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// 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 <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/metrics/histogram_base.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"
namespace base {
class ConditionVariable;
class HistogramBase;
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 (RunNextTask).
// TaskTracker also sets up the environment to run a task (RunNextTask) and
// records metrics and trace events. This class is thread-safe.
// Life of a sequence:
// 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::RunNextTask |
// | 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 {
// |max_num_scheduled_background_sequences| is the maximum number of
// background sequences that be scheduled concurrently.
TaskTracker(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 pending 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 Flush();
// 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 RunNextTask() will soon be called
// with |sequence| as argument. Otherwise, RunNextTask() 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(), RunNextTask()
// or CanScheduleSequenceObserver::OnCanScheduleSequence() must have allowed
// |sequence| to be (re)scheduled.
scoped_refptr<Sequence> RunNextTask(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();
// 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(std::unique_ptr<Task> task,
Sequence* sequence,
bool can_run_task);
// Returns true if this context should be exempt from blocking shutdown
// TODO(robliao): Remove when is fixed.
virtual bool IsPostingBlockShutdownTaskAfterShutdownAllowed();
// Called at the very end of RunNextTask() after the completion of all task
// metrics accounting.
virtual void OnRunNextTaskCompleted() {}
// Returns the number of undelayed tasks that haven't completed their
// execution.
int GetNumPendingUndelayedTasksForTesting() const;
class State;
struct PreemptedBackgroundSequence;
void PerformShutdown();
// 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 pending undelayed tasks and signals |flush_cv_| if
// it reaches zero.
void DecrementNumPendingUndelayedTasks();
// 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 RunNextTask(),
// 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 (RunNextTask() 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);
// Records the TaskScheduler.TaskLatency.[task priority].[may block] histogram
// for |task|.
void RecordTaskLatencyHistogram(Task* task);
// 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 Flush(). The memory barriers ensure that the
// memory written by flushed tasks is visible when Flush() returns.
subtle::Atomic32 num_pending_undelayed_tasks_ = 0;
// Lock associated with |flush_cv_|. Partially synchronizes access to
// |num_pending_undelayed_tasks_|. Full synchronization isn't needed because
// it's atomic, but synchronization is needed to coordinate waking and
// sleeping at the right time.
mutable SchedulerLock flush_lock_;
// Signaled when |num_pending_undelayed_tasks_| is zero or when shutdown
// completes.
const std::unique_ptr<ConditionVariable> flush_cv_;
// 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_;
// Maximum number of background sequences that can that be scheduled
// concurrently.
const int max_num_scheduled_background_sequences_;
// Synchronizes accesses to |preempted_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.
// Number of currently scheduled background sequences.
int num_scheduled_background_sequences_ = 0;
// TaskScheduler.TaskLatency.[task priority].[may block] histograms. The first
// index is a TaskPriority. The second index is 0 for non-blocking tasks, 1
// for blocking tasks. Intentionally leaked.
HistogramBase* const
task_latency_histograms_[static_cast<int>(TaskPriority::HIGHEST) + 1][2];
// Number of BLOCK_SHUTDOWN tasks posted during shutdown.
HistogramBase::Sample num_block_shutdown_tasks_posted_during_shutdown_ = 0;
} // namespace internal
} // namespace base