Prioritizing input and compositor tasks in the blink scheduler.

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/TraceLocation.h"
+#include "wtf/DoubleBufferedDeque.h"
 #include "wtf/Functional.h"
 #include "wtf/Noncopyable.h"
+#include "wtf/ThreadingPrimitives.h"
 
 namespace blink {
 class WebThread;
 }
 
 namespace blink {
 
 class TraceLocation;
 
 // 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;
 
     static Scheduler* shared();
     static void initializeOnMainThread();
     static void shutdown();
 
     // 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 postTask(const TraceLocation&, const Task&); // For generic (low priority) tasks.
-    void postIdleTask(const IdleTask&); // For non-critical tasks which may be reordered relative to other task types.
+    void postIdleTask(const TraceLocation&, const IdleTask&); // For non-critical tasks which may be reordered relative to other task types.
 
     // 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 the main thread.
+    // Can be called on any thread.
     bool shouldYieldForHighPriorityWork();
 
     // 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();
 
 private:
+    class MainThreadPendingTaskRunner;
+    class MainThreadPendingHighPriorityTaskRunner;
+    friend class MainThreadPendingTaskRunner;
+    friend class MainThreadPendingHighPriorityTaskRunner;
+
     Scheduler();
     ~Scheduler();
 
-    void scheduleTask(const TraceLocation&, const Task&);
-    void scheduleIdleTask(const IdleTask&);
+    void scheduleIdleTask(const TraceLocation&, const IdleTask&);
 
     static void sharedTimerAdapter();
     void tickSharedTimer();
 
     static Scheduler* s_sharedScheduler;
-    blink::WebThread* m_mainThread;
 
+    class TracedTask {
+    public:
+        TracedTask(const Task& task, const TraceLocation& location)
+            : m_task(task)
+            , m_location(location) { }
+
+        void run();
+
+    private:
+        Task m_task;
+        TraceLocation m_location;
+    };
+
+    void maybePostMainThreadPendingHighPriorityTaskRunner();
+    void runHighPriorityTasks();
+
+    void incrementMainThreadTaskRunnerCount();

[eseidel, 2014/08/11 17:02:36]

Please add a comment to explain what these are for.

[alexclarke, 2014/08/12 11:37:03]

They've gone.

+    void decrementMainThreadTaskRunnerCount();

[eseidel, 2014/08/11 17:02:36]

we should also add a comment that the only reason thse are not a bool is due to
our limited WTF atomics.

[alexclarke, 2014/08/12 11:37:03]

They've gone.

+
+    // These members can be accessed from any thread.
+    WebThread* m_mainThread;
     void (*m_sharedTimerFunction)();
+    volatile int m_mainThreadTaskRunnerCount;
+
+    // This mutex protects calls to the pending task queues.
+    Mutex m_pendingTasksMutex;
+    DoubleBufferedDeque<TracedTask> m_pendingInputTasks;
+    DoubleBufferedDeque<TracedTask> m_pendingCompositorTasks;
+
+    volatile int m_highPriotityTaskCount;
 };
 
 } // namespace blink
 
 #endif // Scheduler_h