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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 <stddef.h>
#include <memory>
#include <string>
#include <vector>
#include "base/base_export.h"
#include "base/compiler_specific.h"
#include "base/containers/stack.h"
#include "base/gtest_prod_util.h"
#include "base/logging.h"
#include "base/macros.h"
#include "base/memory/ref_counted.h"
#include "base/optional.h"
#include "base/strings/string_piece.h"
#include "base/synchronization/atomic_flag.h"
#include "base/synchronization/condition_variable.h"
#include "base/synchronization/waitable_event.h"
#include "base/task/task_scheduler/priority_queue.h"
#include "base/task/task_scheduler/scheduler_lock.h"
#include "base/task/task_scheduler/scheduler_worker.h"
#include "base/task/task_scheduler/scheduler_worker_pool.h"
#include "base/task/task_scheduler/scheduler_worker_stack.h"
#include "base/task/task_scheduler/sequence.h"
#include "base/task/task_scheduler/task.h"
#include "base/task/task_scheduler/tracked_ref.h"
#include "base/task_runner.h"
#include "base/time/time.h"
#include "build/build_config.h"
namespace base {
class HistogramBase;
class SchedulerWorkerObserver;
class SchedulerWorkerPoolParams;
namespace internal {
class TaskTracker;
// A pool of workers that run Tasks.
// The pool doesn't create threads until Start() is called. Tasks can be posted
// at any time but will not run until after Start() is called.
// This class is thread-safe.
class BASE_EXPORT SchedulerWorkerPoolImpl : public SchedulerWorkerPool {
enum class WorkerEnvironment {
// No special worker environment required.
#if defined(OS_WIN)
// Initialize a COM MTA on the worker.
#endif // defined(OS_WIN)
// Constructs a pool without workers.
// |histogram_label| is used to label the pool's histograms ("TaskScheduler."
// + histogram_name + "." + |histogram_label| + extra suffixes), it must not
// be empty. |pool_label| is used to label the pool's threads, it must not be
// empty. |priority_hint| is the preferred thread priority; the actual thread
// priority depends on shutdown state and platform capabilities.
// |task_tracker| keeps track of tasks.
SchedulerWorkerPoolImpl(StringPiece histogram_label,
StringPiece pool_label,
ThreadPriority priority_hint,
TrackedRef<TaskTracker> task_tracker,
TrackedRef<Delegate> delegate);
// Creates workers following the |params| specification, allowing existing and
// future tasks to run. The pool runs at most |max_best_effort_tasks|
// unblocked BEST_EFFORT tasks concurrently, uses |service_thread_task_runner|
// to monitor for blocked tasks, and, if specified, notifies
// |scheduler_worker_observer| when a worker enters and exits its main
// function (the observer must not be destroyed before JoinForTesting() has
// returned). |worker_environment| specifies the environment in which tasks
// are executed. |may_block_threshold| is the timeout after which a task in a
// MAY_BLOCK ScopedBlockingCall is considered blocked (the pool will choose an
// appropriate value if none is specified). Can only be called once. CHECKs on
// failure.
void Start(const SchedulerWorkerPoolParams& params,
int max_best_effort_tasks,
scoped_refptr<TaskRunner> service_thread_task_runner,
SchedulerWorkerObserver* scheduler_worker_observer,
WorkerEnvironment worker_environment,
Optional<TimeDelta> may_block_threshold = Optional<TimeDelta>());
// Destroying a SchedulerWorkerPoolImpl returned by Create() is not allowed in
// production; it is always leaked. In tests, it can only be destroyed after
// JoinForTesting() has returned.
