Add a feature flag for reducing the number of concurrent marking tasks

The V8ReduceConcurrentMarkingTasks flag will be used in Finch experiment.

Bug: v8:10442
Change-Id: I2cb6d0a8445f4eeb9fcbad9ecb8595542d50fc8b
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2283219
Reviewed-by: Ross McIlroy <rmcilroy@chromium.org>
Commit-Queue: Ulan Degenbaev <ulan@chromium.org>
Cr-Commit-Position: refs/heads/master@{#786242}

gin/gin_features.cc

@@ -22,15 +22,20 @@ const base::Feature kV8OffThreadFinalization{"V8OffThreadFinalization",
 const base::Feature kV8LazyFeedbackAllocation{"V8LazyFeedbackAllocation",
                                               base::FEATURE_ENABLED_BY_DEFAULT};
 
-// Enable concurrent inlining in TurboFan.
+// Enables concurrent inlining in TurboFan.
 const base::Feature kV8ConcurrentInlining{"V8ConcurrentInlining",
                                           base::FEATURE_DISABLED_BY_DEFAULT};
 
-// Enable per-context marking worklists in V8 GC.
+// Enables per-context marking worklists in V8 GC.
 const base::Feature kV8PerContextMarkingWorklist{
     "V8PerContextMarkingWorklist", base::FEATURE_DISABLED_BY_DEFAULT};
 
-// Enable flushing of the instruction cache for the embedded blob.
+// Enables flushing of the instruction cache for the embedded blob.
 const base::Feature kV8FlushEmbeddedBlobICache{
     "kV8FlushEmbeddedBlobICache", base::FEATURE_DISABLED_BY_DEFAULT};
+
+// Enables reduced number of concurrent marking tasks.
+const base::Feature kV8ReduceConcurrentMarkingTasks{
+    "kV8ReduceConcurrentMarkingTasks", base::FEATURE_DISABLED_BY_DEFAULT};
+
 }  // namespace features

gin/gin_features.h

@@ -17,6 +17,7 @@ GIN_EXPORT extern const base::Feature kV8LazyFeedbackAllocation;
 GIN_EXPORT extern const base::Feature kV8ConcurrentInlining;
 GIN_EXPORT extern const base::Feature kV8PerContextMarkingWorklist;
 GIN_EXPORT extern const base::Feature kV8FlushEmbeddedBlobICache;
+GIN_EXPORT extern const base::Feature kV8ReduceConcurrentMarkingTasks;
 
 }  // namespace features
 

gin/v8_initializer.cc

@@ -248,6 +248,14 @@ void V8Initializer::Initialize(IsolateHolder::ScriptMode mode) {
         sizeof(experimental_flush_embedded_blob_icache) - 1);
   }
 
+  if (base::FeatureList::IsEnabled(features::kV8ReduceConcurrentMarkingTasks)) {
+    static const char gc_experiment_reduce_concurrent_marking_tasks[] =
+        "--gc-experiment-reduce-concurrent-marking-tasks";
+    v8::V8::SetFlagsFromString(
+        gc_experiment_reduce_concurrent_marking_tasks,
+        sizeof(gc_experiment_reduce_concurrent_marking_tasks) - 1);
+  }
+
   if (IsolateHolder::kStrictMode == mode) {
     static const char use_strict[] = "--use_strict";
     v8::V8::SetFlagsFromString(use_strict, sizeof(use_strict) - 1);