heap: Fix metric around corner case of fast marking

Bug: v8:10956
Change-Id: Iab17e3f0163aa81cfb978d72e0f0cd9b15c14743
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2454716Reviewed-by: Ulan Degenbaev <ulan@chromium.org>
Commit-Queue: Michael Lippautz <mlippautz@chromium.org>
Cr-Commit-Position: refs/heads/master@{#70371}

src/heap/gc-tracer.cc

@@ -1229,14 +1229,18 @@ void GCTracer::RecordGCPhasesHistograms(TimedHistogram* gc_timer) {
     heap_->isolate()->counters()->gc_marking_sum()->AddSample(
         static_cast<int>(overall_marking_time));
 
+    // Filter out samples where
+    // - we don't have high-resolution timers;
+    // - size of marked objects is very small;
+    // - marking time is rounded to 0;
     constexpr size_t kMinObjectSizeForReportingThroughput = 1024 * 1024;
     if (base::TimeTicks::IsHighResolution() &&
-        heap_->SizeOfObjects() > kMinObjectSizeForReportingThroughput) {
-      DCHECK_GT(overall_marking_time, 0.0);
+        heap_->SizeOfObjects() > kMinObjectSizeForReportingThroughput &&
+        overall_marking_time > 0) {
       const double overall_v8_marking_time =
           overall_marking_time -
           current_.scopes[Scope::MC_MARK_EMBEDDER_TRACING];
-      DCHECK_GT(overall_v8_marking_time, 0.0);
+      if (overall_v8_marking_time > 0) {
         const int main_thread_marking_throughput_mb_per_s =
             static_cast<int>(static_cast<double>(heap_->SizeOfObjects()) /
                              overall_v8_marking_time * 1000 / 1024 / 1024);
@@ -1246,6 +1250,7 @@ void GCTracer::RecordGCPhasesHistograms(TimedHistogram* gc_timer) {
             ->AddSample(
                 static_cast<int>(main_thread_marking_throughput_mb_per_s));
       }
+    }
 
     DCHECK_EQ(Scope::LAST_TOP_MC_SCOPE, Scope::MC_SWEEP);
   } else if (gc_timer == counters->gc_scavenger()) {