~SchedulerWorkerPoolImpl() override;
// SchedulerWorkerPool:
void JoinForTesting() override;
void ReEnqueueSequenceChangingPool(
SequenceAndTransaction sequence_and_transaction) override;
const HistogramBase* num_tasks_before_detach_histogram() const {
return num_tasks_before_detach_histogram_;
const HistogramBase* num_tasks_between_waits_histogram() const {
return num_tasks_between_waits_histogram_;
const HistogramBase* num_workers_histogram() const {
return num_workers_histogram_;
void GetHistograms(std::vector<const HistogramBase*>* histograms) const;
// Returns the maximum number of non-blocked tasks that can run concurrently
// in this pool.
// TODO(fdoray): Remove this method.
size_t GetMaxConcurrentNonBlockedTasksDeprecated() const;
// Waits until at least |n| workers are idle. Note that while workers are
// disallowed from cleaning up during this call: tests using a custom
// |suggested_reclaim_time_| need to be careful to invoke this swiftly after
// unblocking the waited upon workers as: if a worker is already detached by
// the time this is invoked, it will never make it onto the idle stack and
// this call will hang.
void WaitForWorkersIdleForTesting(size_t n);
// Waits until at least |n| workers are idle.
void WaitForWorkersIdleLockRequiredForTesting(size_t n)
// Waits until all workers are idle.
void WaitForAllWorkersIdleForTesting();
// Waits until |n| workers have cleaned up (since the last call to
// WaitForWorkersCleanedUpForTesting() or Start() if it wasn't called yet).
void WaitForWorkersCleanedUpForTesting(size_t n);
// Returns the number of workers in this worker pool.
size_t NumberOfWorkersForTesting() const;
// Returns |max_tasks_|.
size_t GetMaxTasksForTesting() const;
// Returns the number of workers that are idle (i.e. not running tasks).
size_t NumberOfIdleWorkersForTesting() const;
// Records number of worker and active workers.
void ReportHeartbeatMetrics() const;
// Updates the position of the Sequence in |sequence_and_transaction| in
// |shared_priority_queue| based on the Sequence's current traits.
void UpdateSortKey(SequenceAndTransaction sequence_and_transaction);
// Removes |sequence| from |priority_queue_|. Returns true if successful, or
// false if |sequence| is not currently in |priority_queue_|, such as when a
// worker is running a task from it.
bool RemoveSequence(scoped_refptr<Sequence> sequence);
class SchedulerWorkerActionExecutor;
class SchedulerWorkerDelegateImpl;
// Friend tests so that they can access |blocked_workers_poll_period| and
// may_block_threshold().
friend class TaskSchedulerWorkerPoolBlockingTest;
friend class TaskSchedulerWorkerPoolMayBlockTest;
// SchedulerWorkerPool:
void OnCanScheduleSequence(scoped_refptr<Sequence> sequence) override;
void OnCanScheduleSequence(
SequenceAndTransaction sequence_and_transaction) override;
// Pushes the Sequence in |sequence_and_transaction| to |priority_queue_| and
// wakes up workers as appropriate.
void PushSequenceAndWakeUpWorkers(
SequenceAndTransaction sequence_and_transaction);
// Creates a worker and schedules its start, if needed, to maintain one idle
// worker, |max_tasks_| permitting.
void MaintainAtLeastOneIdleWorkerLockRequired(
SchedulerWorkerActionExecutor* executor) EXCLUSIVE_LOCKS_REQUIRED(lock_);
// Returns true if worker cleanup is permitted.
bool CanWorkerCleanupForTestingLockRequired() EXCLUSIVE_LOCKS_REQUIRED(lock_);
// Creates a worker, adds it to the pool, schedules its start and returns it.
