Prioritizing input and compositor tasks in the blink scheduler.

Source/platform/scheduler/Scheduler.cpp

 // 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.
 
 #include "config.h"
 #include "platform/scheduler/Scheduler.h"
 
+#include "platform/PlatformThreadData.h"
 #include "platform/Task.h"
+#include "platform/ThreadTimers.h"
 #include "platform/TraceEvent.h"
-#include "platform/TraceLocation.h"
 #include "public/platform/Platform.h"
-#include "public/platform/WebThread.h"
+#include "wtf/ThreadingPrimitives.h"
 
 namespace blink {
 
 namespace {
 
-class MainThreadTaskAdapter : public blink::WebThread::Task {
-public:
-    explicit MainThreadTaskAdapter(const TraceLocation& location, const Scheduler::Task& task)
-        : m_location(location)
-        , m_task(task)
-    {
-    }
-
-    // WebThread::Task implementation.
-    virtual void run() OVERRIDE
-    {
-        TRACE_EVENT2("blink", "MainThreadTaskAdapter::run",
-            "src_file", m_location.fileName(),
-            "src_func", m_location.functionName());
-        m_task();
-    }
-
-private:
-    const TraceLocation m_location;
-    Scheduler::Task m_task;
-};
-
 class MainThreadIdleTaskAdapter : public blink::WebThread::Task {
 public:
     MainThreadIdleTaskAdapter(const Scheduler::IdleTask& idleTask, double allottedTimeMs)
         : m_idleTask(idleTask)
         , m_allottedTimeMs(allottedTimeMs)
     {
     }
 
     // WebThread::Task implementation.
     virtual void run() OVERRIDE
     {
         TRACE_EVENT1("blink", "MainThreadIdleTaskAdapter::run", "allottedTime", m_allottedTimeMs);
         m_idleTask(m_allottedTimeMs);
     }
 
 private:
     Scheduler::IdleTask m_idleTask;

[eseidel, 2014/08/08 17:31:06]

Why are idle tasks different from tasks?

[alexclarke, 2014/08/12 11:37:01]

Done.

     double m_allottedTimeMs;

[eseidel, 2014/08/08 17:31:06]

Filed cbug.com/402027 about having real types for these.

[alexclarke, 2014/08/12 11:37:01]

Acknowledged.

 };
 
-}
+} // namespace
+
+class Scheduler::MainThreadPendingHighPriorityTaskRunner : public blink::WebThread::Task {

[eseidel, 2014/08/08 17:31:06]

Don't need blink::

[alexclarke, 2014/08/12 11:37:01]

Done.

+public:
+    MainThreadPendingHighPriorityTaskRunner()
+    {
+        ASSERT(Scheduler::shared());
+        atomicIncrement(&Scheduler::shared()->m_mainThreadTaskRunnerCount);

[eseidel, 2014/08/08 17:31:06]

Why are we grabbing at a m_?  Instead of using a function?

[alexclarke, 2014/08/12 11:37:01]

Done.

+    }
+
+    ~MainThreadPendingHighPriorityTaskRunner()
+    {
+        Scheduler* scheduler = Scheduler::shared();
+        if (!scheduler)
+            return;
+        atomicDecrement(&scheduler->m_mainThreadTaskRunnerCount);
+    }
+
+    // WebThread::Task implementation.
+    virtual void run() OVERRIDE
+    {
+        Scheduler* scheduler = Scheduler::shared();
+        if (!scheduler)
+            return;
+        scheduler->runHighPriorityTasks();
+    }
+};
+
+class Scheduler::MainThreadPendingTaskRunner : public blink::WebThread::Task {
+public:
+    MainThreadPendingTaskRunner(
+        const Scheduler::Task& task, const TraceLocation& location)
+        : m_task(task)
+        , m_location(location)
+    {
+        ASSERT(Scheduler::shared());
+        atomicIncrement(&Scheduler::shared()->m_mainThreadTaskRunnerCount);
+    }
+
+    ~MainThreadPendingTaskRunner()
+    {
+        Scheduler* scheduler = Scheduler::shared();
+        if (!scheduler)
+            return;
+        atomicDecrement(&scheduler->m_mainThreadTaskRunnerCount);
+    }
+
+    // WebThread::Task implementation.
+    virtual void run() OVERRIDE
+    {
+        TRACE_EVENT2("blink", "MainThreadPendingTaskRunner::run",
+            "src_file", m_location.fileName(),
+            "src_func", m_location.functionName());
+        Scheduler* scheduler = Scheduler::shared();
+        if (scheduler)
+            Scheduler::shared()->runHighPriorityTasks();
+        m_task();
+    }
+
+    Scheduler::Task m_task;
+    TraceLocation m_location;
+};
 
 Scheduler* Scheduler::s_sharedScheduler = nullptr;
 
 void Scheduler::initializeOnMainThread()
 {
     s_sharedScheduler = new Scheduler();
 }
 
 void Scheduler::shutdown()
 {
     delete s_sharedScheduler;
     s_sharedScheduler = nullptr;
 }
 
