[PM] Utilize MaxSimultaneousLoad threshold

Add usage of MaxSimultaneousLoad threshold in
BackgroundTabLoadingPolicy's loading logic.

Original CL: https://chromium-review.googlesource.com/c/chromium/src/+/2122479
Reland due to test failing because of dependencies on system core
number. Removed these dependencies in unit tests.

Bug: 1059341
Change-Id: I20fd569aa13c2912ba8f6366bf5d3916f72e8f58
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2134190Reviewed-by: Sébastien Marchand <sebmarchand@chromium.org>
Reviewed-by: François Doray <fdoray@chromium.org>
Commit-Queue: Hailey Wang <haileywang@google.com>
Cr-Commit-Position: refs/heads/master@{#756267}

chrome/browser/performance_manager/graph/policies/background_tab_loading_policy.cc

@@ -64,7 +64,7 @@ BackgroundTabLoadingPolicy::BackgroundTabLoadingPolicy()
     : page_loader_(std::make_unique<mechanism::PageLoader>()) {
   DCHECK(!g_background_tab_loading_policy);
   g_background_tab_loading_policy = this;
-  simultaneous_tab_loads_ = CalculateMaxSimultaneousTabLoads(
+  max_simultaneous_tab_loads_ = CalculateMaxSimultaneousTabLoads(
       kMinSimultaneousTabLoads, kMaxSimultaneousTabLoads,
       kCoresPerSimultaneousTabLoad, base::SysInfo::NumberOfProcessors());
 }
@@ -86,6 +86,9 @@ void BackgroundTabLoadingPolicy::OnIsLoadingChanged(const PageNode* page_node) {
   if (!page_node->IsLoading()) {
     // Once the PageNode finishes loading, stop tracking it within this policy.
     RemovePageNode(page_node);
+
+    // Since there is a free loading slot, load more tab if needed.
+    MaybeLoadSomeTabs();
     return;
   }
   // The PageNode started loading, either because of this policy or because of
@@ -105,6 +108,10 @@ void BackgroundTabLoadingPolicy::OnIsLoadingChanged(const PageNode* page_node) {
 void BackgroundTabLoadingPolicy::OnBeforePageNodeRemoved(
     const PageNode* page_node) {
   RemovePageNode(page_node);
+
+  // There may be free loading slots, check and load more tabs if that's the
+  // case.
+  MaybeLoadSomeTabs();
 }
 
 void BackgroundTabLoadingPolicy::ScheduleLoadForRestoredTabs(
@@ -115,8 +122,8 @@ void BackgroundTabLoadingPolicy::ScheduleLoadForRestoredTabs(
     page_nodes_to_load_.push_back(page_node);
     DCHECK(
         TabPropertiesDecorator::Data::FromPageNode(page_node)->IsInTabStrip());
-    InitiateLoad(page_node);
   }
+  MaybeLoadSomeTabs();
 }
 
 void BackgroundTabLoadingPolicy::SetMockLoaderForTesting(
@@ -124,11 +131,16 @@ void BackgroundTabLoadingPolicy::SetMockLoaderForTesting(
   page_loader_ = std::move(loader);
 }
 
+void BackgroundTabLoadingPolicy::SetMaxSimultaneousLoadsForTesting(
+    size_t loading_slots) {
+  max_simultaneous_tab_loads_ = loading_slots;
+}
+
 BackgroundTabLoadingPolicy* BackgroundTabLoadingPolicy::GetInstance() {
   return g_background_tab_loading_policy;
 }
 
-void BackgroundTabLoadingPolicy::InitiateLoad(PageNode* page_node) {
+void BackgroundTabLoadingPolicy::InitiateLoad(const PageNode* page_node) {
   // Mark |page_node| as load initiated. Ensure that InitiateLoad is only called
   // for a PageNode that is tracked by the policy.
   size_t num_removed = base::Erase(page_nodes_to_load_, page_node);
@@ -145,6 +157,44 @@ void BackgroundTabLoadingPolicy::RemovePageNode(const PageNode* page_node) {
   base::Erase(page_nodes_loading_, page_node);
 }
 
+void BackgroundTabLoadingPolicy::MaybeLoadSomeTabs() {
+  // Continue to load tabs while possible. This is in a loop with a
+  // recalculation of GetMaxNewTabLoads() as reentrancy can cause conditions
+  // to change as each tab load is initiated.
+  while (GetMaxNewTabLoads() > 0)
+    LoadNextTab();
+}
+
+size_t BackgroundTabLoadingPolicy::GetMaxNewTabLoads() const {
+  // This takes into account all tabs currently loading across the browser,
+  // including ones that BackgroundTabLoadingPolicy isn't explicitly managing.
+  // This ensures that BackgroundTabLoadingPolicy respects user interaction
+  // first and foremost. There's a small race between when we initiated loading
+  // and when PageNodeObserver notifies us that it has actually started, so we
+  // also make use of |page_nodes_initiated_| to track these.
+  size_t loading_tab_count =
+      page_nodes_load_initiated_.size() + page_nodes_loading_.size();
+
+  // Determine the number of free loading slots available.
+  size_t page_nodes_to_load = 0;
+  if (loading_tab_count < max_simultaneous_tab_loads_)
+    page_nodes_to_load = max_simultaneous_tab_loads_ - loading_tab_count;
+
+  // Cap the number of loads by the actual number of tabs remaining.
+  page_nodes_to_load = std::min(page_nodes_to_load, page_nodes_to_load_.size());
+
+  return page_nodes_to_load;
+}
+
+void BackgroundTabLoadingPolicy::LoadNextTab() {
+  DCHECK(!page_nodes_to_load_.empty());
+
+  // Find the next PageNode to load.
+  const PageNode* page_node = page_nodes_to_load_.front();
+
+  InitiateLoad(page_node);
+}
+
 }  // namespace policies
 
