[Sessions] Add detailed logging of SessionRestore events.

chrome/browser/sessions/session_restore_stats_collector.cc

Index: chrome/browser/sessions/session_restore_stats_collector.cc
diff --git a/chrome/browser/sessions/session_restore_stats_collector.cc b/chrome/browser/sessions/session_restore_stats_collector.cc
index f892d1ad107b4bdbb27b03cb5d161c475de593a2..34b3781d42be5caad6ce320934fb6b105c247441 100644
--- a/chrome/browser/sessions/session_restore_stats_collector.cc
+++ b/chrome/browser/sessions/session_restore_stats_collector.cc
@@ -13,53 +13,117 @@
 #include "content/public/browser/render_widget_host_view.h"
 #include "content/public/browser/web_contents.h"
 
+namespace {
+
 using content::NavigationController;
 using content::RenderWidgetHost;
+using content::Source;
 using content::WebContents;
 
-// static
-void SessionRestoreStatsCollector::TrackTabs(
-    const std::vector<SessionRestoreDelegate::RestoredTab>& tabs,
-    const base::TimeTicks& restore_started) {
-  if (!shared_collector_)
-    shared_collector_ = new SessionRestoreStatsCollector(restore_started);
+const char kSessionRestoreActions[] = "SessionRestore.Actions";
 
-  shared_collector_->AddTabs(tabs);
-}
+// The enumeration values stored in the "SessionRestore.Actions" histogram.
+enum SessionRestoreActionsUma {
+  // Counts the total number of session restores that have occurred.
+  SESSION_RESTORE_ACTIONS_UMA_INITIATED = 0,
+  // Counts the number of session restores that have seen deferred tab loadings
+  // for whatever reason (almost certainly due to memory pressure).
+  SESSION_RESTORE_ACTIONS_UMA_DEFERRED_TABS = 1,
+  // The size of this enum. Must be the last entry.
+  SESSION_RESTORE_ACTIONS_UMA_MAX,
+};
 
-// static
-void SessionRestoreStatsCollector::TrackActiveTabs(
-    const std::vector<SessionRestoreDelegate::RestoredTab>& tabs,
-    const base::TimeTicks& restore_started) {
-  if (!shared_collector_)
-    shared_collector_ = new SessionRestoreStatsCollector(restore_started);
+// The enumeration of values stored in the "SessionRestore.TabActions"
+// histogram.
+enum  SessionRestoreTabActionsUma {
+  // Incremented for each tab created in a session restore.
+  SESSION_RESTORE_TAB_ACTIONS_UMA_TAB_CREATED = 0,
+  // Incremented for each tab that session restore decides not to load.
+  SESSION_RESTORE_TAB_ACTIONS_UMA_TAB_LOADING_DEFERRED = 1,
+  // Incremented for each tab that is successfully loaded.
+  SESSION_RESTORE_TAB_ACTIONS_UMA_TAB_LOADED = 2,
+  // Incremented for each session-restore-deferred tab that is subsequently
+  // loaded.
+  SESSION_RESTORE_TAB_ACTIONS_UMA_DEFERRED_TAB_LOADED = 3,
+  // The size of this enum. Must be the last entry.
+  SESSION_RESTORE_TAB_ACTIONS_UMA_MAX,
+};
 
-  std::vector<SessionRestoreDelegate::RestoredTab> active_tabs;
-  for (auto tab : tabs) {
-    if (tab.is_active())
-      active_tabs.push_back(tab);
-  }
-  shared_collector_->AddTabs(active_tabs);
+}  // namespace
+
+SessionRestoreStatsCollector::TabLoaderStats::TabLoaderStats()
+    : tab_count(0u), tabs_loaded(0u), parallel_tab_loads(0u) {
+}
+
+SessionRestoreStatsCollector::TabState::TabState(
+    NavigationController* controller)
+    : controller(controller),
+      render_widget_host(nullptr),
+      is_deferred(false),
+      loading_state(TAB_IS_NOT_LOADING) {
 }
 