// Cannot be called before Start().
scoped_refptr<SchedulerWorker> CreateAndRegisterWorkerLockRequired(
SchedulerWorkerActionExecutor* executor) EXCLUSIVE_LOCKS_REQUIRED(lock_);
// Returns the number of workers that are awake (i.e. not on the idle stack).
size_t GetNumAwakeWorkersLockRequired() const EXCLUSIVE_LOCKS_REQUIRED(lock_);
// Returns the desired number of awake workers, given current workload and
// concurrency limits.
size_t GetDesiredNumAwakeWorkersLockRequired() const
// Ensures that there are at least GetDesiredNumAwakeWorkersLockRequired()
// awake workers.
void EnsureEnoughWorkersLockRequired(SchedulerWorkerActionExecutor* executor)
// Examines the list of SchedulerWorkers and increments |max_tasks_| for each
// worker that has been within the scope of a MAY_BLOCK ScopedBlockingCall for
// more than BlockedThreshold().
void AdjustMaxTasks();
// Returns the threshold after which the max tasks is increased to compensate
// for a worker that is within a MAY_BLOCK ScopedBlockingCall.
TimeDelta may_block_threshold_for_testing() const {
return after_start().may_block_threshold;
// Interval at which the service thread checks for workers in this pool
// that have been in a MAY_BLOCK ScopedBlockingCall for more than
// may_block_threshold().
TimeDelta blocked_workers_poll_period_for_testing() const {
return after_start().blocked_workers_poll_period;
// Starts calling AdjustMaxTasks() periodically on
// |service_thread_task_runner_|.
void ScheduleAdjustMaxTasks();
// Returns true if ScheduleAdjustMaxTasks() must be called.
bool MustScheduleAdjustMaxTasksLockRequired() EXCLUSIVE_LOCKS_REQUIRED(lock_);
// Calls AdjustMaxTasks() and schedules it again as necessary. May only be
// called from the service thread.
void AdjustMaxTasksFunction();
// Returns true if AdjustMaxTasks() should periodically be called on
// |service_thread_task_runner_|.
bool ShouldPeriodicallyAdjustMaxTasksLockRequired()
// Increments/decrements the number of tasks that can run in this pool.
// |is_running_best_effort_task| indicates whether the worker causing the
// change is currently running a TaskPriority::BEST_EFFORT task.
void DecrementMaxTasksLockRequired(bool is_running_best_effort_task)
void IncrementMaxTasksLockRequired(bool is_running_best_effort_task)
// Values set at Start() and never modified afterwards.
struct InitializedInStart {
// Set after all members of this struct are set.
bool initialized = false;
// Initial value of |max_tasks_|.
size_t initial_max_tasks = 0;
// Suggested reclaim time for workers.
TimeDelta suggested_reclaim_time;
SchedulerBackwardCompatibility backward_compatibility;
// Environment to be initialized per worker.
WorkerEnvironment worker_environment = WorkerEnvironment::NONE;
scoped_refptr<TaskRunner> service_thread_task_runner;
// Optional observer notified when a worker enters and exits its main.
SchedulerWorkerObserver* scheduler_worker_observer = nullptr;
bool may_block_without_delay_;
// Threshold after which the max tasks is increased to compensate for a
// worker that is within a MAY_BLOCK ScopedBlockingCall.
TimeDelta may_block_threshold;
// The period between calls to AdjustMaxTasks() when the pool is at
// capacity.
TimeDelta blocked_workers_poll_period;
} initialized_in_start_;
InitializedInStart& in_start() {
return initialized_in_start_;
const InitializedInStart& after_start() const {
return initialized_in_start_;
const std::string pool_label_;
const ThreadPriority priority_hint_;
// Synchronizes accesses to all members of this class which are neither const,
// atomic, nor InitializedInStart. Since this lock is a bottleneck to post and
// schedule work, only simple data structure manipulations are allowed within
// its scope (no thread creation or wake up).
mutable SchedulerLock lock_;
// PriorityQueue from which all threads of this worker pool get work.
