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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/debug/task_annotator.h"
#include "base/logging.h"
#include "base/macros.h"
#include "base/metrics/histogram_base.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;
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 (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:
// 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.
// TODO(fdoray): We want to allow disabling TaskPriority::BACKGROUND tasks in a
// scope (e.g. during startup or page load), but we don't need a dynamic maximum
// number of background tasks. The code could probably be simplified if it
// didn't support that.
class BASE_EXPORT TaskTracker {
// |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).
// This method may also modify metadata on |task| if desired.
bool WillPostTask(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;
enum class LatencyHistogramType {
// Records the latency of each individual task posted through TaskTracker.
// Records the latency of heartbeat tasks which are independent of current
// workload. These avoid a bias towards TASK_LATENCY reporting that high-
// priority tasks are "slower" than regular tasks because high-priority
// tasks tend to be correlated with heavy workloads.
// 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();
// Records |Now() - posted_time| to the appropriate |latency_histogram_type|
// based on |task_traits|.
void RecordLatencyHistogram(LatencyHistogramType latency_histogram_type,
TaskTraits task_traits,
TimeTicks posted_time) const;
TrackedRef<TaskTracker> GetTrackedRef() {
return tracked_ref_factory_.GetTrackedRef();
// 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);
// Returns true if this context should be exempt from blocking shutdown
// TODO(robliao): Remove when is fixed.
virtual bool IsPostingBlockShutdownTaskAfterShutdownAllowed();
// 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;
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.
// Schedules |sequence_to_schedule.sequence| using
// ||. 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();
debug::TaskAnnotator task_annotator_;
// 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.
// 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;
// TaskScheduler.TaskLatencyMicroseconds.* and
// TaskScheduler.HeartbeatLatencyMicroseconds.* histograms. The first index is
// a TaskPriority. The second index is 0 for non-blocking tasks, 1 for
// blocking tasks. Intentionally leaked.
// TODO(scheduler-dev): Consider using STATIC_HISTOGRAM_POINTER_GROUP for
// these.
static constexpr int kNumTaskPriorities =
static_cast<int>(TaskPriority::HIGHEST) + 1;
HistogramBase* const task_latency_histograms_[kNumTaskPriorities][2];
HistogramBase* const heartbeat_latency_histograms_[kNumTaskPriorities][2];
// Number of BLOCK_SHUTDOWN tasks posted during shutdown.
HistogramBase::Sample num_block_shutdown_tasks_posted_during_shutdown_ = 0;
// Ensures all state (e.g. dangling cleaned up workers) is coalesced before
// destroying the TaskTracker (e.g. in test environments).
// Ref.
TrackedRefFactory<TaskTracker> tracked_ref_factory_;
} // namespace internal
} // namespace base