 SessionRestoreStatsCollector::SessionRestoreStatsCollector(
     const base::TimeTicks& restore_started)
-    : got_first_foreground_load_(false),
+    : got_done_tracking_non_deferred_tabs_(false),
+      got_first_foreground_load_(false),
       got_first_paint_(false),
+      got_report_tab_deferred_(false),
       restore_started_(restore_started),
-      tab_count_(0),
-      max_parallel_tab_loads_(0) {
+      waiting_for_load_tab_count_(0u),
+      loading_tab_count_(0u) {
   this_retainer_ = this;
-  registrar_.Add(
-      this, content::NOTIFICATION_RENDER_WIDGET_HOST_DID_UPDATE_BACKING_STORE,
-      content::NotificationService::AllSources());
 }
 
 SessionRestoreStatsCollector::~SessionRestoreStatsCollector() {
-  DCHECK((got_first_paint_ || render_widget_hosts_to_paint_.empty()) &&
-         tabs_tracked_.empty() && render_widget_hosts_loading_.empty());
-  DCHECK(shared_collector_ == this);
-  shared_collector_ = nullptr;
+  // The collector should only be destroyed when tracking is entirely

[sky, 2015/05/27 15:18:39]

Is this true if shutdown happens before everything is loaded?

[chrisha, 2015/06/03 21:16:36]

Very good point. Removed.

+  // finished.
+  DCHECK(DoneTracking());
+}
+
+void SessionRestoreStatsCollector::TrackTabs(
+    const std::vector<SessionRestoreDelegate::RestoredTab>& tabs) {
+  DCHECK(!got_done_tracking_non_deferred_tabs_);
+
+  // If this is the first call to TrackTabs then start observing events.
+  if (tab_loader_stats_.tab_count == 0) {
+    registrar_.Add(
+        this, content::NOTIFICATION_RENDER_WIDGET_HOST_DID_UPDATE_BACKING_STORE,
+        content::NotificationService::AllSources());
+  }
+
+  tab_loader_stats_.tab_count += tabs.size();
+  waiting_for_load_tab_count_ += tabs.size();
+  for (auto& tab : tabs) {
+    TabState* tab_state = RegisterForNotifications(
+        &tab.contents()->GetController());
+    // Active tabs have already started loading, kicked by the TabLoader.

[sky, 2015/05/27 15:18:39]

Actually, don't they start loading because they are active?

[chrisha, 2015/06/03 21:16:36]

Acknowledged.

+    if (tab.is_active())
+      MarkTabAsLoading(tab_state);
+  }
+}
+
+void SessionRestoreStatsCollector::DeferTab(NavigationController* tab) {
+  TabState* tab_state = GetTabState(tab);
+
+  // If the tab is no longer being tracked it has already finished loading.
+  // This can occur if the user forces the tab to load before the entire session
+  // restore is over, and the TabLoader then decides it would defer loading of
+  // that tab.
+  if (!tab_state)
+    return;
+
+  // Mark this tab as deferred, if its still being tracked. A tab should not be
+  // marked as deferred twice.
+  DCHECK(!tab_state->is_deferred);
+  tab_state->is_deferred = true;
+
+  // A tab that didn't start loading before it was deferred is not to be
+  // actively monitored for loading.
+  if (tab_state->loading_state == TAB_IS_NOT_LOADING)
+    --waiting_for_load_tab_count_;

[sky, 2015/05/27 15:18:39]

Might waiting_for_load_tab_count_ be 0 now?

[chrisha, 2015/06/03 21:16:36]

Yeah, it might. I thought it simpler to keep all of the state transition logic
in Observe (the 15 lines or so at the end of that function). This can only cause
the collector to wait a little bit longer before it invokes
ReportTabLoaderStats.

I can also move that logic to a helper function, and call it both here and from
Observe. Thoughts?

[sky, 2015/06/03 22:00:07]

Helper function SGTM.

[chrisha, 2015/06/04 14:04:47]

Done.

