popping a task from it but it can't be rescheduled immediately, it
  // will be handed back to |observer| when it can be rescheduled.
  // WillPostTask() must have allowed the task in front of |sequence| to be
  // posted before this is called. Also, WillScheduleSequence(),
  // RunAndPopNextTask() or CanScheduleSequenceObserver::OnCanScheduleSequence()
  // must have allowed |sequence| to be (re)scheduled.
  scoped_refptr<Sequence> RunAndPopNextTask(
      scoped_refptr<Sequence> sequence,
      CanScheduleSequenceObserver* observer);

  // Returns true once shutdown has started (Shutdown() has been called but
  // might not have returned). Note: sequential consistency with the thread
  // calling Shutdown() (or SetHasShutdownStartedForTesting()) isn't guaranteed
  // by this call.
  bool HasShutdownStarted() const;

  // Returns true if shutdown has completed (Shutdown() has returned).
  bool IsShutdownComplete() const;

  // Causes HasShutdownStarted() to return true. Unlike when Shutdown() returns,
  // IsShutdownComplete() won't return true after this returns. Shutdown()
  // cannot be called after this.
  void SetHasShutdownStartedForTesting();

  TrackedRef<TaskTracker> GetTrackedRef() {
    return tracked_ref_factory_.GetTrackedRef();
  }

 protected:
  // Runs and deletes |task| if |can_run_task| is true. Otherwise, just deletes
  // |task|. |task| is always deleted in the environment where it runs or would
  // have run. |sequence| is the sequence from which |task| was extracted. An
  // override is expected to call its parent's implementation but is free to
  // perform extra work before and after doing so.
  virtual void RunOrSkipTask(Task task, Sequence* sequence, bool can_run_task);

#if DCHECK_IS_ON()
  // Returns true if this context should be exempt from blocking shutdown
  // DCHECKs.
  // TODO(robliao): Remove when http://crbug.com/698140 is fixed.
  virtual bool IsPostingBlockShutdownTaskAfterShutdownAllowed();
#endif

  // Returns true if there are undelayed tasks that haven't completed their
  // execution (still queued or in progress). If it returns false: the side-
  // effects of all completed tasks are guaranteed to be visible to the caller.
  bool HasIncompleteUndelayedTasksForTesting() const;

 private:
  class State;
  struct PreemptedBackgroundSequence;

  void PerformShutdown();

  // Updates the maximum number of background sequences that can be scheduled
  // concurrently to |max_num_scheduled_background_sequences|. Then, schedules
  // as many preempted background sequences as allowed by the new value.
  void SetMaxNumScheduledBackgroundSequences(
      int max_num_scheduled_background_sequences);

  // Pops the next sequence in |preempted_background_sequences_| and increments
  // |num_scheduled_background_sequences_|. Must only be called in the scope of
  // |background_lock_|, with |preempted_background_sequences_| non-empty. The
  // caller must forward the returned sequence to the associated
  // CanScheduleSequenceObserver as soon as |background_lock_| is released.
  PreemptedBackgroundSequence
  GetPreemptedBackgroundSequenceToScheduleLockRequired();

  // Schedules |sequence_to_schedule.sequence| using
  // |sequence_to_schedule.observer|. Does not verify that the sequence is
  // allowed to be scheduled.
  void SchedulePreemptedBackgroundSequence(
      PreemptedBackgroundSequence sequence_to_schedule);

  // Called before WillPostTask() informs the tracing system that a task has
  // been posted. Updates |num_tasks_blocking_shutdown_| if necessary and
  // returns true if the current shutdown state allows the task to be posted.
  bool BeforePostTask(TaskShutdownBehavior shutdown_behavior);

  // Called before a task with |shutdown_behavior| is run by RunTask(). Updates
  // |num_tasks_blocking_shutdown_| if necessary and returns true if the current
  // shutdown state allows the task to be run.
  bool BeforeRunTask(TaskShutdownBehavior shutdown_behavior);

  // Called after a task with |shutdown_behavior| has been run by RunTask().
  // Updates |num_tasks_blocking_shutdown_| and signals |shutdown_cv_| if
  // necessary.
  void AfterRunTask(TaskShutdownBehavior shutdown_behavior);

