More detailed tracing for smooth scrolling.

This cl adds additional tracing for smooth scrolling path in cc. A
NOTREACHED is also added to the invalid case that the scrolling node
exists in layer tree host impl, but the animation update fails.

Bug: 797708
Cq-Include-Trybots: luci.chromium.try:android_optional_gpu_tests_rel;master.tryserver.blink:linux_trusty_blink_rel
Change-Id: If17af64781e78d02f09433e1c0e479231e69b4c2
Reviewed-on: https://chromium-review.googlesource.com/1145736Reviewed-by: weiliangc <weiliangc@chromium.org>
Commit-Queue: Sahel Sharify <sahel@chromium.org>
Cr-Commit-Position: refs/heads/master@{#579069}

cc/animation/scroll_offset_animations_impl.cc

@@ -122,16 +122,25 @@ void ScrollOffsetAnimationsImpl::ScrollAnimationApplyAdjustment(
     ElementId element_id,
     const gfx::Vector2dF& adjustment) {
   DCHECK(scroll_offset_animation_);
-  if (element_id != scroll_offset_animation_->element_id())
+  if (element_id != scroll_offset_animation_->element_id()) {
+    TRACE_EVENT_INSTANT0("cc", "no scroll adjustment different element_ids",
+                         TRACE_EVENT_SCOPE_THREAD);
     return;
+  }
 
-  if (!scroll_offset_animation_->has_element_animations())
+  if (!scroll_offset_animation_->has_element_animations()) {
+    TRACE_EVENT_INSTANT0("cc", "no scroll adjustment no element animation",
+                         TRACE_EVENT_SCOPE_THREAD);
     return;
+  }
 
   KeyframeModel* keyframe_model =
       scroll_offset_animation_->GetKeyframeModel(TargetProperty::SCROLL_OFFSET);
-  if (!keyframe_model)
+  if (!keyframe_model) {
+    TRACE_EVENT_INSTANT0("cc", "no scroll adjustment no keyframe model",
+                         TRACE_EVENT_SCOPE_THREAD);
     return;
+  }
 
   std::unique_ptr<ScrollOffsetAnimationCurve> new_curve =
       keyframe_model->curve()
@@ -151,12 +160,16 @@ void ScrollOffsetAnimationsImpl::ScrollAnimationApplyAdjustment(
 
   // Start a new one with the adjusment.
   scroll_offset_animation_->AddKeyframeModel(std::move(new_keyframe_model));
+  TRACE_EVENT_INSTANT0("cc", "scroll animation adjusted",
+                       TRACE_EVENT_SCOPE_THREAD);
 }
 
 void ScrollOffsetAnimationsImpl::ScrollAnimationAbort(bool needs_completion) {
   DCHECK(scroll_offset_animation_);
   scroll_offset_animation_->AbortKeyframeModelsWithProperty(
       TargetProperty::SCROLL_OFFSET, needs_completion);
+  TRACE_EVENT_INSTANT1("cc", "ScrollAnimationAbort", TRACE_EVENT_SCOPE_THREAD,
+                       "needs_completion", needs_completion);
 }
 
 void ScrollOffsetAnimationsImpl::NotifyAnimationFinished(
@@ -166,15 +179,23 @@ void ScrollOffsetAnimationsImpl::NotifyAnimationFinished(
   DCHECK_EQ(target_property, TargetProperty::SCROLL_OFFSET);
   DCHECK(animation_host_->mutator_host_client());
   animation_host_->mutator_host_client()->ScrollOffsetAnimationFinished();
+  TRACE_EVENT_INSTANT0("cc", "NotifyAnimationFinished",
+                       TRACE_EVENT_SCOPE_THREAD);
 }
 
 void ScrollOffsetAnimationsImpl::ReattachScrollOffsetAnimationIfNeeded(
     ElementId element_id) {
   if (scroll_offset_animation_->element_id() != element_id) {
-    if (scroll_offset_animation_->element_id())
+    if (scroll_offset_animation_->element_id()) {
+      TRACE_EVENT_INSTANT0("cc", "scroll offset animation detached element",
+                           TRACE_EVENT_SCOPE_THREAD);
       scroll_offset_animation_->DetachElement();
-    if (element_id)
+    }
+    if (element_id) {
+      TRACE_EVENT_INSTANT0("cc", "scroll offset animation attached element",
+                           TRACE_EVENT_SCOPE_THREAD);
       scroll_offset_animation_->AttachElement(element_id);
+    }
   }
 }
 

cc/trees/layer_tree_host_impl.cc

@@ -3639,6 +3639,8 @@ bool LayerTreeHostImpl::ScrollAnimationCreate(ScrollNode* scroll_node,
       (std::abs(delta.x()) > kEpsilon || std::abs(delta.y()) > kEpsilon);
   if (!scroll_animated) {
     scroll_tree.ScrollBy(scroll_node, delta, active_tree());
+    TRACE_EVENT_INSTANT0("cc", "no scroll animation due to small delta",
+                         TRACE_EVENT_SCOPE_THREAD);
     return false;
   }
 
@@ -3706,6 +3708,14 @@ InputHandler::ScrollStatus LayerTreeHostImpl::ScrollAnimated(
       scroll_status.thread = SCROLL_IGNORED;
       scroll_status.main_thread_scrolling_reasons =
           MainThreadScrollingReason::kNotScrollable;
+      // Adding NOTREACHED to debug https://crbug.com/797708, based on the
+      // traces on the bug scrolling gets stuck in a situation where the
+      // layout_tree_host_impl assumes that there is an ongoing scroll animation
+      // since scroll_node exists but the
+      // ScrollOffsetAnimationsImpl::ScrollAnimationUpdateTarget returns false
+      // since no keyframe_model exists. TODO(sahel):remove this once the issue
+      // is fixed.
+      NOTREACHED();
     }
     return scroll_status;
   }
@@ -3758,6 +3768,8 @@ InputHandler::ScrollStatus LayerTreeHostImpl::ScrollAnimated(
         did_scroll_y_for_scroll_gesture_ |= scrolled.y() != 0;
         if (scrolled == pending_delta) {
           scroll_animating_latched_element_id_ = scroll_node->element_id;
+          TRACE_EVENT_INSTANT0("cc", "Viewport scroll animated",
+                               TRACE_EVENT_SCOPE_THREAD);
           return scroll_status;
         }
         break;
@@ -3769,6 +3781,8 @@ InputHandler::ScrollStatus LayerTreeHostImpl::ScrollAnimated(
         did_scroll_x_for_scroll_gesture_ |= scroll_delta.x() != 0;
         did_scroll_y_for_scroll_gesture_ |= scroll_delta.y() != 0;
         scroll_animating_latched_element_id_ = scroll_node->element_id;
+        TRACE_EVENT_INSTANT0("cc", "created scroll animation",
+                             TRACE_EVENT_SCOPE_THREAD);
         // Flash the overlay scrollbar even if the scroll dalta is 0.
         if (settings_.scrollbar_flash_after_any_scroll_update) {
           FlashAllScrollbars(false);