Create a new action triggered when a BeginMainFrame is not expected before vsync

cc/scheduler/scheduler_state_machine.cc

Index: cc/scheduler/scheduler_state_machine.cc
diff --git a/cc/scheduler/scheduler_state_machine.cc b/cc/scheduler/scheduler_state_machine.cc
index e73508ce1ab1fb198c9b69ff0e65609a3e637e8e..f8a828476ad9f8f0ecb2b654776f085a8b30fb04 100644
--- a/cc/scheduler/scheduler_state_machine.cc
+++ b/cc/scheduler/scheduler_state_machine.cc
@@ -42,9 +42,11 @@ SchedulerStateMachine::SchedulerStateMachine(const SchedulerSettings& settings)
       last_frame_number_begin_main_frame_sent_(-1),
       last_frame_number_invalidate_compositor_frame_sink_performed_(-1),
       draw_funnel_(false),
-      send_begin_main_frame_funnel_(true),
+      did_send_begin_main_frame_for_current_frame_(true),
       invalidate_compositor_frame_sink_funnel_(false),
       impl_side_invalidation_funnel_(false),
+      did_notify_begin_main_frame_not_sent_(false),
+      did_commit_during_frame_(false),
       prepare_tiles_funnel_(0),
       consecutive_checkerboard_animations_(0),
       pending_submit_frames_(0),
@@ -194,6 +196,8 @@ const char* SchedulerStateMachine::ActionToString(Action action) {
       return "ACTION_INVALIDATE_COMPOSITOR_FRAME_SINK";
     case ACTION_PERFORM_IMPL_SIDE_INVALIDATION:
       return "ACTION_PERFORM_IMPL_SIDE_INVALIDATION";
+    case ACTION_NOTIFY_BEGIN_MAIN_FRAME_NOT_SENT:
+      return "ACTION_NOTIFY_BEGIN_MAIN_FRAME_NOT_SENT";
   }
   NOTREACHED();
   return "???";
@@ -244,8 +248,12 @@ void SchedulerStateMachine::AsValueInto(
       "last_begin_frame_sequence_number_compositor_frame_was_fresh",
       last_begin_frame_sequence_number_compositor_frame_was_fresh_);
   state->SetBoolean("funnel: draw_funnel", draw_funnel_);
-  state->SetBoolean("funnel: send_begin_main_frame_funnel",
-                    send_begin_main_frame_funnel_);
+  state->SetBoolean("funnel: did_send_begin_main_frame",
+                    did_send_begin_main_frame_for_current_frame_);
+  state->SetBoolean("funnel: did_notify_begin_main_frame_not_sent",
+                    did_notify_begin_main_frame_not_sent_);
+  state->SetBoolean("funnel: did_commit_during_frame",
+                    did_commit_during_frame_);
   state->SetInteger("funnel: prepare_tiles_funnel", prepare_tiles_funnel_);
   state->SetBoolean("funnel: invalidate_compositor_frame_sink_funnel",
                     invalidate_compositor_frame_sink_funnel_);
@@ -428,6 +436,44 @@ bool SchedulerStateMachine::ShouldActivatePendingTree() const {
   return pending_tree_is_ready_for_activation_;
 }
 
+bool SchedulerStateMachine::ShouldNotifyBeginMainFrameNotSent() const {

[brianderson, 2017/04/07 22:03:16]

Is it okay that we'll be sending this at the start of every frame while the
compositor is active?

Should blink explicitly request this signal?

[Dan Elphick, 2017/04/10 15:53:22]

We're aware this might cause some problems, but we'd like to assess the impact
first and then add the extra complexity later if/when we determine this is
having a negative impact.

Assuming it is a problem, I think we can modify the renderer scheduler to signal
the cc scheduler that it wants this signal whenever the idle queue changes from
empty to non-empty and vice versa.

+  // This method returns true if most of the conditions for sending a
+  // BeginMainFrame are met, but one is not actually requested. This gives the
+  // main thread the chance to do something else.
+
+  // Don't notify if a BeginMainFrame has already been requested.
+  if (needs_begin_main_frame_ || did_send_begin_main_frame_for_current_frame_)

[brianderson, 2017/04/07 22:03:16]

I think the did_send_begin_main_frame_for_current_frame_ check can be removed
since it is redundant with the combination of checks on did_commit_during_frame_
and begin_main_frame_state_.

[Dan Elphick, 2017/04/10 15:53:22]

It is actually necessary as it's the flag is initialized to true which stops it
sending this notification straight away (which caused every single state machine
test to fail).

However I can remove this check and fix that by just tweaking the initialization
of did_notify_begin_main_frame_not_sent_ to true.

