Implement idle task scheduling.

Source/platform/scheduler/Scheduler.h

 // Copyright 2014 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.
 
 #ifndef Scheduler_h
 #define Scheduler_h
 
 #include "platform/PlatformExport.h"
 #include "platform/scheduler/TracedTask.h"
 #include "wtf/DoubleBufferedDeque.h"
 #include "wtf/Functional.h"
 #include "wtf/Noncopyable.h"
 #include "wtf/ThreadingPrimitives.h"
 
 namespace blink {
 class WebThread;
 struct WebBeginFrameArgs;
 
 // The scheduler is an opinionated gateway for arranging work to be run on the
 // main thread. It decides which tasks get priority over others based on a
 // scheduling policy and the overall system state.
 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();
 
     // The following entrypoints are used to schedule different types of tasks
     // to be run on the main thread. They can be called from any thread.
     void postInputTask(const TraceLocation&, const Task&);
     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.
     void didReceiveInputEvent();
 
     // Returns true if there is high priority work pending on the main thread
     // and the caller should yield to let the scheduler service that work.
     // Can be called on any thread.
     bool shouldYieldForHighPriorityWork() const;
 
     // The shared timer can be used to schedule a periodic callback which may
     // get preempted by higher priority work.
     void setSharedTimerFiredFunction(void (*function)());
     void setSharedTimerFireInterval(double);
     void stopSharedTimer();
 
+    // Returns the deadline, in CLOCK_MONOTONIC seconds, which an idle task should
+    // finish by for the current frame.
+    double currentFrameDeadlineForIdleTasks() const;
+
 protected:
     class MainThreadPendingTaskRunner;
     class MainThreadPendingHighPriorityTaskRunner;
+    class MainThreadPendingIdleTaskRunner;
     friend class MainThreadPendingTaskRunner;
     friend class MainThreadPendingHighPriorityTaskRunner;
 
     enum SchedulerPolicy {
         Normal,
         CompositorPriority,
     };
 
     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();
 
     // Start of main thread only members -----------------------------------
 
     // 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;
 
     void maybeEnterNormalSchedulerPolicy();
 
     // 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)();
 
     // End of main thread only members -------------------------------------
 
     bool hasPendingHighPriorityWork() const;
 
     void enterSchedulerPolicyLocked(SchedulerPolicy);
 
     void enterSchedulerPolicy(SchedulerPolicy);
 
     static Scheduler* s_sharedScheduler;
 
     WebThread* m_mainThread;
 
     // 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;
 
     bool m_highPriorityTaskRunnerPosted;
 
     // Don't access m_schedulerPolicy directly, use enterSchedulerPolicyLocked and SchedulerPolicy instead.
     volatile int m_schedulerPolicy;
 };
 
 } // namespace blink
 
 #endif // Scheduler_h