Calculate Histogram diffs rather reporting all to Traces

We currently measure Histograms in perf benchmarks. This is done by
emitting a trace of all histograms from startup. Then emitting another
at the end of tracing.

Then in Catapult we calculate differences in uma_metric.html

However this has led to issues in the past with mis-calculating buckets
of histograms. It also currently breaks down when histogram emitting
changes occur near the startup.

To help clear this, we want to not record the initial histograms. But
instead calculate the differences. Thus emitting only one sample for
Catapult to process. We will then reduce the complexity of uma_metric's
processing in a follow up change in Catapult.

Bug: 1145731

Change-Id: Ia6452984a16914aaf19e66c22d583592835f46df
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2521106Reviewed-by: oysteine <oysteine@chromium.org>
Commit-Queue: Jonathan Ross <jonross@chromium.org>
Cr-Commit-Position: refs/heads/master@{#827962}

services/tracing/public/cpp/perfetto/trace_event_data_source.cc

@@ -958,7 +958,17 @@ void TraceEventDataSource::LogHistogram(base::HistogramBase* histogram) {
   if (!histogram) {
     return;
   }
+  // For the purpose of calculating metrics from histograms we only want the
+  // delta of the events.
   auto samples = histogram->SnapshotSamples();
+
+  // If there were HistogramSamples recorded during startup, then those should
+  // be subtracted from the overall set. This way we only report the samples
+  // that occured during the run.
+  auto it = startup_histogram_samples_.find(histogram->histogram_name());
+  if (it != startup_histogram_samples_.end()) {
+    samples->Subtract(*it->second.get());
+  }
   base::Pickle pickle;
   samples->Serialize(&pickle);
   std::string buckets;
@@ -972,9 +982,22 @@ void TraceEventDataSource::LogHistogram(base::HistogramBase* histogram) {
 
 void TraceEventDataSource::ResetHistograms(const TraceConfig& trace_config) {
   histograms_.clear();
+  startup_histogram_samples_.clear();
   for (const std::string& histogram_name : trace_config.histogram_names()) {
     histograms_.push_back(histogram_name);
-    LogHistogram(base::StatisticsRecorder::FindHistogram(histogram_name));
+    auto* histogram = base::StatisticsRecorder::FindHistogram(histogram_name);
+    if (!histogram) {
+      continue;
+    }
+
+    // For the purpose of calculating metrics from histograms we only want the
+    // delta of the events. However we do not want to emit the results when
+    // resetting. This will allow LogHistogram to emit one UMAHistogramSamples
+    // which encompasses only the histograms recorded during the trace. We
+    // cache the initial HistogramSamples so that they can be subtracted from
+    // the full snapshot at the end.
+    startup_histogram_samples_.emplace(histogram_name,
+                                       histogram->SnapshotSamples());
   }
 }
 

services/tracing/public/cpp/perfetto/trace_event_data_source.h

@@ -27,11 +27,15 @@
 #include "third_party/perfetto/protos/perfetto/trace/chrome/chrome_trace_event.pbzero.h"
 
 namespace base {
+
+class HistogramSamples;
+
 namespace trace_event {
 class ThreadInstructionCount;
 class TraceEvent;
 struct TraceEventHandle;
 }  // namespace trace_event
+
 }  // namespace base
 
 namespace perfetto {
@@ -280,6 +284,11 @@ class COMPONENT_EXPORT(TRACING_CPP) TraceEventDataSource
   bool flushing_trace_log_ = false;
   base::OnceClosure flush_complete_task_;
   std::vector<std::string> histograms_;
+  // For each of the Histogram that we are tracking, cache the snapshot of their
+  // HistogramSamples from before tracing began. So that we can calculate the
+  // delta when we go to LogHistograms.
+  std::map<std::string, std::unique_ptr<base::HistogramSamples>>
+      startup_histogram_samples_;
   // Stores all histogram names for which OnMetricsSampleCallback was set as an
   // OnSampleCallback. This is done in order to avoid clearing callbacks for the
   // other histograms.