+    return false;
+
+  // Only notify when we're visible.
+  if (!visible_)
+    return false;
+
+  // There are no BeginImplFrames while BeginFrameSource is paused, meaning
+  // the scheduler should send SendBeginMainFrameNotExpectedSoon instead,
+  // indicating a longer period of inactivity.
+  if (begin_frame_source_paused_)
+    return false;
+
+  // Do not notify that no BeginMainFrame was sent too many times in a single
+  // frame.
+  if (did_notify_begin_main_frame_not_sent_)
+    return false;
+
+  // Do not notify if a commit happened during this frame as the main thread
+  // will already be active and does not need to be woken up to make further
+  // actions. (This occurs if the main frame was scheduled but didn't complete
+  // before the vsync deadline).
+  if (did_commit_during_frame_)
+    return false;
+
+  // If a BeginMainFrame is in progress, there's no need to activate it.
+  if (begin_main_frame_state_ != BEGIN_MAIN_FRAME_STATE_IDLE)

[brianderson, 2017/04/07 22:03:16]

Group this with the needs_begin_main_frame_ check on line 445.

[Dan Elphick, 2017/04/10 15:53:22]

Done.

+    return false;
+
+  return true;
+}
+
 bool SchedulerStateMachine::CouldSendBeginMainFrame() const {
   if (!needs_begin_main_frame_)
     return false;
@@ -454,7 +500,7 @@ bool SchedulerStateMachine::ShouldSendBeginMainFrame() const {
 
   // Do not send begin main frame too many times in a single frame or before
   // the first BeginFrame.
-  if (send_begin_main_frame_funnel_)
+  if (did_send_begin_main_frame_for_current_frame_)
     return false;
 
   // Only send BeginMainFrame when there isn't another commit pending already.
@@ -598,6 +644,8 @@ SchedulerStateMachine::Action SchedulerStateMachine::NextAction() const {
     return ACTION_INVALIDATE_COMPOSITOR_FRAME_SINK;
   if (ShouldBeginCompositorFrameSinkCreation())
     return ACTION_BEGIN_COMPOSITOR_FRAME_SINK_CREATION;
+  if (ShouldNotifyBeginMainFrameNotSent())
+    return ACTION_NOTIFY_BEGIN_MAIN_FRAME_NOT_SENT;
   return ACTION_NONE;
 }
 
@@ -683,15 +731,22 @@ void SchedulerStateMachine::WillSendBeginMainFrame() {
   DCHECK(!has_pending_tree_ || settings_.main_frame_before_activation_enabled);
   DCHECK(visible_);
   DCHECK(!begin_frame_source_paused_);
-  DCHECK(!send_begin_main_frame_funnel_);
+  DCHECK(!did_send_begin_main_frame_for_current_frame_);
   begin_main_frame_state_ = BEGIN_MAIN_FRAME_STATE_SENT;
   needs_begin_main_frame_ = false;
-  send_begin_main_frame_funnel_ = true;
+  did_send_begin_main_frame_for_current_frame_ = true;
   last_frame_number_begin_main_frame_sent_ = current_frame_number_;
   last_begin_frame_sequence_number_begin_main_frame_sent_ =
       begin_frame_sequence_number_;
 }
 
+void SchedulerStateMachine::WillNotifyBeginMainFrameNotSent() {
+  DCHECK(visible_);
+  DCHECK(!begin_frame_source_paused_);
+  DCHECK(!did_notify_begin_main_frame_not_sent_);
+  did_notify_begin_main_frame_not_sent_ = true;
+}
+
 void SchedulerStateMachine::WillCommit(bool commit_has_no_updates) {
   bool can_have_pending_tree =
       commit_has_no_updates &&
@@ -701,6 +756,7 @@ void SchedulerStateMachine::WillCommit(bool commit_has_no_updates) {
   commit_count_++;
   last_commit_had_no_updates_ = commit_has_no_updates;
   begin_main_frame_state_ = BEGIN_MAIN_FRAME_STATE_IDLE;
+  did_commit_during_frame_ = true;
 
   if (commit_has_no_updates) {
     // Pending tree might still exist from prior commit.
@@ -1047,7 +1103,9 @@ void SchedulerStateMachine::OnBeginImplFrame(uint32_t source_id,
   needs_one_begin_impl_frame_ = false;
 
   // Clear funnels for any actions we perform during the frame.
-  send_begin_main_frame_funnel_ = false;
+  did_notify_begin_main_frame_not_sent_ = false;
+  did_send_begin_main_frame_for_current_frame_ = false;
+  did_commit_during_frame_ = false;
   invalidate_compositor_frame_sink_funnel_ = false;
   impl_side_invalidation_funnel_ = false;
 
@@ -1085,7 +1143,7 @@ void SchedulerStateMachine::OnBeginImplFrameIdle() {
   // If we're entering a state where we won't get BeginFrames set all the
   // funnels so that we don't perform any actions that we shouldn't.
   if (!BeginFrameNeeded())
-    send_begin_main_frame_funnel_ = true;
+    did_send_begin_main_frame_for_current_frame_ = true;
 
   // Synchronous compositor finishes BeginFrames before triggering their
   // deadline. Therefore, we update sequence numbers when becoming idle, before