Add metrics for DelayNavigationThrottle

chrome/browser/page_load_metrics/experiments/delay_navigation_throttle.cc

 // Copyright 2017 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include "chrome/browser/page_load_metrics/experiments/delay_navigation_throttle.h"
 
 #include "base/memory/ptr_util.h"
 #include "base/metrics/field_trial_params.h"
+#include "base/metrics/histogram_macros.h"
 #include "base/rand_util.h"
 #include "base/threading/thread_task_runner_handle.h"
 #include "chrome/browser/profiles/profile.h"
 #include "chrome/browser/search/search.h"
 #include "content/public/browser/navigation_handle.h"
 #include "content/public/browser/web_contents.h"
 
 namespace {
 
+const char kHistogramNavigationDelaySpecified[] =
+    "DelayNavigationThrottle.Delay.Specified";
+const char kHistogramNavigationDelayActual[] =
+    "DelayNavigationThrottle.Delay.Actual";
+const char kHistogramNavigationDelayDelta[] =
+    "DelayNavigationThrottle.Delay.Delta";
+
 base::TimeDelta GetNavigationDelayFromParams() {
   double delay_probability = base::GetFieldTrialParamByFeatureAsDouble(
       kDelayNavigationFeature,
       DelayNavigationThrottle::kParamDelayNavigationProbability,
       0 /* default value */);
 
   DCHECK_GE(delay_probability, 0.0);
   DCHECK_LE(delay_probability, 1.0);
   if (delay_probability == 0 || delay_probability < base::RandDouble())
     return base::TimeDelta();
@@ skipping 65 matching lines @@
     base::TimeDelta navigation_delay)
     : content::NavigationThrottle(handle),
       task_runner_(task_runner),
       navigation_delay_(navigation_delay),
       weak_ptr_factory_(this) {}
 
 DelayNavigationThrottle::~DelayNavigationThrottle() {}
 
 content::NavigationThrottle::ThrottleCheckResult
 DelayNavigationThrottle::WillStartRequest() {
+  UMA_HISTOGRAM_TIMES(kHistogramNavigationDelaySpecified, navigation_delay_);
+  delay_start_time_ = base::TimeTicks::Now();
   task_runner_->PostDelayedTask(
       FROM_HERE,
       base::Bind(&DelayNavigationThrottle::OnDelayComplete,
                  weak_ptr_factory_.GetWeakPtr()),
       navigation_delay_);
   return content::NavigationThrottle::DEFER;
 }
 
 void DelayNavigationThrottle::OnDelayComplete() {
+  base::TimeDelta actual_delay = base::TimeTicks::Now() - delay_start_time_;
+  base::TimeDelta delay_delta = actual_delay - navigation_delay_;
+  UMA_HISTOGRAM_TIMES(kHistogramNavigationDelayActual, actual_delay);
+  UMA_HISTOGRAM_TIMES(kHistogramNavigationDelayDelta, delay_delta.magnitude());

[Charlie Harrison, 2017/04/05 16:54:37]

if actual_delay < navigation_delay_ this could be problematic. If we expect this
to not happen very often, could we just let negative values clamp to 0?

[Bryan McQuade, 2017/04/05 20:11:05]

copying the relevant part of my general reply here:

I'm hesitant to clamp to zero as this prevents us from seeing the delta in cases
where it's negative. The goal of the delta histo is just to give us insight into
the accuracy of the delays overall.

   navigation_handle()->Resume();
 }