[PM] Add ExecutionContextPriorityDecorator

This is a new class that takes care of setting up the voting tree that
decides which priority an execution context will have depending on the
votes of many voters.

Bug: 971272
Change-Id: Id4536e2180d7147357f80b706a8821eb44465145
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2538310Reviewed-by: Chris Hamilton <chrisha@chromium.org>
Commit-Queue: Patrick Monette <pmonette@chromium.org>
Cr-Commit-Position: refs/heads/master@{#833184}

components/performance_manager/BUILD.gn

@@ -33,6 +33,8 @@ static_library("performance_manager") {
     "execution_context_priority/boosting_vote_aggregator.cc",
     "execution_context_priority/boosting_vote_aggregator.h",
     "execution_context_priority/execution_context_priority.cc",
+    "execution_context_priority/execution_context_priority_decorator.cc",
+    "execution_context_priority/execution_context_priority_decorator.h",
     "execution_context_priority/frame_visibility_voter.cc",
     "execution_context_priority/frame_visibility_voter.h",
     "execution_context_priority/max_vote_aggregator.cc",

components/performance_manager/embedder/graph_features_helper.h

@@ -24,6 +24,7 @@ class GraphFeaturesHelper {
       // (1) Add a corresponding EnableFeatureFoo() member function.
       // (2) Add the feature to EnableDefault() if necessary.
       // (3) Add the feature to the implementation of ConfigureGraph().
+      bool execution_context_priority_decorator : 1;
       bool execution_context_registry : 1;
       bool frame_node_impl_describer : 1;
       bool frame_visibility_decorator : 1;
@@ -42,6 +43,12 @@ class GraphFeaturesHelper {
   constexpr GraphFeaturesHelper(const GraphFeaturesHelper& other) = default;
   GraphFeaturesHelper& operator=(const GraphFeaturesHelper& other) = default;
 
+  constexpr GraphFeaturesHelper& EnableExecutionContextPriorityDecorator() {
+    EnableExecutionContextRegistry();
+    flags_.execution_context_priority_decorator = true;
+    return *this;
+  }
+
   constexpr GraphFeaturesHelper& EnableExecutionContextRegistry() {
     flags_.execution_context_registry = true;
     return *this;

components/performance_manager/execution_context_priority/execution_context_priority_decorator.cc

+// Copyright 2020 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 "components/performance_manager/execution_context_priority/execution_context_priority_decorator.h"
+
+namespace performance_manager {
+namespace execution_context_priority {
+
+ExecutionContextPriorityDecorator::ExecutionContextPriorityDecorator() {
+  // The following schema describes the structure of the voting tree. Arrows are
+  // voting channels.
+  //
+  // Note: |ad_frame_voter_| is currently the only downvoter but there could
+  // possibly be more should the need arise. Their votes would be aggregated
+  // using some sort of MinVoteAggregator.
+  //
+  //            |root_vote_observer_|
+  //                     ^
+  //                     |
+  //         |override_vote_aggregator_|
+  //            ^                  ^
+  //            | (override)       | (default)
+  //            |                  |
+  //        Downvoter         |max_vote_aggregator_|
+  //                              ^    ^        ^
+  //                             /     |         \
+  //                            /      |          \
+  //                         Voter1, Voter2, ..., VoterN
+  //
+  // Set up the voting tree from top to bottom.
+  override_vote_aggregator_.SetUpstreamVotingChannel(
+      root_vote_observer_.GetVotingChannel());
+  max_vote_aggregator_.SetUpstreamVotingChannel(
+      override_vote_aggregator_.GetDefaultVotingChannel());
+
+  // Set up downvoter.
+  ad_frame_voter_.SetVotingChannel(
+      override_vote_aggregator_.GetOverrideVotingChannel());
+
+  // Set up voters.
+  frame_visibility_voter_.SetVotingChannel(
+      max_vote_aggregator_.GetVotingChannel());
+}
+ExecutionContextPriorityDecorator::~ExecutionContextPriorityDecorator() =
+    default;
+
+void ExecutionContextPriorityDecorator::OnPassedToGraph(Graph* graph) {
+  // Subscribe voters to the graph.
+  graph->AddFrameNodeObserver(&ad_frame_voter_);
+  graph->AddFrameNodeObserver(&frame_visibility_voter_);
+}
+
+void ExecutionContextPriorityDecorator::OnTakenFromGraph(Graph* graph) {
+  // Unsubscribe voters from the graph.
+  graph->RemoveFrameNodeObserver(&frame_visibility_voter_);
+  graph->RemoveFrameNodeObserver(&ad_frame_voter_);
+}
+
+}  // namespace execution_context_priority
+}  // namespace performance_manager

components/performance_manager/execution_context_priority/execution_context_priority_decorator.h