  // Called when the number of tasks blocking shutdown becomes zero after
  // shutdown has started.
  void OnBlockingShutdownTasksComplete();

  // Decrements the number of incomplete undelayed tasks and signals |flush_cv_|
  // if it reaches zero.
  void DecrementNumIncompleteUndelayedTasks();

  // To be called after running a background task from |just_ran_sequence|.
  // Performs the following actions:
  //  - If |just_ran_sequence| is non-null:
  //    - returns it if it should be rescheduled by the caller of
  //      RunAndPopNextTask(), i.e. its next task is set to run earlier than the
  //      earliest currently preempted sequence.
  //    - Otherwise |just_ran_sequence| is preempted and the next preempted
  //      sequence is scheduled (|observer| will be notified when
  //      |just_ran_sequence| should be scheduled again).
  //  - If |just_ran_sequence| is null (RunAndPopNextTask() just popped the last
  //    task from it):
  //    - the next preempeted sequence (if any) is scheduled.
  //  - In all cases: adjusts the number of scheduled background sequences
  //    accordingly.
  scoped_refptr<Sequence> ManageBackgroundSequencesAfterRunningTask(
      scoped_refptr<Sequence> just_ran_sequence,
      CanScheduleSequenceObserver* observer);

  // Calls |flush_callback_for_testing_| if one is available in a lock-safe
  // manner.
  void CallFlushCallbackForTesting();

  // Number of tasks blocking shutdown and boolean indicating whether shutdown
  // has started.
  const std::unique_ptr<State> state_;

  // Number of undelayed tasks that haven't completed their execution. Is
  // decremented with a memory barrier after a task runs. Is accessed with an
  // acquire memory barrier in FlushForTesting(). The memory barriers ensure
  // that the memory written by flushed tasks is visible when FlushForTesting()
  // returns.
  subtle::Atomic32 num_incomplete_undelayed_tasks_ = 0;

  // Lock associated with |flush_cv_|. Partially synchronizes access to
  // |num_incomplete_undelayed_tasks_|. Full synchronization isn't needed
  // because it's atomic, but synchronization is needed to coordinate waking and
  // sleeping at the right time. Fully synchronizes access to
  // |flush_callback_for_testing_|.
  mutable SchedulerLock flush_lock_;

  // Signaled when |num_incomplete_undelayed_tasks_| is or reaches zero or when
  // shutdown completes.
  const std::unique_ptr<ConditionVariable> flush_cv_;

  // Invoked if non-null when |num_incomplete_undelayed_tasks_| is zero or when
  // shutdown completes.
  OnceClosure flush_callback_for_testing_;

  // Synchronizes access to shutdown related members below.
  mutable SchedulerLock shutdown_lock_;

  // Event instantiated when shutdown starts and signaled when shutdown
  // completes.
  std::unique_ptr<WaitableEvent> shutdown_event_;

  // Synchronizes accesses to |preempted_background_sequences_|,
  // |max_num_scheduled_background_sequences_| and
  // |num_scheduled_background_sequences_|.
  SchedulerLock background_lock_;

  // A priority queue of sequences that are waiting to be scheduled. Use
  // std::greater so that the sequence which contains the task that has been
  // posted the earliest is on top of the priority queue.
  std::priority_queue<PreemptedBackgroundSequence,
                      std::vector<PreemptedBackgroundSequence>,
                      std::greater<PreemptedBackgroundSequence>>
      preempted_background_sequences_;

  // Maximum number of background sequences that can that be scheduled
  // concurrently.
  int max_num_scheduled_background_sequences_;

  // Number of currently scheduled background sequences.
  int num_scheduled_background_sequences_ = 0;

  // Ensures all state (e.g. dangling cleaned up workers) is coalesced before
  // destroying the TaskTracker (e.g. in test environments).
  // Ref. https://crbug.com/827615.
  TrackedRefFactory<TaskTracker> tracked_ref_factory_;

  DISALLOW_COPY_AND_ASSIGN(TaskTracker);
};

}  // namespace internal
}  // namespace base

#endif  // BASE_TASK_SCHEDULER_TASK_TRACKER_H_