Prioritizing input and compositor tasks in the blink scheduler.

Source/platform/scheduler/Scheduler.cpp

Index: Source/platform/scheduler/Scheduler.cpp
diff --git a/Source/platform/scheduler/Scheduler.cpp b/Source/platform/scheduler/Scheduler.cpp
index 61ae7aef1d486bd618417a2d2d62c14c08db06bc..e343d57cb9a8b282257dd3f0b2c08f0e52a689b1 100644
--- a/Source/platform/scheduler/Scheduler.cpp
+++ b/Source/platform/scheduler/Scheduler.cpp
@@ -5,59 +5,103 @@
 #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 {
+class MainThreadIdleTaskAdapter : public blink::WebThread::Task {
 public:
-    explicit MainThreadTaskAdapter(const TraceLocation& location, const Scheduler::Task& task)
-        : m_location(location)
-        , m_task(task)
+    MainThreadIdleTaskAdapter(const Scheduler::IdleTask& idleTask, double allottedTimeMs)
+        : m_idleTask(idleTask)
+        , m_allottedTimeMs(allottedTimeMs)
     {
     }
 
     // WebThread::Task implementation.
     virtual void run() OVERRIDE
     {
-        TRACE_EVENT2("blink", "MainThreadTaskAdapter::run",
-            "src_file", m_location.fileName(),
-            "src_func", m_location.functionName());
-        m_task();
+        TRACE_EVENT1("blink", "MainThreadIdleTaskAdapter::run", "allottedTime", m_allottedTimeMs);
+        m_idleTask(m_allottedTimeMs);
     }
 
 private:
-    const TraceLocation m_location;
-    Scheduler::Task m_task;
+    Scheduler::IdleTask m_idleTask;
+    double m_allottedTimeMs;
 };
 
-class MainThreadIdleTaskAdapter : public blink::WebThread::Task {
+} // namespace
+
+class Scheduler::MainThreadPendingHighPriorityTaskRunner : public blink::WebThread::Task {
 public:
-    MainThreadIdleTaskAdapter(const Scheduler::IdleTask& idleTask, double allottedTimeMs)
-        : m_idleTask(idleTask)
-        , m_allottedTimeMs(allottedTimeMs)
+    explicit MainThreadPendingHighPriorityTaskRunner()

[Sami, 2014/08/08 15:31:13]

No need for |explicit| since there are no parameters.

[alexclarke, 2014/08/08 15:47:46]

Done.

+    {
+        Scheduler* scheduler = Scheduler::shared();
+        if (!scheduler)

[Sami, 2014/08/08 15:31:13]

This condition should never fail since the scheduler is the one creating this
class, right?

[alexclarke, 2014/08/08 15:47:46]

I'll dcheck it instead

+            return;
+        atomicIncrement(&scheduler->m_mainThreadTaskRunnerCount);
+    }
+
+    ~MainThreadPendingHighPriorityTaskRunner()
     {
+        Scheduler* scheduler = Scheduler::shared();
+        if (!scheduler)
+            return;
+        atomicDecrement(&scheduler->m_mainThreadTaskRunnerCount);
     }
 
     // WebThread::Task implementation.
     virtual void run() OVERRIDE
     {
-        TRACE_EVENT1("blink", "MainThreadIdleTaskAdapter::run", "allottedTime", m_allottedTimeMs);
-        m_idleTask(m_allottedTimeMs);
+        Scheduler* scheduler = Scheduler::shared();
+        if (!scheduler)

[Sami, 2014/08/08 15:31:13]

Do we really need these safe guards? I guess these could get posted right before
shutdown so I guess we do...

[alexclarke, 2014/08/08 15:47:45]

Unfortunately yes for that reason :(

+            return;
+        scheduler->runHighPriorityTasks();
     }
-
-private:
-    Scheduler::IdleTask m_idleTask;
-    double m_allottedTimeMs;
 };
 
-}
+class Scheduler::MainThreadPendingTaskRunner : public blink::WebThread::Task {
+public:
+    explicit MainThreadPendingTaskRunner(

[Sami, 2014/08/08 15:31:13]

No need for |explicit| since there are two parameters.

[alexclarke, 2014/08/08 15:47:45]

Done.

+        const Scheduler::Task& task, const TraceLocation& location)
+        : m_task(task)
+        , m_location(location)
+    {
+        Scheduler* scheduler = Scheduler::shared();
+        if (!scheduler)

[Sami, 2014/08/08 15:31:13]

Ditto about scheduler always being there.

[alexclarke, 2014/08/08 15:47:45]

Done.

+            return;
+        atomicIncrement(&scheduler->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;
 
@@ -80,16 +124,16 @@ Scheduler* Scheduler::shared()
 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();
+        runHighPriorityTasks();

[Sami, 2014/08/08 15:31:13]

Why two calls?

[alexclarke, 2014/08/08 15:47:45]

Done.

+    }
 }
 
 void Scheduler::scheduleIdleTask(const IdleTask& idleTask)
@@ -100,17 +144,19 @@ void Scheduler::scheduleIdleTask(const IdleTask& idleTask)
 
 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));
+    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)
@@ -121,7 +167,38 @@ void Scheduler::postIdleTask(const IdleTask& idleTask)
 void Scheduler::tickSharedTimer()
 {
     TRACE_EVENT0("blink", "Scheduler::tickSharedTimer");
+
+    runHighPriorityTasks();
     m_sharedTimerFunction();
+    runHighPriorityTasks();
+}
+
+void Scheduler::runHighPriorityTasks()
+{
+    TRACE_EVENT0("blink", "Scheduler::runHighPriorityTasks");
+
+    WTF::Deque<TracedTask>& inputTasks = m_pendingInputTasks.swapBuffers();
+    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()
@@ -147,7 +224,15 @@ void Scheduler::stopSharedTimer()
 
 bool Scheduler::shouldYieldForHighPriorityWork()
 {
-    return false;
+    return !m_pendingInputTasks.isEmpty() || !m_pendingCompositorTasks.isEmpty();

[Sami, 2014/08/08 15:31:13]

If we're worried about the polling here, we could add another atomic
(m_pendingHighPriorityTaskCount) that we check here. This is probably good
enough for now though.

[alexclarke, 2014/08/08 15:47:46]

Lets see if this pops up on the profiles.  If it does we can do that.

+}
+
+void Scheduler::TracedTask::run()
+{
+    TRACE_EVENT2("blink", "TracedTask::run",
+        "src_file", m_location.fileName(),
+        "src_func", m_location.functionName());
+    m_task();
 }
 
 } // namespace blink