+
+  ReportTabDeferred();
 }
 
 void SessionRestoreStatsCollector::Observe(
@@ -68,99 +132,110 @@ void SessionRestoreStatsCollector::Observe(
     const content::NotificationDetails& details) {
   switch (type) {
     case content::NOTIFICATION_LOAD_START: {
-      // Add this render_widget_host to the set of those we're waiting for
-      // paints on. We want to only record stats for paints that occur after
-      // a load has finished.
-      NavigationController* tab =
-          content::Source<NavigationController>(source).ptr();
-      RenderWidgetHost* render_widget_host = GetRenderWidgetHost(tab);
-      DCHECK(render_widget_host);
-      render_widget_hosts_loading_.insert(render_widget_host);
+      // This occurs when a tab has started to load. This can be because of
+      // the tab loader (only for non-deferred tabs) or because the user clicked
+      // on the tab.
+      NavigationController* tab = Source<NavigationController>(source).ptr();
+      TabState* tab_state = GetTabState(tab);
+      MarkTabAsLoading(tab_state);
       break;
     }
     case content::NOTIFICATION_WEB_CONTENTS_DESTROYED: {
-      WebContents* web_contents = content::Source<WebContents>(source).ptr();
-      RemoveTab(&web_contents->GetController());
+      // This happens when a tab has been closed. A tab can be in any state
+      // when this occurs. Simply stop tracking the tab.
+      WebContents* web_contents = Source<WebContents>(source).ptr();

[sky, 2015/05/27 15:18:39]

Code seems to assume we'll get a LOAD_STOP before a DESTROYED. Is that
guaranteed?

[chrisha, 2015/06/03 21:16:36]

Another great catch.

I'm not sure if that's guaranteed or not, but it's easier to make this tolerate
the case where that *doesn't* happen.

+      NavigationController* tab = &web_contents->GetController();
+      RemoveTab(tab);
       break;
     }
     case content::NOTIFICATION_LOAD_STOP: {
-      NavigationController* tab =
-          content::Source<NavigationController>(source).ptr();
-      RenderWidgetHost* render_widget_host = GetRenderWidgetHost(tab);
-      render_widget_hosts_to_paint_.insert(render_widget_host);
-      RemoveTab(tab);
-      if (!got_first_foreground_load_ && render_widget_host &&
-          render_widget_host->GetView() &&
-          render_widget_host->GetView()->IsShowing()) {
+      // This occurs to loading tabs when they have finished loading. The tab
+      // may or may not already have painted at this point.
+
+      // Update the tab state and any global state as necessary.
+      NavigationController* tab = Source<NavigationController>(source).ptr();
+      TabState* tab_state = GetTabState(tab);
+      DCHECK(tab_state);
+      tab_state->loading_state = TAB_IS_LOADED;
+      --loading_tab_count_;
+      if (!tab_state->is_deferred)
+        --waiting_for_load_tab_count_;
+
+      // Update statistics.

[sky, 2015/05/27 15:18:39]

nit: this comment isn't really useful, I would nuke it.

[chrisha, 2015/06/03 21:16:36]

Done.

+      if (tab_state->is_deferred) {
+        ReportDeferredTabLoaded();
+      } else {
+        DCHECK(!got_done_tracking_non_deferred_tabs_);
+        ++tab_loader_stats_.tabs_loaded;
+      }
+
+      // Update UMA stats for foreground tabs.
+      if (!got_first_foreground_load_ &&
+          tab_state->render_widget_host &&
+          tab_state->render_widget_host->GetView() &&
+          tab_state->render_widget_host->GetView()->IsShowing()) {
         got_first_foreground_load_ = true;
         base::TimeDelta time_to_load =
             base::TimeTicks::Now() - restore_started_;
-        UMA_HISTOGRAM_CUSTOM_TIMES("SessionRestore.ForegroundTabFirstLoaded",
-                                   time_to_load,
-                                   base::TimeDelta::FromMilliseconds(10),
-                                   base::TimeDelta::FromSeconds(100), 100);
-        // Record a time for the number of tabs, to help track down
-        // contention.
-        std::string time_for_count = base::StringPrintf(
-            "SessionRestore.ForegroundTabFirstLoaded_%d", tab_count_);
-        base::HistogramBase* counter_for_count =
-            base::Histogram::FactoryTimeGet(
-                time_for_count, base::TimeDelta::FromMilliseconds(10),
-                base::TimeDelta::FromSeconds(100), 100,
-                base::Histogram::kUmaTargetedHistogramFlag);
-        counter_for_count->AddTime(time_to_load);
+        DCHECK(!got_done_tracking_non_deferred_tabs_);
+        tab_loader_stats_.foreground_tab_first_loaded = time_to_load;
       }
+
+      // If first paint has already been observed then there's no need to keep

[sky, 2015/05/27 15:18:39]

This comment explains what the code does not why. The 'why' is the interesting
part to document.

