[Throughput UKM] Correct sampling logic

We have identified that the reason of not getting throughput UKM is
due to the sampling logic, via this debug CL:
https://chromium-review.googlesource.com/c/chromium/src/+/1946872

This CL changes the sampling logic.

Bug: 1029964
Change-Id: Icf5fc750102d0e3631047913d77adb4e37589721
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1958195
Reviewed-by: Sadrul Chowdhury <sadrul@chromium.org>
Reviewed-by: Stephen Chenney <schenney@chromium.org>
Commit-Queue: Xida Chen <xidachen@chromium.org>
Cr-Commit-Position: refs/heads/master@{#723642}

cc/metrics/throughput_ukm_reporter.cc

@@ -10,7 +10,7 @@ namespace cc {
 
 namespace {
 // Collect UKM once per kNumberOfSamplesToReport UMA reports.
-constexpr unsigned kNumberOfSamplesToReport = 2000u;
+constexpr unsigned kNumberOfSamplesToReport = 100u;
 }  // namespace
 
 void ThroughputUkmReporter::ReportThroughputUkm(
@@ -19,17 +19,12 @@ void ThroughputUkmReporter::ReportThroughputUkm(
     const base::Optional<int>& impl_throughput_percent,
     const base::Optional<int>& main_throughput_percent,
     FrameSequenceTrackerType type) {
-  // Sampling control. We sample the event here to not throttle the UKM system.
-  // Currently, the same sampling rate is applied to all existing trackers. We
-  // might want to iterate on this based on the collected data.
-  static uint32_t samples_to_next_event = 0;
-
-  if (samples_to_next_event == 0) {
+  if (samples_to_next_event_ == 0) {
     // Sample every 2000 events. Using the Universal tracker as an example
     // which reports UMA every 5s, then the system collects UKM once per
     // 2000*5 = 10000 seconds, which is about 3 hours. This number may need to
     // be tuned to not throttle the UKM system.
-    samples_to_next_event = kNumberOfSamplesToReport;
+    samples_to_next_event_ = kNumberOfSamplesToReport;
     if (impl_throughput_percent) {
       ukm_manager->RecordThroughputUKM(
           type, FrameSequenceTracker::ThreadType::kCompositor,
@@ -44,8 +39,8 @@ void ThroughputUkmReporter::ReportThroughputUkm(
                                      FrameSequenceTracker::ThreadType::kSlower,
                                      slower_throughput_percent.value());
   }
-  DCHECK_GT(samples_to_next_event, 0u);
-  samples_to_next_event--;
+  DCHECK_GT(samples_to_next_event_, 0u);
+  samples_to_next_event_--;
 }
 
 }  // namespace cc

cc/metrics/throughput_ukm_reporter.h

@@ -24,6 +24,12 @@ class CC_EXPORT ThroughputUkmReporter {
                            const base::Optional<int>& impl_throughput_percent,
                            const base::Optional<int>& main_throughput_percent,
                            FrameSequenceTrackerType type);
+
+ private:
+  // Sampling control. We sample the event here to not throttle the UKM system.
+  // Currently, the same sampling rate is applied to all existing trackers. We
+  // might want to iterate on this based on the collected data.
+  uint32_t samples_to_next_event_ = 0;
 };
 
 }  // namespace cc