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 "platform/scheduler/AtomicRingbuffer.h"
+#include "wtf/Deque.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.
 
+    // For generic (low priority) tasks, can only be called from the main thread.
+    void postTask(const TraceLocation&, const Task&);
+
     // 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.
     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();
 
+    void runPendingTasks();
+    void allowMainThreadTaskPosting();
+
 private:
     Scheduler();
     ~Scheduler();
 
-    void scheduleTask(const TraceLocation&, const Task&);
     void scheduleIdleTask(const IdleTask&);
 
     static void sharedTimerAdapter();
     void tickSharedTimer();
 
     static Scheduler* s_sharedScheduler;
     blink::WebThread* m_mainThread;
 
+    class TracedTask {
+    public:
+        TracedTask(Task task, TraceLocation location) : m_task(task), m_location(location) { }

[Sami, 2014/08/06 10:19:52]

nit: Initializer style.

[alexclarke, 2014/08/07 12:08:06]

Done.

+
+        void run();
+
+    private:
+        Task m_task;
+        TraceLocation m_location;
+    };
+
+    // The input and compositor task queues can be posted to from any thread.
+    AtomicRingBuffer<TracedTask, 128> m_pendingInputTasks;

[Sami, 2014/08/06 10:19:52]

One pattern we could steal from cc/ is to put members into different structs
depending on which thread they are accessed from. For example, there's a
MainThreadOnly struct and a main() function that returns a reference to it and
also asserts the caller is on the main thread. Maybe here we could have a struct
for the shared state and everything else would be main thread only. WDYT?

[alexclarke, 2014/08/07 12:08:06]

I quite like that pattern, however I don't think we need it yet.  I've found a
way to eliminate the need for the WTF::Deque - if one of the ring buffers
becomes full, we'll fall back to using the MainThreadTaskAdapter to process
tasks, so it's now safe for m_pendingLowPriorityTasks to be an AtomicRingBuffer
since it no longer spinlocks when full.

+    AtomicRingBuffer<TracedTask, 128> m_pendingCompositorTasks;
+
+    // Blink tasks can only be posted on the main thread.
+    WTF::Deque<TracedTask> m_pendingBlinkTasks;

[Sami, 2014/08/06 10:19:52]

m_pendingLowPriorityTasks or something?

[alexclarke, 2014/08/07 12:08:06]

Done.

+
+    volatile int m_mainThreadTaskPosted;
+    volatile int m_blinkShouldNotYield;
+
+    void setBlinkShouldYield();

[Sami, 2014/08/06 10:19:52]

How about setHighPriorityWorkPending to match shouldYieldForHighPriorityWork?

[alexclarke, 2014/08/07 12:08:06]

I've decided not to use atomics, nailing down all the corner cases was getting
too complicated.

+    void clearBlinkShouldYield();
+    void maybePostTaskLoopOnMainThread();
+
     void (*m_sharedTimerFunction)();
 };
 
 } // namespace blink
 
 #endif // Scheduler_h