[oilpan] Introduce incremental marking stress flag

This frees up the regular HeapIncrementalMarking flag for implementing an
initial incremental marking strategy.

Bug: chromium:757440
Change-Id: Iee924ef718f450b4d1e865ef6880242f501376fb
Reviewed-on: https://chromium-review.googlesource.com/1028273Reviewed-by: John Budorick <jbudorick@chromium.org>
Reviewed-by: Kentaro Hara <haraken@chromium.org>
Reviewed-by: Keishi Hattori <keishi@chromium.org>
Commit-Queue: Michael Lippautz <mlippautz@chromium.org>
Cr-Commit-Position: refs/heads/master@{#553935}

testing/buildbot/chromium.fyi.json

@@ -8322,7 +8322,7 @@
       {
         "args": [
           "--debug",
-          "--additional-driver-flag=--enable-blink-features=HeapIncrementalMarking"
+          "--additional-driver-flag=--enable-blink-features=HeapIncrementalMarkingStress"
         ],
         "isolate_name": "webkit_layout_tests_exparchive",
         "merge": {

testing/buildbot/test_suite_exceptions.pyl

@@ -3339,7 +3339,7 @@
       'linux-blink-heap-incremental-marking': {
         'args': [
           '--debug',
-          '--additional-driver-flag=--enable-blink-features=HeapIncrementalMarking',
+          '--additional-driver-flag=--enable-blink-features=HeapIncrementalMarkingStress',
         ],
       },
       'WebKit Linux layout_ng Dummy Builder': {

third_party/blink/renderer/platform/heap/thread_state.cc

@@ -426,10 +426,10 @@ bool ThreadState::JudgeGCThreshold(size_t allocated_object_size_threshold,
 
 bool ThreadState::ShouldScheduleIncrementalMarking() const {
 #if BUILDFLAG(BLINK_HEAP_INCREMENTAL_MARKING)
-  // TODO(mlippautz): For now immediately schedule incremental marking if
-  // the runtime flag is provided, basically exercising a stress test.
+  // TODO(mlippautz): For now only schedule incremental marking if
+  // the runtime stress flag is provided.
   return GcState() == kNoGCScheduled &&
-         RuntimeEnabledFeatures::HeapIncrementalMarkingEnabled();
+         RuntimeEnabledFeatures::HeapIncrementalMarkingStressEnabled();
 #else
   return false;
 #endif  // BUILDFLAG(BLINK_HEAP_INCREMENTAL_MARKING)
@@ -1465,6 +1465,15 @@ bool ThreadState::MarkPhaseAdvanceMarking(double deadline_seconds) {
   return complete;
 }
 
+bool ThreadState::ShouldVerifyMarking() const {
+  bool should_verify_marking =
+      RuntimeEnabledFeatures::HeapIncrementalMarkingStressEnabled();
+#if BUILDFLAG(BLINK_HEAP_VERIFICATION)
+  should_verify_marking = true;
+#endif  // BLINK_HEAP_VERIFICATION
+  return should_verify_marking;
+}
+
 void ThreadState::MarkPhaseEpilogue(BlinkGC::MarkingType marking_type) {
   Visitor* visitor = current_gc_data_.visitor.get();
   // Finish marking of not-fully-constructed objects.
@@ -1483,9 +1492,8 @@ void ThreadState::MarkPhaseEpilogue(BlinkGC::MarkingType marking_type) {
 
   current_gc_data_.visitor.reset();
 
-#if BUILDFLAG(BLINK_HEAP_VERIFICATION)
-  VerifyMarking(marking_type);
-#endif  // BLINK_HEAP_VERIFICATION
+  if (ShouldVerifyMarking())
+    VerifyMarking(marking_type);
 
   Heap().HeapStats().SetEstimatedMarkingTimePerByte(
       current_gc_data_.marked_object_size

third_party/blink/renderer/platform/heap/thread_state.h

@@ -613,6 +613,8 @@ class PLATFORM_EXPORT ThreadState {
     safe_point_scope_marker_ = nullptr;
   }
 
+  bool ShouldVerifyMarking() const;
+
   // shouldScheduleIdleGC and shouldForceConservativeGC
   // implement the heuristics that are used to determine when to collect
   // garbage.

third_party/blink/renderer/platform/runtime_enabled_features.json5

@@ -542,6 +542,9 @@
     {
       name: "HeapIncrementalMarking",
     },
+    {
+      name: "HeapIncrementalMarkingStress"
+    },
     // https://crbug.com/766694 for testing disabling the feature.
     {
       name: "HTMLImports",