 }  // namespace performance_manager

chrome/browser/performance_manager/graph/policies/background_tab_loading_policy.h

@@ -42,6 +42,7 @@ class BackgroundTabLoadingPolicy : public GraphOwned,
   void ScheduleLoadForRestoredTabs(std::vector<PageNode*> page_nodes);
 
   void SetMockLoaderForTesting(std::unique_ptr<mechanism::PageLoader> loader);
+  void SetMaxSimultaneousLoadsForTesting(size_t loading_slots);
 
   // Returns the instance of BackgroundTabLoadingPolicy within the graph.
   static BackgroundTabLoadingPolicy* GetInstance();
@@ -49,12 +50,25 @@ class BackgroundTabLoadingPolicy : public GraphOwned,
  private:
   // Move the PageNode from |page_nodes_to_load_| to
   // |page_nodes_load_initiated_| and make the call to load the PageNode.
-  void InitiateLoad(PageNode* page_node);
+  void InitiateLoad(const PageNode* page_node);
 
   // Removes the PageNode from all the sets of PageNodes that the policy is
   // tracking.
   void RemovePageNode(const PageNode* page_node);
 
+  // Initiates the load of enough tabs to fill all loading slots. No-ops if all
+  // loading slots are occupied.
+  void MaybeLoadSomeTabs();
+
+  // Determines the number of tab loads that can be started at the moment to
+  // avoid exceeding the number of loading slots.
+  size_t GetMaxNewTabLoads() const;
+
+  // Loads the next tab. This should only be called if there is a next tab to
+  // load. This will always start loading a next tab even if the number of
+  // simultaneously loading tabs is exceeded.
+  void LoadNextTab();
+
   // The mechanism used to load the pages.
   std::unique_ptr<performance_manager::mechanism::PageLoader> page_loader_;
 
@@ -72,7 +86,7 @@ class BackgroundTabLoadingPolicy : public GraphOwned,
 
   // The number of simultaneous tab loads that are permitted by policy. This
   // is computed based on the number of cores on the machine.
-  size_t simultaneous_tab_loads_;
+  size_t max_simultaneous_tab_loads_;
 };
 
 }  // namespace policies

chrome/browser/performance_manager/graph/policies/background_tab_loading_policy_unittest.cc

@@ -49,6 +49,10 @@ class BackgroundTabLoadingPolicyTest : public GraphTestHarness {
     auto mock_loader = std::make_unique<MockPageLoader>();
     mock_loader_ = mock_loader.get();
     policy_->SetMockLoaderForTesting(std::move(mock_loader));
+
+    // Set a value explicitly for MaxSimultaneousLoad threshold to avoid a
+    // dependency on the number of cores of the machine on which the test runs.
+    policy_->SetMaxSimultaneousLoadsForTesting(4);
   }
 
   void TearDown() override { graph()->TakeFromGraph(policy_); }
@@ -82,6 +86,45 @@ TEST_F(BackgroundTabLoadingPolicyTest, ScheduleLoadForRestoredTabs) {
   policy()->ScheduleLoadForRestoredTabs(raw_page_nodes);
 }
 
+TEST_F(BackgroundTabLoadingPolicyTest, AllLoadingSlotsUsed) {
+  // Create 4 PageNode to restore.
+  std::vector<
+      performance_manager::TestNodeWrapper<performance_manager::PageNodeImpl>>
+      page_nodes;
+  std::vector<PageNode*> raw_page_nodes;
+
+  // Create vector of PageNode to restore.
+  for (int i = 0; i < 4; i++) {
+    page_nodes.push_back(CreateNode<performance_manager::PageNodeImpl>());
+    raw_page_nodes.push_back(page_nodes.back().get());
+
+    // Set |is_tab| property as this is a requirement to pass the PageNode to
+    // ScheduleLoadForRestoredTabs().
+    TabPropertiesDecorator::SetIsTabForTesting(raw_page_nodes.back(), true);
+  }
+  PageNodeImpl* page_node_impl = page_nodes[0].get();
+
+  EXPECT_CALL(*loader(), LoadPageNode(raw_page_nodes[0]));
+  EXPECT_CALL(*loader(), LoadPageNode(raw_page_nodes[1]));
+
+  // Use 2 loading slots, which means only 2 of the PageNodes should immediately
+  // be scheduled to load.
+  policy()->SetMaxSimultaneousLoadsForTesting(2);
+
+  policy()->ScheduleLoadForRestoredTabs(raw_page_nodes);
+  testing::Mock::VerifyAndClear(loader());
+
+  // Simulate load start of a PageNode that initiated load.
+  page_node_impl->SetIsLoading(true);
+
+  // The policy should allow one more PageNode to load after a PageNode finishes
+  // loading.
+  EXPECT_CALL(*loader(), LoadPageNode(raw_page_nodes[2]));
+
+  // Simulate load finish of a PageNode.
+  page_node_impl->SetIsLoading(false);
+}
+
 }  // namespace policies
 
 }  // namespace performance_manager