 Scheduler* Scheduler::shared()
 {
     return s_sharedScheduler;
 }
 
 Scheduler::Scheduler()
     : m_mainThread(blink::Platform::current()->currentThread())
     , m_sharedTimerFunction(nullptr)
+    , m_mainThreadTaskRunnerCount(0)
 {
 }
 
 Scheduler::~Scheduler()
 {
-}
-
-void Scheduler::scheduleTask(const TraceLocation& location, const Task& task)
-{
-    m_mainThread->postTask(new MainThreadTaskAdapter(location, task));
+    while (!m_pendingInputTasks.isEmpty() || !m_pendingCompositorTasks.isEmpty()) {
+        runHighPriorityTasks();
+    }
 }
 
 void Scheduler::scheduleIdleTask(const IdleTask& idleTask)
 {
     // TODO: send a real allottedTime here.
     m_mainThread->postTask(new MainThreadIdleTaskAdapter(idleTask, 0));
 }
 
 void Scheduler::postTask(const TraceLocation& location, const Task& task)
 {
-    scheduleTask(location, task);
+    m_mainThread->postTask(new MainThreadPendingTaskRunner(task, location));
 }
 
 void Scheduler::postInputTask(const TraceLocation& location, const Task& task)
 {
-    scheduleTask(location, task);
+    m_pendingInputTasks.append(TracedTask(task, location));

[eseidel, 2014/08/08 17:31:06]

I really think we want all tasks to be traced.  Tracing gives us a huge
debuggability benifit.

[alexclarke, 2014/08/12 11:37:01]

Done.

+    maybePostMainThreadPendingHighPriorityTaskRunner();
 }
 
 void Scheduler::postCompositorTask(const TraceLocation& location, const Task& task)
 {
-    scheduleTask(location, task);
+    m_pendingCompositorTasks.append(TracedTask(task, location));
+    maybePostMainThreadPendingHighPriorityTaskRunner();
 }
 
 void Scheduler::postIdleTask(const IdleTask& idleTask)
 {
     scheduleIdleTask(idleTask);
 }
 
 void Scheduler::tickSharedTimer()
 {
     TRACE_EVENT0("blink", "Scheduler::tickSharedTimer");
+
+    runHighPriorityTasks();
     m_sharedTimerFunction();
+    runHighPriorityTasks();

[eseidel, 2014/08/08 17:31:06]

Why twice?  If the sharedTimer function has chosen to yield due to spending too
much time are we sure we want to run these now?

[alexclarke, 2014/08/12 11:37:01]

I've added some comments to explain this.

+}
+
+void Scheduler::runHighPriorityTasks()
+{
+    TRACE_EVENT0("blink", "Scheduler::runHighPriorityTasks");
+
+    WTF::Deque<TracedTask>& inputTasks = m_pendingInputTasks.swapBuffers();

[eseidel, 2014/08/08 17:31:06]

WTF:: should not be needed.

[alexclarke, 2014/08/12 11:37:02]

Done.

+    WTF::Deque<TracedTask>& compositorTasks = m_pendingCompositorTasks.swapBuffers();
+
+    for (;;) {
+        if (!inputTasks.isEmpty()) {
+            inputTasks.takeFirst().run();
+            continue;
+        }
+
+        if (compositorTasks.isEmpty())
+            break;
+        compositorTasks.takeFirst().run();
+    }
+}
+
+void Scheduler::maybePostMainThreadPendingHighPriorityTaskRunner()
+{
+    // Only post a task if there isn't a task already in flight.
+    if (acquireLoad(&m_mainThreadTaskRunnerCount))
+        return;
+
+    m_mainThread->postTask(new MainThreadPendingHighPriorityTaskRunner());
 }
 
 void Scheduler::sharedTimerAdapter()
 {
     shared()->tickSharedTimer();
 }
 
 void Scheduler::setSharedTimerFiredFunction(void (*function)())
 {
     m_sharedTimerFunction = function;
     blink::Platform::current()->setSharedTimerFiredFunction(function ? &Scheduler::sharedTimerAdapter : nullptr);
 }
 
 void Scheduler::setSharedTimerFireInterval(double interval)
 {
     blink::Platform::current()->setSharedTimerFireInterval(interval);
 }
 
 void Scheduler::stopSharedTimer()
 {
     blink::Platform::current()->stopSharedTimer();
 }
 
 bool Scheduler::shouldYieldForHighPriorityWork()
 {
-    return false;
+    return !m_pendingInputTasks.isEmpty() || !m_pendingCompositorTasks.isEmpty();
+}
+
+void Scheduler::TracedTask::run()
+{
+    TRACE_EVENT2("blink", "TracedTask::run",
+        "src_file", m_location.fileName(),
+        "src_func", m_location.functionName());
+    m_task();
 }
 
 } // namespace blink