PriorityQueue priority_queue_ GUARDED_BY(lock_);
// All workers owned by this worker pool.
std::vector<scoped_refptr<SchedulerWorker>> workers_ GUARDED_BY(lock_);
// Maximum number of tasks of any priority / BEST_EFFORT priority that can run
// concurrently in this pool.
size_t max_tasks_ GUARDED_BY(lock_) = 0;
size_t max_best_effort_tasks_ GUARDED_BY(lock_) = 0;
// Number of tasks of any priority / BEST_EFFORT priority that are currently
// running in this pool.
size_t num_running_tasks_ GUARDED_BY(lock_) = 0;
size_t num_running_best_effort_tasks_ GUARDED_BY(lock_) = 0;
// Number of workers running a task of any priority / BEST_EFFORT priority
// that are within the scope of a MAY_BLOCK ScopedBlockingCall but haven't
// caused a max tasks increase yet.
int num_unresolved_may_block_ GUARDED_BY(lock_) = 0;
int num_unresolved_best_effort_may_block_ GUARDED_BY(lock_) = 0;
// Stack of idle workers. Initially, all workers are on this stack. A worker
// is removed from the stack before its WakeUp() function is called and when
// it receives work from GetWork() (a worker calls GetWork() when its sleep
// timeout expires, even if its WakeUp() method hasn't been called). A worker
// is pushed on this stack when it receives nullptr from GetWork().
SchedulerWorkerStack idle_workers_stack_ GUARDED_BY(lock_);
// Signaled when a worker is added to the idle workers stack.
std::unique_ptr<ConditionVariable> idle_workers_stack_cv_for_testing_
// Stack that contains the timestamps of when workers get cleaned up.
// Timestamps get popped off the stack as new workers are added.
base::stack<TimeTicks, std::vector<TimeTicks>> cleanup_timestamps_
// Whether we are currently polling for necessary adjustments to |max_tasks_|.
bool polling_max_tasks_ GUARDED_BY(lock_) = false;
// Indicates to the delegates that workers are not permitted to cleanup.
bool worker_cleanup_disallowed_for_testing_ GUARDED_BY(lock_) = false;
// Counts the number of workers cleaned up since the last call to
// WaitForWorkersCleanedUpForTesting() (or Start() if it wasn't called yet).
// |some_workers_cleaned_up_for_testing_| is true if this was ever
// incremented. Tests with a custom |suggested_reclaim_time_| can wait on a
// specific number of workers being cleaned up via
// WaitForWorkersCleanedUpForTesting().
size_t num_workers_cleaned_up_for_testing_ GUARDED_BY(lock_) = 0;
bool some_workers_cleaned_up_for_testing_ GUARDED_BY(lock_) = false;
// Signaled, if non-null, when |num_workers_cleaned_up_for_testing_| is
// incremented.
std::unique_ptr<ConditionVariable> num_workers_cleaned_up_for_testing_cv_
// Set at the start of JoinForTesting().
AtomicFlag join_for_testing_started_;
// TaskScheduler.DetachDuration.[worker pool name] histogram. Intentionally
// leaked.
HistogramBase* const detach_duration_histogram_;
// TaskScheduler.NumTasksBeforeDetach.[worker pool name] histogram.
// Intentionally leaked.
HistogramBase* const num_tasks_before_detach_histogram_;
// TaskScheduler.NumTasksBetweenWaits.[worker pool name] histogram.
// Intentionally leaked.
HistogramBase* const num_tasks_between_waits_histogram_;
// TaskScheduler.NumWorkers.[worker pool name] histogram.
// Intentionally leaked.
HistogramBase* const num_workers_histogram_;
// TaskScheduler.NumActiveWorkers.[worker pool name] histogram.
// Intentionally leaked.
HistogramBase* const num_active_workers_histogram_;
// Ensures recently cleaned up workers (ref.
// SchedulerWorkerDelegateImpl::CleanupLockRequired()) had time to exit as
// they have a raw reference to |this| (and to TaskTracker) which can
// otherwise result in racy use-after-frees per no longer being part of
// |workers_| and hence not being explicitly joined in JoinForTesting():
// Uses AtomicRefCount to make its only public
// method thread-safe.
TrackedRefFactory<SchedulerWorkerPoolImpl> tracked_ref_factory_;
} // namespace internal
} // namespace base