[chrisha, 2015/06/03 21:16:36]

Done.

+      // tracking this tab.
+      if (got_first_paint_)
+        RemoveTab(tab);
+
       break;
     }
     case content::NOTIFICATION_RENDER_WIDGET_HOST_DID_UPDATE_BACKING_STORE: {
+      // This notification is across all tabs in the browser so notifications
+      // will arrive for tabs that the collector is not explicitly tracking.
+
+      // Only process this event if first paint hasn't been seen and this is a
+      // paint of a visible tab.
       RenderWidgetHost* render_widget_host =
-          content::Source<RenderWidgetHost>(source).ptr();
-      if (!got_first_paint_ && render_widget_host->GetView() &&
+          Source<RenderWidgetHost>(source).ptr();
+      if (!got_first_paint_ &&
+          render_widget_host->GetView() &&
           render_widget_host->GetView()->IsShowing()) {
-        if (render_widget_hosts_to_paint_.find(render_widget_host) !=
-            render_widget_hosts_to_paint_.end()) {
-          // Got a paint for one of our renderers, so record time.
+        TabState* tab_state = GetTabState(render_widget_host);
+        if (tab_state) {
+          // This is a paint for a tab that is explicitly being tracked so
+          // update the statistics.
           got_first_paint_ = true;
           base::TimeDelta time_to_paint =
               base::TimeTicks::Now() - restore_started_;
-          // TODO(danduong): to remove this with 467680, to make sure we
-          // don't forget to clean this up.
-          UMA_HISTOGRAM_CUSTOM_TIMES("SessionRestore.ForegroundTabFirstPaint",
-                                     time_to_paint,
-                                     base::TimeDelta::FromMilliseconds(10),
-                                     base::TimeDelta::FromSeconds(100), 100);
-          // Record a time for the number of tabs, to help track down
-          // contention.
-          std::string time_for_count = base::StringPrintf(
-              "SessionRestore.ForegroundTabFirstPaint_%d", tab_count_);
-          base::HistogramBase* counter_for_count =
-              base::Histogram::FactoryTimeGet(
-                  time_for_count, base::TimeDelta::FromMilliseconds(10),
-                  base::TimeDelta::FromSeconds(100), 100,
-                  base::Histogram::kUmaTargetedHistogramFlag);
-          counter_for_count->AddTime(time_to_paint);
-          UMA_HISTOGRAM_CUSTOM_TIMES("SessionRestore.ForegroundTabFirstPaint2",
-                                     time_to_paint,
-                                     base::TimeDelta::FromMilliseconds(100),
-                                     base::TimeDelta::FromMinutes(16), 50);
-          // Record a time for the number of tabs, to help track down
-          // contention.
-          std::string time_for_count2 = base::StringPrintf(
-              "SessionRestore.ForegroundTabFirstPaint2_%d", tab_count_);
-          base::HistogramBase* counter_for_count2 =
-              base::Histogram::FactoryTimeGet(
-                  time_for_count2, base::TimeDelta::FromMilliseconds(100),
-                  base::TimeDelta::FromMinutes(16), 50,
-                  base::Histogram::kUmaTargetedHistogramFlag);
-          counter_for_count2->AddTime(time_to_paint);
-        } else if (render_widget_hosts_loading_.find(render_widget_host) ==
-                   render_widget_hosts_loading_.end()) {
-          // If this is a host for a tab we're not loading some other tab
+          DCHECK(!got_done_tracking_non_deferred_tabs_);
+          tab_loader_stats_.foreground_tab_first_paint = time_to_paint;
+        } else {
+          // If this is a host for a tab we're not tracking then some other tab
           // has rendered and there's no point tracking the time. This could
           // happen because the user opened a different tab or restored tabs
-          // to an already existing browser and an existing tab painted.
+          // to an already existing browser and an existing tab was in the
+          // foreground.
           got_first_paint_ = true;

[sky, 2015/05/27 15:18:39]

AFAICT both paths set got_first_paint_ to true. Set it outside of conditional
and remove 220 as it's always true.

