Implement idle task scheduling.

Source/platform/scheduler/Scheduler.h

Index: Source/platform/scheduler/Scheduler.h
diff --git a/Source/platform/scheduler/Scheduler.h b/Source/platform/scheduler/Scheduler.h
index 7f7835abcf76d6ac90c6780e6fdad520e3808d80..6cf752c988aa03279af476548429e7ab30bec717 100644
--- a/Source/platform/scheduler/Scheduler.h
+++ b/Source/platform/scheduler/Scheduler.h
@@ -23,15 +23,15 @@ class PLATFORM_EXPORT Scheduler {
     WTF_MAKE_NONCOPYABLE(Scheduler);
 public:
     typedef Function<void()> Task;
-    // An IdleTask is passed an allotted time in CLOCK_MONOTONIC milliseconds and is expected to complete within this timeframe.
-    typedef Function<void(double allottedTimeMs)> IdleTask;
+    // An IdleTask is passed a deadline in CLOCK_MONOTONIC seconds and is expected to complete before this deadline.
+    typedef Function<void(double deadlineSeconds)> IdleTask;
 
     static Scheduler* shared();
     static void initializeOnMainThread();
     static void shutdown();
 
     // Called to notify about the start of a new frame.
-    void willBeginFrame(const WebBeginFrameArgs&);
+    void willBeginFrame(double frameDeadlineSeconds);
 
     // Called to notify that a previously begun frame was committed.
     void didCommitFrameToCompositor();
@@ -42,7 +42,9 @@ public:
     void postCompositorTask(const TraceLocation&, const Task&);
     void postIpcTask(const TraceLocation&, const Task&);
     void postTask(const TraceLocation&, const Task&); // For generic (low priority) tasks.
-    void postIdleTask(const TraceLocation&, const IdleTask&); // For non-critical tasks which may be reordered relative to other task types.
+    // For non-critical tasks which may be reordered relative to other task types and may be starved
+    // for an arbitrarily long time if no idle time is available.
+    void postIdleTask(const TraceLocation&, const IdleTask&);
 
     // Tells the scheduler that the system received an input event. This causes the scheduler to go into
     // Compositor Priority mode for a short duration.
@@ -59,9 +61,14 @@ public:
     void setSharedTimerFireInterval(double);
     void stopSharedTimer();
 
+    // Returns the time in CLOCK_MONOTONIC seconds until the deadline for running idle tasks during
+    // the current frame expires.
+    double currentFrameDeadlineForIdleTasks() const;

[Sami, 2014/10/06 09:48:30]

The name makes it sound like this is a deadline but the description sounds like
it gives the time *until* the deadline. Please fix the one that is wrong :)

[rmcilroy, 2014/10/06 11:29:42]

Comment was unclear - fixed, thanks.

+
 protected:
     class MainThreadPendingTaskRunner;
     class MainThreadPendingHighPriorityTaskRunner;
+    class MainThreadPendingIdleTaskRunner;
     friend class MainThreadPendingTaskRunner;
     friend class MainThreadPendingHighPriorityTaskRunner;
 
@@ -73,8 +80,8 @@ protected:
     Scheduler();
     virtual ~Scheduler();
 
-    void scheduleIdleTask(const TraceLocation&, const IdleTask&);
     void postHighPriorityTaskInternal(const TraceLocation&, const Task&, const char* traceName);
+    void postIdleTaskInternal(const TraceLocation&, const IdleTask&, const char* traceName);
 
     static void sharedTimerAdapter();
 
@@ -83,13 +90,21 @@ protected:
     // Only does work in CompositorPriority mode. Returns true if any work was done.
     bool runPendingHighPriorityTasksIfInCompositorPriority();
 
+    // Only does work if canRunIdleTask. Returns true if any work was done.
+    bool maybeRunPendingIdleTask();
+
+    TracedIdleTask takeFirstPendingIdleTask();
+
     // Returns true if any work was done.
     bool swapQueuesAndRunPendingTasks();
 
     void swapQueuesRunPendingTasksAndAllowHighPriorityTaskRunnerPosting();
 
     // Returns true if any work was done.
-    bool executeHighPriorityTasks(Deque<TracedTask>&);
+    bool executeHighPriorityTasks(Deque<TracedStandardTask>&);
+
+    // Returns true if the scheduler can run idle tasks at this time.
+    bool canRunIdleTask() const;
 
     // Return the current SchedulerPolicy.
     SchedulerPolicy schedulerPolicy() const;
@@ -99,6 +114,9 @@ protected:
     // Must be called while m_pendingTasksMutex is locked.
     void maybePostMainThreadPendingHighPriorityTaskRunner();
 
+    // Returns true if an idle task was posted to the main thread for execution.
+    bool maybePostMainThreadPendingIdleTask();
+
     void tickSharedTimer();
 
     void (*m_sharedTimerFunction)();
@@ -118,9 +136,13 @@ protected:
     // This mutex protects calls to the pending task queue, m_highPriorityTaskRunnerPosted and
     // m_compositorPriorityPolicyEndTimeSeconds.
     Mutex m_pendingTasksMutex;
-    DoubleBufferedDeque<TracedTask> m_pendingHighPriorityTasks;
+    DoubleBufferedDeque<TracedStandardTask> m_pendingHighPriorityTasks;
+    Deque<TracedIdleTask> m_pendingIdleTasks;
     double m_compositorPriorityPolicyEndTimeSeconds;
 
+    bool m_currentFrameCommitted;
+    double m_currentFrameDeadlineSeconds;
+
     // Declared volatile as it is atomically incremented.
     volatile int m_highPriorityTaskCount;