Update the animation callback histogram to measure only the delay between post and invocation

chrome/renderer/render_widget.cc

Index: chrome/renderer/render_widget.cc
diff --git a/chrome/renderer/render_widget.cc b/chrome/renderer/render_widget.cc
index 96f33c912435d9478d6f0b9b1b7b5e37adce4ff1..7342c2d05c299576acfbe430b12021a5395cf248 100644
--- a/chrome/renderer/render_widget.cc
+++ b/chrome/renderer/render_widget.cc
@@ -483,8 +483,21 @@ void RenderWidget::PaintDebugBorder(const gfx::Rect& rect,
 
 void RenderWidget::AnimationCallback() {
   animation_task_posted_ = false;
-  UMA_HISTOGRAM_CUSTOM_COUNTS("Renderer4.AnimationDelayTime", 0, 0, 16, 17);
-  CallDoDeferredUpdate();
+  if (animation_update_pending_) {

[jar (doing other things), 2011/02/18 07:31:29]

nit: Suggest an early return, rather than indenting the rest of the method.

+    if (!animation_floor_time_.is_null()) {
+      // Record when we fired (according to base::Time::Now()) relative to when
+      // we posted the task to quantify how much the base::Time/base::TimeTicks
+      // skew is affecting animations.
+      base::TimeDelta animation_callback_delay = base::Time::Now() -
+          (animation_floor_time_ - base::TimeDelta::FromMilliseconds(16));
+      UMA_HISTOGRAM_CUSTOM_TIMES("Renderer4.AnimationCallbackDelayTime",
+                                 animation_callback_delay,
+                                 base::TimeDelta::FromMilliseconds(0),
+                                 base::TimeDelta::FromMilliseconds(30),
+                                 25);
+    }
+    CallDoDeferredUpdate();
+  }
 }
 
 void RenderWidget::AnimateIfNeeded() {
@@ -510,11 +523,9 @@ void RenderWidget::AnimateIfNeeded() {
       // avoid exposing this delay to javascript, we keep posting delayed
       // tasks until base::Time::Now() has advanced far enough.
       int64 delay = (animation_floor_time_ - now).InMillisecondsRoundedUp();

[jar (doing other things), 2011/02/18 07:31:29]

Comment: It is interesting that delay could be as small as 1 ms.  Considering
that the resolution of the time is rarely that fine, it is not clear that
setting such a small delay is helpful (though I know you're worried about early
firing, rather than late firing).  

-      UMA_HISTOGRAM_CUSTOM_COUNTS("Renderer4.AnimationDelayTime",
-                                  static_cast<int>(delay), 0, 16, 17);
       animation_task_posted_ = true;
-      MessageLoop::current()->PostDelayedTask(FROM_HERE, NewRunnableMethod(
-          this, &RenderWidget::AnimationCallback), delay);
+      MessageLoop::current()->PostDelayedTask(FROM_HERE,
+          NewRunnableMethod(this, &RenderWidget::AnimationCallback), delay);
     }
   }
 }