[chrisha, 2015/06/03 21:16:36]

Indeed... remnant of the previous more tortured logic.

Done.

         }
+
+        if (got_first_paint_) {
+          // If the first paint has been observed the entire to-paint tracking
+          // mechanism is no longer needed.
+          registrar_.Remove(
+              this,
+              content::NOTIFICATION_RENDER_WIDGET_HOST_DID_UPDATE_BACKING_STORE,
+              content::NotificationService::AllSources());
+
+          // Remove any tabs that have loaded. These were only being kept around
+          // while waiting for a paint event.
+          std::vector<NavigationController*> loaded_tabs;
+          for (auto& map_entry : tabs_tracked_) {
+            TabState& tab_state = map_entry.second;
+            if (tab_state.loading_state == TAB_IS_LOADED)
+              loaded_tabs.push_back(tab_state.controller);
+          }
+          for (auto& tab : loaded_tabs)
+            RemoveTab(tab);
+        }
       }
       break;
     }
@@ -169,72 +244,59 @@ void SessionRestoreStatsCollector::Observe(
       break;
   }
 
-  // Check if we are done and if so, reset |this_retainer_| as the collector no
-  // longer needs to stay alive.
-  if ((got_first_paint_ || render_widget_hosts_to_paint_.empty()) &&
-      tabs_tracked_.empty() && render_widget_hosts_loading_.empty())
-    this_retainer_ = nullptr;
-}
+  // If non-deferred tabs are no longer being tracked then detach this collector
+  // from being the shared collector.
+  if (!got_done_tracking_non_deferred_tabs_ && DoneTrackingNonDeferredTabs()) {
+    got_done_tracking_non_deferred_tabs_ = true;
 
-void SessionRestoreStatsCollector::AddTabs(
-    const std::vector<SessionRestoreDelegate::RestoredTab>& tabs) {
-  tab_count_ += tabs.size();
-  for (auto& tab : tabs) {
-    RegisterForNotifications(&tab.contents()->GetController());
-    if (tab.is_active()) {
-      RenderWidgetHost* render_widget_host =
-          GetRenderWidgetHost(&tab.contents()->GetController());
-      render_widget_hosts_loading_.insert(render_widget_host);
-    }
+    // Mark the point at which all tabs have loaded and log the UMA metrics.

[sky, 2015/05/27 15:18:39]

Again, not a useful comment.

[chrisha, 2015/06/03 21:16:36]

Acknowledged.

+    tab_loader_stats_.all_tabs_loaded =
+        base::TimeTicks::Now() - restore_started_;
+    ReportTabLoaderStats();
   }
+
+  // If tracking is completely finished then emit collected metrics and destroy
+  // this stats collector.
+  if (got_done_tracking_non_deferred_tabs_ && DoneTracking())
+    this_retainer_ = nullptr;
 }
 