+// Copyright 2020 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.
+
+#ifndef COMPONENTS_PERFORMANCE_MANAGER_EXECUTION_CONTEXT_PRIORITY_EXECUTION_CONTEXT_PRIORITY_DECORATOR_H_
+#define COMPONENTS_PERFORMANCE_MANAGER_EXECUTION_CONTEXT_PRIORITY_EXECUTION_CONTEXT_PRIORITY_DECORATOR_H_
+
+#include "components/performance_manager/execution_context_priority/ad_frame_voter.h"
+#include "components/performance_manager/execution_context_priority/frame_visibility_voter.h"
+#include "components/performance_manager/execution_context_priority/max_vote_aggregator.h"
+#include "components/performance_manager/execution_context_priority/override_vote_aggregator.h"
+#include "components/performance_manager/execution_context_priority/root_vote_observer.h"
+#include "components/performance_manager/public/graph/graph.h"
+
+namespace performance_manager {
+namespace execution_context_priority {
+
+// The ExecutionContextPriorityDecorator's responsibility is to own the voting
+// system that assigns the priority of every frame and worker in the graph.
+//
+// See the README.md for more details on the voting system.
+class ExecutionContextPriorityDecorator final : public GraphOwned {
+ public:
+  ExecutionContextPriorityDecorator();
+  ~ExecutionContextPriorityDecorator() override;
+
+  ExecutionContextPriorityDecorator(const ExecutionContextPriorityDecorator&) =
+      delete;
+  ExecutionContextPriorityDecorator& operator=(
+      const ExecutionContextPriorityDecorator&) = delete;
+
+ private:
+  void OnPassedToGraph(Graph* graph) override;
+  void OnTakenFromGraph(Graph* graph) override;
+
+  // Takes in the aggregated votes and applies them to the execution contexts in
+  // the graph.
+  RootVoteObserver root_vote_observer_;
+
+  // Used to cast a negative vote that overrides the vote from
+  // |max_vote_aggregator_|.
+  OverrideVoteAggregator override_vote_aggregator_;
+
+  // Aggregates all the votes from the voters.
+  MaxVoteAggregator max_vote_aggregator_;
+
+  // Note: Voters should be added below this line so that they are destroyed
+  //       before the aggregators.
+
+  // Casts a downvote for ad frames.
+  AdFrameVoter ad_frame_voter_;
+
+  // Casts a USER_VISIBLE vote when a frame is visible.
+  FrameVisibilityVoter frame_visibility_voter_;
+};
+
+}  // namespace execution_context_priority
+}  // namespace performance_manager
+
+#endif  // COMPONENTS_PERFORMANCE_MANAGER_EXECUTION_CONTEXT_PRIORITY_EXECUTION_CONTEXT_PRIORITY_DECORATOR_H_

components/performance_manager/graph_features_helper.cc

@@ -10,6 +10,7 @@
 #include "components/performance_manager/decorators/frame_visibility_decorator.h"
 #include "components/performance_manager/decorators/page_load_tracker_decorator.h"
 #include "components/performance_manager/execution_context/execution_context_registry_impl.h"
+#include "components/performance_manager/execution_context_priority/execution_context_priority_decorator.h"
 #include "components/performance_manager/graph/frame_node_impl_describer.h"
 #include "components/performance_manager/graph/page_node_impl_describer.h"
 #include "components/performance_manager/graph/process_node_impl_describer.h"
@@ -59,10 +60,14 @@ void GraphFeaturesHelper::ConfigureGraph(Graph* graph) const {
     Install<SiteDataRecorder>(graph);
 #endif
 
-  // This class has a dependency on ExecutionContextRegistry, so must be
+  // These classes have a dependency on ExecutionContextRegistry, so must be
   // installed after it.
+  if (flags_.execution_context_priority_decorator) {
+    Install<execution_context_priority::ExecutionContextPriorityDecorator>(
+        graph);
+  }
   if (flags_.v8_context_tracker)
     Install<v8_memory::V8ContextTracker>(graph);
 }
 
-}  // namespace performance_manager
\ No newline at end of file
+}  // namespace performance_manager