Oilpan: move "forced GC" handling back to gcEpilogue().

Source/bindings/core/v8/V8GCController.cpp

 /*
  * Copyright (C) 2009 Google Inc. All rights reserved.
  *
  * Redistribution and use in source and binary forms, with or without
  * modification, are permitted provided that the following conditions are
  * met:
  *
  *     * Redistributions of source code must retain the above copyright
  * notice, this list of conditions and the following disclaimer.
  *     * Redistributions in binary form must reproduce the above
@@ skipping 376 matching lines @@
     // for tests that force GCs from/ JavaScript to verify that objects die
     // when expected, or when handling memory pressure notifications.
     bool forceGC = flags & v8::kGCCallbackFlagForced;
 
     // FIXME: It would be nice if the GC callbacks passed the Isolate directly....
     v8::Isolate* isolate = v8::Isolate::GetCurrent();
     if (type == v8::kGCTypeScavenge) {
         minorGCEpilogue(isolate);
     } else if (type == v8::kGCTypeMarkSweepCompact) {
         majorGCEpilogue(isolate);
-        ThreadState::current()->didV8MajorGC(forceGC);
+        if (!forceGC)
+            ThreadState::current()->didV8MajorGC();
     }
 
+    if (forceGC) {
+        // This single GC is not enough for two reasons:
+        //   (1) The GC is not precise because the GC scans on-stack pointers conservatively.
+        //   (2) One GC is not enough to break a chain of persistent handles. It's possible that
+        //       some heap allocated objects own objects that contain persistent handles
+        //       pointing to other heap allocated objects. To break the chain, we need multiple GCs.
+        //
+        // Regarding (1), we force a precise GC at the end of the current event loop. So if you want
+        // to collect all garbage, you need to wait until the next event loop.
+        // Regarding (2), it would be OK in practice to trigger only one GC per gcEpilogue, because
+        // GCController.collectAll() forces 7 V8's GC.
+        Heap::collectGarbage(ThreadState::HeapPointersOnStack, ThreadState::GCWithSweep, Heap::ForcedGC);
+
+        // Forces a precise GC at the end of the current event loop.
+        ThreadState::current()->setGCState(ThreadState::FullGCScheduled);
+    }
     TRACE_EVENT_INSTANT1(TRACE_DISABLED_BY_DEFAULT("devtools.timeline"), "UpdateCounters", TRACE_EVENT_SCOPE_THREAD, "data", InspectorUpdateCountersEvent::data());
 }
 
 void V8GCController::minorGCEpilogue(v8::Isolate* isolate)
 {
     TRACE_EVENT_END1("devtools.timeline,v8", "MinorGC", "usedHeapSizeAfter", usedHeapSize(isolate));
     if (isMainThread()) {
         TRACE_EVENT_SET_NONCONST_SAMPLING_STATE(V8PerIsolateData::from(isolate)->previousSamplingState());
         ScriptForbiddenScope::exit();
     }
@@ skipping 53 matching lines @@
     Visitor* m_visitor;
 };
 
 void V8GCController::traceDOMWrappers(v8::Isolate* isolate, Visitor* visitor)
 {
     DOMWrapperTracer tracer(visitor);
     v8::V8::VisitHandlesWithClassIds(isolate, &tracer);
 }
 
 } // namespace blink