 void SessionRestoreStatsCollector::RemoveTab(NavigationController* tab) {
+  // Stop observing this tab.
   registrar_.Remove(this, content::NOTIFICATION_WEB_CONTENTS_DESTROYED,
-                    content::Source<WebContents>(tab->GetWebContents()));
+                    Source<WebContents>(tab->GetWebContents()));
   registrar_.Remove(this, content::NOTIFICATION_LOAD_STOP,
-                    content::Source<NavigationController>(tab));
+                    Source<NavigationController>(tab));
   registrar_.Remove(this, content::NOTIFICATION_LOAD_START,
-                    content::Source<NavigationController>(tab));
-  if (render_widget_hosts_loading_.size() > max_parallel_tab_loads_)
-    max_parallel_tab_loads_ = render_widget_hosts_loading_.size();
-  RenderWidgetHost* render_widget_host = GetRenderWidgetHost(tab);
-  render_widget_hosts_loading_.erase(render_widget_host);
-  tabs_tracked_.erase(tab);
-
-  // If there are no more tabs loading or being tracked, restore is done and
-  // record the time. Note that we are not yet finished, as we might still be
-  // waiting for our first paint, which can happen after all tabs are done
-  // loading.
-  // TODO(georgesak): review behaviour of ForegroundTabFirstPaint.
-  if (tabs_tracked_.empty() && render_widget_hosts_loading_.empty()) {
-    base::TimeDelta time_to_load = base::TimeTicks::Now() - restore_started_;
-    UMA_HISTOGRAM_CUSTOM_TIMES("SessionRestore.AllTabsLoaded", time_to_load,
-                               base::TimeDelta::FromMilliseconds(10),
-                               base::TimeDelta::FromSeconds(100), 100);
-    // Record a time for the number of tabs, to help track down contention.
-    std::string time_for_count =
-        base::StringPrintf("SessionRestore.AllTabsLoaded_%d", tab_count_);
-    base::HistogramBase* counter_for_count = base::Histogram::FactoryTimeGet(
-        time_for_count, base::TimeDelta::FromMilliseconds(10),
-        base::TimeDelta::FromSeconds(100), 100,
-        base::Histogram::kUmaTargetedHistogramFlag);
-    counter_for_count->AddTime(time_to_load);
+                    Source<NavigationController>(tab));
 
-    UMA_HISTOGRAM_COUNTS_100("SessionRestore.ParallelTabLoads",
-                             max_parallel_tab_loads_);
-  }
+  // Get the tab state.

[sky, 2015/05/27 15:18:39]

Again, this comment isn't useful. It just documents what the code is doing, and
the code isn't doing anything tricky here.

[chrisha, 2015/06/03 21:16:36]

Acknowledged.

+  auto tab_it = tabs_tracked_.find(tab);
+  DCHECK(tab_it != tabs_tracked_.end());
+  TabState& tab_state = tab_it->second;
+
+  // Remove the tab from the secondary index if it's there.
+  auto rwh_it = render_widget_map_.find(tab_state.render_widget_host);
+  if (rwh_it != render_widget_map_.end())
+    render_widget_map_.erase(rwh_it);
+
+  // Remove the tab from the |tracked_tabs_| map.
+  tabs_tracked_.erase(tab_it);
 }
 
-void SessionRestoreStatsCollector::RegisterForNotifications(
+SessionRestoreStatsCollector::TabState*
+SessionRestoreStatsCollector::RegisterForNotifications(
     NavigationController* tab) {
   registrar_.Add(this, content::NOTIFICATION_WEB_CONTENTS_DESTROYED,
-                 content::Source<WebContents>(tab->GetWebContents()));
+                 Source<WebContents>(tab->GetWebContents()));
   registrar_.Add(this, content::NOTIFICATION_LOAD_STOP,
-                 content::Source<NavigationController>(tab));
+                 Source<NavigationController>(tab));
   registrar_.Add(this, content::NOTIFICATION_LOAD_START,
-                 content::Source<NavigationController>(tab));
-  tabs_tracked_.insert(tab);
+                 Source<NavigationController>(tab));
+  auto result = tabs_tracked_.insert(std::make_pair(tab, TabState(tab)));
+  DCHECK(result.second);
+  TabState* tab_state = &result.first->second;
+  return tab_state;
 }
 
 RenderWidgetHost* SessionRestoreStatsCollector::GetRenderWidgetHost(
@@ -249,6 +311,154 @@ RenderWidgetHost* SessionRestoreStatsCollector::GetRenderWidgetHost(
   return nullptr;
 }
 
-// static
-SessionRestoreStatsCollector* SessionRestoreStatsCollector::shared_collector_ =
-    nullptr;
+SessionRestoreStatsCollector::TabState*
+SessionRestoreStatsCollector::GetTabState(NavigationController* tab) {
+  // This lookup can fail because DeferTab calls can arrive for tabs that have
+  // already loaded (user forced) and are no longer tracked.
+  auto it = tabs_tracked_.find(tab);
+  if (it == tabs_tracked_.end())
+    return nullptr;
+  return &it->second;
+}
+
+SessionRestoreStatsCollector::TabState*
+SessionRestoreStatsCollector::GetTabState(RenderWidgetHost* tab) {
+  // This lookup can fail as events are received for tabs that aren't

[sky, 2015/05/27 15:18:39]

You can figure out the NavigationController the RWH is associated with by
iterating over all the NavigationControllers. As this is only called once, and
there aren't a ton of tabs I would rather see you do that then maintain a second
map that we have to worry about getting out of sync.

