Add clear trace events per ItemParallelJob::Items instead of reusing their top-level category.

This is a follow-up to https://chromium-review.googlesource.com/904164
now that I've realized raw TRACE_EVENTs work in v8 (initially thought
had to go through heap()->tracer()).

After adding events to EvacuatePage(), the subitem event for evacuation tasks
became redundant. Since this results in a better output this CL applies
this approach to all paralell work items.

Not using TRACE_BACKGROUND_GC also avoids polluting logs with subitems
when using v8's tracing flags.

R=mlippautz@chromium.org

Bug: chromium:651354
Change-Id: I62efbde69da2a0a9bd255e34093d64924e339974
Reviewed-on: https://chromium-review.googlesource.com/934263
Commit-Queue: Gabriel Charette <gab@chromium.org>
Reviewed-by: Michael Lippautz <mlippautz@chromium.org>
Cr-Commit-Position: refs/heads/master@{#51544}

src/heap/heap.cc

@@ -1952,9 +1952,6 @@ class ScavengingTask final : public ItemParallelJob::Task {
       TimedScope scope(&scavenging_time);
       PageScavengingItem* item = nullptr;
       while ((item = GetItem<PageScavengingItem>()) != nullptr) {
-        TRACE_BACKGROUND_GC(
-            heap_->tracer(),
-            GCTracer::BackgroundScope::SCAVENGER_BACKGROUND_SCAVENGE_PARALLEL);
         item->Process(scavenger_);
         item->MarkFinished();
       }

src/heap/mark-compact.cc

@@ -1835,9 +1835,6 @@ class YoungGenerationMarkingTask : public ItemParallelJob::Task {
       TimedScope scope(&marking_time);
       MarkingItem* item = nullptr;
       while ((item = GetItem<MarkingItem>()) != nullptr) {
-        TRACE_BACKGROUND_GC(
-            collector_->heap()->tracer(),
-            GCTracer::BackgroundScope::MINOR_MC_BACKGROUND_MARKING);
         item->Process(this);
         item->MarkFinished();
         EmptyLocalMarkingWorklist();
@@ -1903,6 +1900,8 @@ class BatchedRootMarkingItem : public MarkingItem {
   virtual ~BatchedRootMarkingItem() {}
 
   void Process(YoungGenerationMarkingTask* task) override {
+    TRACE_EVENT0(TRACE_DISABLED_BY_DEFAULT("v8.gc"),
+                 "BatchedRootMarkingItem::Process");
     for (Object* object : objects_) {
       task->MarkObject(object);
     }
@@ -1920,6 +1919,8 @@ class PageMarkingItem : public MarkingItem {
   virtual ~PageMarkingItem() { global_slots_->Increment(slots_); }
 
   void Process(YoungGenerationMarkingTask* task) override {
+    TRACE_EVENT0(TRACE_DISABLED_BY_DEFAULT("v8.gc"),
+                 "PageMarkingItem::Process");
     base::LockGuard<base::Mutex> guard(chunk_->mutex());
     MarkUntypedPointers(task);
     MarkTypedPointers(task);
@@ -1976,6 +1977,8 @@ class GlobalHandlesMarkingItem : public MarkingItem {
   virtual ~GlobalHandlesMarkingItem() {}
 
   void Process(YoungGenerationMarkingTask* task) override {
+    TRACE_EVENT0(TRACE_DISABLED_BY_DEFAULT("v8.gc"),
+                 "GlobalHandlesMarkingItem::Process");
     GlobalHandlesRootMarkingVisitor visitor(task);
     global_handles_
         ->IterateNewSpaceStrongAndDependentRootsAndIdentifyUnmodified(
@@ -3236,7 +3239,6 @@ class PageEvacuationTask : public ItemParallelJob::Task {
     TRACE_BACKGROUND_GC(tracer_, evacuator_->GetBackgroundTracingScope());
     PageEvacuationItem* item = nullptr;
     while ((item = GetItem<PageEvacuationItem>()) != nullptr) {
-      TRACE_BACKGROUND_GC(tracer_, evacuator_->GetBackgroundTracingScope());
       evacuator_->EvacuatePage(item->page());
       item->MarkFinished();
     }
@@ -3560,7 +3562,6 @@ class PointersUpdatingTask : public ItemParallelJob::Task {
     TRACE_BACKGROUND_GC(tracer_, scope_);
     UpdatingItem* item = nullptr;
     while ((item = GetItem<UpdatingItem>()) != nullptr) {
-      TRACE_BACKGROUND_GC(tracer_, scope_);
       item->Process();
       item->MarkFinished();
     }
@@ -3594,6 +3595,8 @@ class ToSpaceUpdatingItem : public UpdatingItem {
 
