[heap] Add histogram counter for young generation handling

BUG=chromium:693413

Change-Id: I6c6bc62e7f2c702be2462e4b0c3704fabf44f9d2
Reviewed-on: https://chromium-review.googlesource.com/445156
Commit-Queue: Michael Lippautz <mlippautz@chromium.org>
Reviewed-by: Ulan Degenbaev <ulan@chromium.org>
Cr-Commit-Position: refs/heads/master@{#43316}

src/counters.h

@@ -941,6 +941,7 @@ class RuntimeCallTimerScope {
   HR(incremental_marking_reason, V8.GCIncrementalMarkingReason, 0, 21, 22)    \
   HR(mark_compact_reason, V8.GCMarkCompactReason, 0, 21, 22)                  \
   HR(scavenge_reason, V8.GCScavengeReason, 0, 21, 22)                         \
+  HR(young_generation_handling, V8.YoungGenerationHandling, 0, 1, 2)          \
   /* Asm/Wasm. */                                                             \
   HR(wasm_functions_per_module, V8.WasmFunctionsPerModule, 1, 10000, 51)
 

src/heap/gc-tracer.cc

@@ -140,6 +140,13 @@ void GCTracer::ResetForTesting() {
   start_counter_ = 0;
 }
 
+void GCTracer::NotifyYoungGenerationHandling(
+    YoungGenerationHandling young_generation_handling) {
+  DCHECK(current_.type == Event::SCAVENGER || start_counter_ > 1);
+  heap_->isolate()->counters()->young_generation_handling()->AddSample(
+      static_cast<int>(young_generation_handling));
+}
+
 void GCTracer::Start(GarbageCollector collector,
                      GarbageCollectionReason gc_reason,
                      const char* collector_reason) {

src/heap/gc-tracer.h

@@ -236,6 +236,9 @@ class V8_EXPORT_PRIVATE GCTracer {
   // Stop collecting data and print results.
   void Stop(GarbageCollector collector);
 
+  void NotifyYoungGenerationHandling(
+      YoungGenerationHandling young_generation_handling);
+
   // Sample and accumulate bytes allocated since the last GC.
   void SampleAllocation(double current_ms, size_t new_space_counter_bytes,
                         size_t old_generation_counter_bytes);

src/heap/heap.cc

@@ -1358,10 +1358,17 @@ bool Heap::PerformGarbageCollection(
         MinorMarkCompact();
         break;
       case SCAVENGER:
-        if (fast_promotion_mode_ && CanExpandOldGeneration(new_space()->Size()))
+        if (fast_promotion_mode_ &&
+            CanExpandOldGeneration(new_space()->Size())) {
+          tracer()->NotifyYoungGenerationHandling(
+              YoungGenerationHandling::kFastPromotionDuringScavenge);
           EvacuateYoungGeneration();
-        else
+        } else {
+          tracer()->NotifyYoungGenerationHandling(
+              YoungGenerationHandling::kRegularScavenge);
+
           Scavenge();
+        }
         break;
     }
 

src/heap/heap.h

@@ -377,6 +377,14 @@ enum class GarbageCollectionReason {
   // Also update src/tools/metrics/histograms/histograms.xml in chromium.
 };
 
+enum class YoungGenerationHandling {
+  kRegularScavenge = 0,
+  kFastPromotionDuringScavenge = 1,
+  // If you add new items here, then update the young_generation_handling in
+  // counters.h.
+  // Also update src/tools/metrics/histograms/histograms.xml in chromium.
+};
+
 // A queue of objects promoted during scavenge. Each object is accompanied by
 // its size to avoid dereferencing a map pointer for scanning. The last page in
 // to-space is used for the promotion queue. On conflict during scavenge, the