[chrisha, 2015/06/03 21:16:36]

Done.

+  // explicitly being tracked.
+  auto it = render_widget_map_.find(tab);
+  if (it == render_widget_map_.end())
+    return nullptr;
+  return it->second;
+}
+
+void SessionRestoreStatsCollector::MarkTabAsLoading(TabState* tab_state) {
+  CHECK(tab_state->loading_state == TAB_IS_NOT_LOADING);

[sky, 2015/05/27 15:18:39]

Why the CHECK?

[chrisha, 2015/06/03 21:16:36]

Made a DCHECK. This really shouldn't be called twice for a tab, but it's
harmless if it is.

+  if (tab_state->loading_state != TAB_IS_NOT_LOADING)
+    return;
+  tab_state->loading_state = TAB_IS_LOADING;
+  ++loading_tab_count_;
+
+  DCHECK(!got_done_tracking_non_deferred_tabs_);
+  tab_loader_stats_.parallel_tab_loads = std::max(
+      tab_loader_stats_.parallel_tab_loads,
+      loading_tab_count_);
+
+  // Get the RenderWidgetHost for the tab and add it to the secondary index.
+  RenderWidgetHost* render_widget_host = GetRenderWidgetHost(
+      tab_state->controller);
+  DCHECK(render_widget_host);
+  tab_state->render_widget_host = render_widget_host;
+  auto result = render_widget_map_.insert(std::make_pair(
+      render_widget_host, tab_state));
+  DCHECK(result.second);
+}
+
+bool SessionRestoreStatsCollector::DoneTrackingNonDeferredTabs() const {
+  // This should only be called when it hasn't yet returned true.
+  DCHECK(!got_done_tracking_non_deferred_tabs_);
+  return got_first_paint_ && waiting_for_load_tab_count_ == 0;
+}
+
+bool SessionRestoreStatsCollector::DoneTracking() const {
+  // This should only be called once DoneTrackingNonDeferredTabs has returned
+  // true.
+  DCHECK(got_done_tracking_non_deferred_tabs_);
+  return tabs_tracked_.empty() && render_widget_map_.empty();
+}
+
+void SessionRestoreStatsCollector::ReportTabLoaderStats() {
+  UMA_HISTOGRAM_COUNTS_100("SessionRestore.TabCount",
+                           tab_loader_stats_.tab_count);
+
+  UMA_HISTOGRAM_ENUMERATION(kSessionRestoreActions,
+                            SESSION_RESTORE_ACTIONS_UMA_INITIATED,
+                            SESSION_RESTORE_ACTIONS_UMA_MAX);
+
+  for (size_t i = 0; i < tab_loader_stats_.tab_count; ++i) {
+    UMA_HISTOGRAM_ENUMERATION("SessionRestore.TabActions",
+                              SESSION_RESTORE_TAB_ACTIONS_UMA_TAB_CREATED,
+                              SESSION_RESTORE_TAB_ACTIONS_UMA_MAX);
+  }
+
+  for (size_t i = 0; i < tab_loader_stats_.tabs_loaded; ++i) {
+    UMA_HISTOGRAM_ENUMERATION("SessionRestore.TabActions",
+                              SESSION_RESTORE_TAB_ACTIONS_UMA_TAB_LOADED,
+                              SESSION_RESTORE_TAB_ACTIONS_UMA_MAX);
+  }
+
+  if (!tab_loader_stats_.foreground_tab_first_loaded.is_zero()) {
+    UMA_HISTOGRAM_CUSTOM_TIMES("SessionRestore.ForegroundTabFirstLoaded",
+                               tab_loader_stats_.foreground_tab_first_loaded,
+                               base::TimeDelta::FromMilliseconds(10),
+                               base::TimeDelta::FromSeconds(100), 100);
+