  private:
   void ProcessVisitAll() {
+    TRACE_EVENT0(TRACE_DISABLED_BY_DEFAULT("v8.gc"),
+                 "ToSpaceUpdatingItem::ProcessVisitAll");
     PointersUpdatingVisitor visitor;
     for (Address cur = start_; cur < end_;) {
       HeapObject* object = HeapObject::FromAddress(cur);
@@ -3605,6 +3608,8 @@ class ToSpaceUpdatingItem : public UpdatingItem {
   }
 
   void ProcessVisitLive() {
+    TRACE_EVENT0(TRACE_DISABLED_BY_DEFAULT("v8.gc"),
+                 "ToSpaceUpdatingItem::ProcessVisitLive");
     // For young generation evacuations we want to visit grey objects, for
     // full MC, we need to visit black objects.
     PointersUpdatingVisitor visitor;
@@ -3633,6 +3638,8 @@ class RememberedSetUpdatingItem : public UpdatingItem {
   virtual ~RememberedSetUpdatingItem() {}
 
   void Process() override {
+    TRACE_EVENT0(TRACE_DISABLED_BY_DEFAULT("v8.gc"),
+                 "RememberedSetUpdatingItem::Process");
     base::LockGuard<base::Mutex> guard(chunk_->mutex());
     UpdateUntypedPointers();
     UpdateTypedPointers();
@@ -3780,6 +3787,8 @@ class GlobalHandlesUpdatingItem : public UpdatingItem {
   virtual ~GlobalHandlesUpdatingItem() {}
 
   void Process() override {
+    TRACE_EVENT0(TRACE_DISABLED_BY_DEFAULT("v8.gc"),
+                 "GlobalHandlesUpdatingItem::Process");
     PointersUpdatingVisitor updating_visitor;
     global_handles_->IterateNewSpaceRoots(&updating_visitor, start_, end_);
   }
@@ -3804,6 +3813,9 @@ class ArrayBufferTrackerUpdatingItem : public UpdatingItem {
   virtual ~ArrayBufferTrackerUpdatingItem() {}
 
   void Process() override {
+    TRACE_EVENT1(TRACE_DISABLED_BY_DEFAULT("v8.gc"),
+                 "ArrayBufferTrackerUpdatingItem::Process", "EvacuationState",
+                 state_);
     switch (state_) {
       case EvacuationState::kRegular:
         ArrayBufferTracker::ProcessBuffers(

src/heap/scavenger.cc

@@ -97,6 +97,7 @@ void Scavenger::AddPageToSweeperIfNecessary(MemoryChunk* page) {
 }
 
 void Scavenger::ScavengePage(MemoryChunk* page) {
+  TRACE_EVENT0(TRACE_DISABLED_BY_DEFAULT("v8.gc"), "Scavenger::ScavengePage");
   CodePageMemoryModificationScope memory_modification_scope(page);
   RememberedSet<OLD_TO_NEW>::Iterate(
       page,
@@ -115,6 +116,7 @@ void Scavenger::ScavengePage(MemoryChunk* page) {
 }
 
 void Scavenger::Process(OneshotBarrier* barrier) {
+  TRACE_EVENT0(TRACE_DISABLED_BY_DEFAULT("v8.gc"), "Scavenger::Process");
   // Threshold when to switch processing the promotion list to avoid
   // allocating too much backing store in the worklist.
   const int kProcessPromotionListThreshold = kPromotionListSegmentSize / 2;