Move logic of recording main thread scrolling reasons from cc to blink::ScrollManager

third_party/WebKit/Source/core/input/ScrollManager.cpp

Index: third_party/WebKit/Source/core/input/ScrollManager.cpp
diff --git a/third_party/WebKit/Source/core/input/ScrollManager.cpp b/third_party/WebKit/Source/core/input/ScrollManager.cpp
index cd8ace43239b75d60b2304179281472207e2a849..76a67a5702c3dd154cfc2454c718d515740b14a8 100644
--- a/third_party/WebKit/Source/core/input/ScrollManager.cpp
+++ b/third_party/WebKit/Source/core/input/ScrollManager.cpp
@@ -22,6 +22,7 @@
 #include "core/page/scrolling/RootScrollerController.h"
 #include "core/page/scrolling/ScrollState.h"
 #include "core/paint/PaintLayer.h"
+#include "platform/Histogram.h"
 #include "platform/RuntimeEnabledFeatures.h"
 #include "wtf/PtrUtil.h"
 
@@ -196,6 +197,67 @@ void ScrollManager::CustomizedScroll(const Node& start_node,
   scroll_state.distributeToScrollChainDescendant();
 }
 
+uint32_t ScrollManager::ComputeNonCompositedMainThreadScrollingReasons() {
+  // When scrolling on the main thread, the scrollableArea may or may not be
+  // composited. Either way, we have recorded either the reasons stored in
+  // its layer or the reason NonFastScrollableRegion from the compositor
+  // side. Here we record scrolls that occurred on main thread due to a
+  // non-composited scroller.
+  if (!scroll_gesture_handling_node_->GetLayoutObject() || !frame_->View())
+    return 0;
+
+  uint32_t non_composited_main_thread_scrolling_reasons = 0;
+
+  for (auto* cur_box =
+           scroll_gesture_handling_node_->GetLayoutObject()->EnclosingBox();
+       cur_box; cur_box = cur_box->ContainingBlock()) {
+    PaintLayerScrollableArea* scrollable_area = cur_box->GetScrollableArea();
+
+    if (!scrollable_area || !scrollable_area->ScrollsOverflow())
+      continue;
+
+    DCHECK(!scrollable_area->UsesCompositedScrolling() ||
+           !scrollable_area->GetNonCompositedMainThreadScrollingReasons());
+    non_composited_main_thread_scrolling_reasons |=
+        scrollable_area->GetNonCompositedMainThreadScrollingReasons();
+  }

[flackr, 2017/04/11 19:35:35]

What is the state when we handle these events? If compositing inputs are clean,
which it seems like they should be if we're checking for compositing reasons,
then I think you can replace this loop with
scroll_gesture_handling_node_->enclosingLayer()->ancestorScrollingLayer()

[flackr, 2017/04/11 20:13:03]

It looks like recomputing the scroll chain actually computes all of the ancestor
scrollers in an upwards walk to the root too, so this shouldn't be too bad for
performance. It is worth noting however that if compositing inputs are dirty the
reasons are probably not current either, but this may be fine.

+
+  return non_composited_main_thread_scrolling_reasons;
+}
+
+void ScrollManager::RecordNonCompositedMainThreadScrollingReasons(
+    const WebGestureDevice device) {
+  if (device != kWebGestureDeviceTouchpad &&
+      device != kWebGestureDeviceTouchscreen) {
+    return;
+  }
+
+  uint32_t reasons = ComputeNonCompositedMainThreadScrollingReasons();
+  if (!reasons)
+    return;
+  DCHECK(MainThreadScrollingReason::HasNonCompositedScrollReasons(reasons));
+
+  uint32_t main_thread_scrolling_reason_enum_max =
+      MainThreadScrollingReason::kMainThreadScrollingReasonCount + 1;
+  for (uint32_t i = MainThreadScrollingReason::kNonCompositedReasonsFirst;
+       i <= MainThreadScrollingReason::kNonCompositedReasonsLast; ++i) {
+    unsigned val = 1 << i;
+    if (reasons & val) {
+      if (device == kWebGestureDeviceTouchscreen) {
+        DEFINE_STATIC_LOCAL(EnumerationHistogram, touch_histogram,
+                            ("Renderer4.MainThreadGestureScrollReason",
+                             main_thread_scrolling_reason_enum_max));
+        touch_histogram.Count(i + 1);
+      } else {
+        DEFINE_STATIC_LOCAL(EnumerationHistogram, wheel_histogram,
+                            ("Renderer4.MainThreadWheelScrollReason",
+                             main_thread_scrolling_reason_enum_max));

[flackr, 2017/04/11 20:13:03]

I notice the device in this case is only kWebGestureDeviceTouchpad, does that
include mouse wheel scrolls?

[yigu, 2017/04/11 21:28:10]

Mouse wheel scrolling uses kWebGestureDeviceTouchpad as device type.

+        wheel_histogram.Count(i + 1);
+      }
+    }
+  }
+}
+
 WebInputEventResult ScrollManager::HandleGestureScrollBegin(
     const WebGestureEvent& gesture_event) {
   Document* document = frame_->GetDocument();
@@ -221,6 +283,8 @@ WebInputEventResult ScrollManager::HandleGestureScrollBegin(
   PassScrollGestureEvent(gesture_event,
                          scroll_gesture_handling_node_->GetLayoutObject());
 
+  RecordNonCompositedMainThreadScrollingReasons(gesture_event.source_device);
+
   current_scroll_chain_.clear();
   std::unique_ptr<ScrollStateData> scroll_state_data =
       WTF::MakeUnique<ScrollStateData>();