+    // Record a time for the number of tabs, to help track down contention.
+    std::string time_for_count = base::StringPrintf(
+        "SessionRestore.ForegroundTabFirstLoaded_%d",
+        tab_loader_stats_.tab_count);
+    base::HistogramBase* counter_for_count = base::Histogram::FactoryTimeGet(
+        time_for_count, base::TimeDelta::FromMilliseconds(10),
+        base::TimeDelta::FromSeconds(100), 100,
+        base::Histogram::kUmaTargetedHistogramFlag);
+    counter_for_count->AddTime(tab_loader_stats_.foreground_tab_first_loaded);
+  }
+
+  if (!tab_loader_stats_.foreground_tab_first_paint.is_zero()) {
+    UMA_HISTOGRAM_CUSTOM_TIMES(
+        "SessionRestore.ForegroundTabFirstPaint3",
+        tab_loader_stats_.foreground_tab_first_paint,
+        base::TimeDelta::FromMilliseconds(100),
+        base::TimeDelta::FromMinutes(16), 50);
+
+    std::string time_for_count = base::StringPrintf(
+        "SessionRestore.ForegroundTabFirstPaint3_%d",
+        tab_loader_stats_.tab_count);
+    base::HistogramBase* counter_for_count = base::Histogram::FactoryTimeGet(
+        time_for_count, base::TimeDelta::FromMilliseconds(100),
+        base::TimeDelta::FromMinutes(16), 50,
+        base::Histogram::kUmaTargetedHistogramFlag);
+    counter_for_count->AddTime(tab_loader_stats_.foreground_tab_first_paint);
+  }
+
+  if (!tab_loader_stats_.all_tabs_loaded.is_zero()) {
+    UMA_HISTOGRAM_CUSTOM_TIMES("SessionRestore.AllTabsLoaded",
+                               tab_loader_stats_.all_tabs_loaded,
+                               base::TimeDelta::FromMilliseconds(10),
+                               base::TimeDelta::FromSeconds(100), 100);
+
+    // Record a time for the number of tabs, to help track down contention.
+    std::string time_for_count =
+        base::StringPrintf("SessionRestore.AllTabsLoaded_%d",
+        tab_loader_stats_.tab_count);
+    base::HistogramBase* counter_for_count = base::Histogram::FactoryTimeGet(
+        time_for_count, base::TimeDelta::FromMilliseconds(10),
+        base::TimeDelta::FromSeconds(100), 100,
+        base::Histogram::kUmaTargetedHistogramFlag);
+    counter_for_count->AddTime(tab_loader_stats_.all_tabs_loaded);
+  }
+
+  UMA_HISTOGRAM_COUNTS_100("SessionRestore.ParallelTabLoads",
+                           tab_loader_stats_.parallel_tab_loads);
+}
+
+void SessionRestoreStatsCollector::ReportTabDeferred() {
+  if (!got_report_tab_deferred_) {
+    got_report_tab_deferred_ = true;
+
+    UMA_HISTOGRAM_ENUMERATION(kSessionRestoreActions,
+                              SESSION_RESTORE_ACTIONS_UMA_DEFERRED_TABS,
+                              SESSION_RESTORE_ACTIONS_UMA_MAX);
+  }
+
+  UMA_HISTOGRAM_ENUMERATION(
+      "SessionRestore.TabActions",
+      SESSION_RESTORE_TAB_ACTIONS_UMA_TAB_LOADING_DEFERRED,
+      SESSION_RESTORE_TAB_ACTIONS_UMA_MAX);
+}
+
+void SessionRestoreStatsCollector::ReportDeferredTabLoaded() {
+  UMA_HISTOGRAM_ENUMERATION(
+      "SessionRestore.TabActions",
+      SESSION_RESTORE_TAB_ACTIONS_UMA_DEFERRED_TAB_LOADED,
+      SESSION_RESTORE_TAB_ACTIONS_UMA_MAX);
+}