Add UMA metric for style invalidation.

We introduce a ScopedHighResUsHistogramTimer which only records samples
if we have a high resolution timer. This is done to avoid a bunch of
samples falling into the 0us bucket when we don't have a high
resolution timer.

Change-Id: Ic57cd34bcd5ceada96fe8f69f9267ce21aef07a7
Reviewed-on: https://chromium-review.googlesource.com/1150020Reviewed-by: Emil A Eklund <eae@chromium.org>
Reviewed-by: Steven Holte <holte@chromium.org>
Reviewed-by: Anders Ruud <andruud@chromium.org>
Commit-Queue: Rune Lillesveen <futhark@chromium.org>
Cr-Commit-Position: refs/heads/master@{#578459}

third_party/blink/renderer/core/dom/document.cc

@@ -1923,6 +1923,7 @@ void Document::UpdateStyleInvalidationIfNeeded() {
   if (!ChildNeedsStyleInvalidation() && !NeedsStyleInvalidation())
     return;
   TRACE_EVENT0("blink", "Document::updateStyleInvalidationIfNeeded");
+  SCOPED_BLINK_UMA_HISTOGRAM_TIMER_HIGHRES("Style.InvalidationTime");
   GetStyleEngine().InvalidateStyle();
 }
 

third_party/blink/renderer/platform/histogram.h

@@ -78,11 +78,32 @@ class PLATFORM_EXPORT ScopedUsHistogramTimer {
   CustomCountHistogram& counter_;
 };
 
+class PLATFORM_EXPORT ScopedHighResUsHistogramTimer {
+ public:
+  explicit ScopedHighResUsHistogramTimer(CustomCountHistogram& counter)
+      : start_time_(CurrentTimeTicks()), counter_(counter) {}
+
+  ~ScopedHighResUsHistogramTimer() {
+    if (TimeTicks::IsHighResolution())
+      counter_.CountMicroseconds(CurrentTimeTicks() - start_time_);
+  }
+
+ private:
+  TimeTicks start_time_;
+  CustomCountHistogram& counter_;
+};
+
 #define SCOPED_BLINK_UMA_HISTOGRAM_TIMER_IMPL(name, allow_cross_thread)  \
   DEFINE_STATIC_LOCAL_IMPL(CustomCountHistogram, scoped_us_counter,      \
                            (name, 0, 10000000, 50), allow_cross_thread); \
   ScopedUsHistogramTimer timer(scoped_us_counter);
 
+#define SCOPED_BLINK_UMA_HISTOGRAM_TIMER_HIGHRES_IMPL(name,               \
+                                                      allow_cross_thread) \
+  DEFINE_STATIC_LOCAL_IMPL(CustomCountHistogram, scoped_us_counter,       \
+                           (name, 0, 10000000, 50), allow_cross_thread);  \
+  ScopedHighResUsHistogramTimer timer(scoped_us_counter);
+
 // Use code like this to record time, in microseconds, to execute a block of
 // code:
 //
@@ -95,6 +116,10 @@ class PLATFORM_EXPORT ScopedUsHistogramTimer {
 #define SCOPED_BLINK_UMA_HISTOGRAM_TIMER(name) \
   SCOPED_BLINK_UMA_HISTOGRAM_TIMER_IMPL(name, false)
 
+// Only record samples when we have a high resolution timer
+#define SCOPED_BLINK_UMA_HISTOGRAM_TIMER_HIGHRES(name) \
+  SCOPED_BLINK_UMA_HISTOGRAM_TIMER_HIGHRES_IMPL(name, false)
+
 // Thread-safe variant of SCOPED_BLINK_UMA_HISTOGRAM_TIMER.
 // Use if the histogram can be accessed by multiple threads.
 #define SCOPED_BLINK_UMA_HISTOGRAM_TIMER_THREAD_SAFE(name) \

third_party/blink/renderer/platform/histogram_test.cc

@@ -23,7 +23,8 @@ class TestCustomCountHistogram : public CustomCountHistogram {
 };
 
 TEST(ScopedUsHistogramTimerTest, Basic) {
-  TestCustomCountHistogram scoped_us_counter("test", 0, 10000000, 50);
+  TestCustomCountHistogram scoped_us_counter("ScopedUsHistogramTimerTest.Basic",
+                                             0, 10000000, 50);
   {
     WTF::ScopedMockClock clock;
     ScopedUsHistogramTimer timer(scoped_us_counter);
@@ -33,4 +34,16 @@ TEST(ScopedUsHistogramTimerTest, Basic) {
   EXPECT_EQ(500000, scoped_us_counter.Histogram()->SnapshotSamples()->sum());
 }
 
+TEST(ScopedHighResUsHistogramTimerTest, Basic) {
+  TestCustomCountHistogram scoped_us_counter(
+      "ScopedHighResUsHistogramTimerTest.Basic", 0, 10000000, 50);
+  {
+    WTF::ScopedMockClock clock;
+    ScopedHighResUsHistogramTimer timer(scoped_us_counter);
+    clock.Advance(TimeDelta::FromMilliseconds(500));
+  }
+  int64_t expected = TimeTicks::IsHighResolution() ? 500000 : 0;
+  EXPECT_EQ(expected, scoped_us_counter.Histogram()->SnapshotSamples()->sum());
+}
+
 }  // namespace blink

tools/metrics/histograms/histograms.xml

@@ -98288,6 +98288,14 @@ uploading your change for review.
   </summary>
 </histogram>
 
+<histogram name="Style.InvalidationTime" units="microseconds">
+  <owner>futhark@chromium.org</owner>
+  <summary>
+    Microseconds spent in StyleEngine::InvalidateStyle. Only samples from high
+    resolution timers are recorded.
+  </summary>
+</histogram>
+
 <histogram name="Style.LazyUsage.Percent" enum="LazyCSSParseUsage">
   <obsolete>
     Deprecated 07/2018, no longer used.