TaskScheduler [5/9] Task Tracker

base/task_scheduler/task_tracker.cc

+// 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 "base/task_scheduler/task_tracker.h"
+
+#include "base/callback.h"
+#include "base/debug/task_annotator.h"
+#include "base/metrics/histogram_macros.h"
+
+namespace base {
+namespace internal {
+
+namespace {
+
+const char kQueueFunctionName[] = "base::PostTask";
+
+// Upper bound for the
+// TaskScheduler.BlockShutdownTasksPostedDuringShutdown histogram.
+const HistogramBase::Sample kMaxBlockShutdownTasksPostedDuringShutdown = 1000;
+
+void RecordNumBlockShutdownTasksPostedDuringShutdown(
+    HistogramBase::Sample value) {
+  UMA_HISTOGRAM_CUSTOM_COUNTS(
+      "TaskScheduler.BlockShutdownTasksPostedDuringShutdown", value, 1,
+      kMaxBlockShutdownTasksPostedDuringShutdown, 50);
+}
+
+}  // namespace
+
+TaskTracker::TaskTracker() = default;
+TaskTracker::~TaskTracker() = default;
+
+void TaskTracker::Shutdown() {
+  AutoSchedulerLock auto_lock(lock_);
+
+  // This method should only be called once.
+  DCHECK(!shutdown_completed_);
+  DCHECK(!shutdown_cv_);
+
+  shutdown_cv_ = lock_.CreateConditionVariable();
+
+  // Wait until the number of tasks blocking shutdown is zero.
+  while (num_tasks_blocking_shutdown_ != 0)
+    shutdown_cv_->Wait();
+
+  shutdown_cv_.reset();
+  shutdown_completed_ = true;
+
+  // Record the TaskScheduler.BlockShutdownTasksPostedDuringShutdown if less
+  // than |kMaxBlockShutdownTasksPostedDuringShutdown| BLOCK_SHUTDOWN tasks were
+  // posted during shutdown. Otherwise, the histogram has already been recorded
+  // in BeforePostTask().
+  if (num_block_shutdown_tasks_posted_during_shutdown_ <
+      kMaxBlockShutdownTasksPostedDuringShutdown) {
+    RecordNumBlockShutdownTasksPostedDuringShutdown(
+        num_block_shutdown_tasks_posted_during_shutdown_);
+  }
+}
+
+void TaskTracker::PostTask(
+    const Callback<void(scoped_ptr<Task>)>& post_task_callback,
+    scoped_ptr<Task> task) {
+  DCHECK(!post_task_callback.is_null());
+  DCHECK(task);
+
+  if (!BeforePostTask(task->traits.shutdown_behavior()))
+    return;
+
+  debug::TaskAnnotator task_annotator;
+  task_annotator.DidQueueTask(kQueueFunctionName, *task);
+
+  post_task_callback.Run(std::move(task));
+}
+
+void TaskTracker::RunTask(const Task* task) {
+  DCHECK(task);
+
+  const TaskShutdownBehavior shutdown_behavior =
+      task->traits.shutdown_behavior();
+  if (!BeforeRunTask(shutdown_behavior))
+    return;
+
+  debug::TaskAnnotator task_annotator;
+  task_annotator.RunTask(kQueueFunctionName, *task);
+
+  AfterRunTask(shutdown_behavior);
+}
+
+bool TaskTracker::IsShuttingDownForTesting() const {
+  AutoSchedulerLock auto_lock(lock_);
+  return !!shutdown_cv_;
+}
+
+bool TaskTracker::BeforePostTask(TaskShutdownBehavior shutdown_behavior) {
+  AutoSchedulerLock auto_lock(lock_);
+
+  if (shutdown_completed_) {
+    // A BLOCK_SHUTDOWN task posted after shutdown has completed is an ordering
+    // bug. This DCHECK aims to catch those early.
+    DCHECK_NE(shutdown_behavior, TaskShutdownBehavior::BLOCK_SHUTDOWN);
+
+    // No task is allowed to be posted after shutdown.
+    return false;
+  }
+
+  if (shutdown_behavior == TaskShutdownBehavior::BLOCK_SHUTDOWN) {
+    // BLOCK_SHUTDOWN tasks block shutdown between the moment they are posted
+    // and the moment they complete their execution.
+    ++num_tasks_blocking_shutdown_;
+
+    if (shutdown_cv_) {
+      ++num_block_shutdown_tasks_posted_during_shutdown_;
+
+      if (num_block_shutdown_tasks_posted_during_shutdown_ ==
+          kMaxBlockShutdownTasksPostedDuringShutdown) {
+        // Record the TaskScheduler.BlockShutdownTasksPostedDuringShutdown
+        // histogram as soon as its upper bound is hit. That way, a value will
+        // be recorded even if an infinite number of BLOCK_SHUTDOWN tasks are
+        // posted, preventing shutdown to complete.
+        RecordNumBlockShutdownTasksPostedDuringShutdown(
+            num_block_shutdown_tasks_posted_during_shutdown_);
+      }
+    }
+
+    // A BLOCK_SHUTDOWN task is allowed to be posted iff shutdown hasn't
+    // completed.
+    return true;
+  }
+
+  // A non BLOCK_SHUTDOWN task is allowed to be posted iff shutdown hasn't
+  // started.
+  return !shutdown_cv_;
+}
+
+bool TaskTracker::BeforeRunTask(TaskShutdownBehavior shutdown_behavior) {
+  AutoSchedulerLock auto_lock(lock_);
+
+  if (shutdown_completed_) {
+    // Trying to run a BLOCK_SHUTDOWN task after shutdown has completed is
+    // unexpected as it either shouldn't have been posted if shutdown completed
+    // or should be blocking shutdown if it was posted before it did.
+    DCHECK_NE(shutdown_behavior, TaskShutdownBehavior::BLOCK_SHUTDOWN);
+
+    // A WorkerThread might extract a non BLOCK_SHUTDOWN task from a
+    // PriorityQueue after shutdown. It shouldn't be allowed to run it.
+    return false;
+  }
+
+  switch (shutdown_behavior) {
+    case TaskShutdownBehavior::BLOCK_SHUTDOWN:
+      DCHECK_GT(num_tasks_blocking_shutdown_, 0U);
+      return true;
+
+    case TaskShutdownBehavior::SKIP_ON_SHUTDOWN:
+      if (shutdown_cv_)
+        return false;
+
+      // SKIP_ON_SHUTDOWN tasks block shutdown while they are running.
+      ++num_tasks_blocking_shutdown_;
+      return true;
+
+    case TaskShutdownBehavior::CONTINUE_ON_SHUTDOWN:
+      return !shutdown_cv_;
+  }
+
+  NOTREACHED();
+  return false;
+}
+
+void TaskTracker::AfterRunTask(TaskShutdownBehavior shutdown_behavior) {
+  if (shutdown_behavior == TaskShutdownBehavior::BLOCK_SHUTDOWN ||
+      shutdown_behavior == TaskShutdownBehavior::SKIP_ON_SHUTDOWN) {
+    AutoSchedulerLock auto_lock(lock_);
+    DCHECK_GT(num_tasks_blocking_shutdown_, 0U);
+    --num_tasks_blocking_shutdown_;
+    if (num_tasks_blocking_shutdown_ == 0 && shutdown_cv_)
+      shutdown_cv_->Signal();
+  }
+}
+
+}  // namespace internal
+}  // namespace base