Allow metadata tagging to be controlled by experiment.

PiperOrigin-RevId: 665513238
Change-Id: Id6de7c01be3baf5668885612177a83fcdc6bbb16

tcmalloc/experiment_config.h

@@ -24,6 +24,7 @@ enum class Experiment : int {
   // clang-format off
   TEST_ONLY_TCMALLOC_POW2_SIZECLASS,
   TEST_ONLY_TCMALLOC_SHARDED_TRANSFER_CACHE,
+  TCMALLOC_METADATA_HUGEPAGE,  // TODO(b/345239704): Complete experiment.
   TEST_ONLY_TCMALLOC_HUGE_CACHE_RELEASE_30S,  // TODO(b/319872040): Complete experiment.
   TEST_ONLY_TCMALLOC_REUSE_SIZE_CLASSES,  // TODO(b/358126781): Complete experiment.
   TCMALLOC_HUGE_REGION_DEMAND_BASED_RELEASE,  // TODO(b/328440160): Complete experiment.
@@ -43,6 +44,7 @@ struct ExperimentConfig {
 inline constexpr ExperimentConfig experiments[] = {
     {Experiment::TEST_ONLY_TCMALLOC_POW2_SIZECLASS, "TEST_ONLY_TCMALLOC_POW2_SIZECLASS"},
     {Experiment::TEST_ONLY_TCMALLOC_SHARDED_TRANSFER_CACHE, "TEST_ONLY_TCMALLOC_SHARDED_TRANSFER_CACHE"},
+    {Experiment::TCMALLOC_METADATA_HUGEPAGE, "TCMALLOC_METADATA_HUGEPAGE"},
     {Experiment::TEST_ONLY_TCMALLOC_HUGE_CACHE_RELEASE_30S, "TEST_ONLY_TCMALLOC_HUGE_CACHE_RELEASE_30S"},
     {Experiment::TEST_ONLY_TCMALLOC_REUSE_SIZE_CLASSES, "TEST_ONLY_TCMALLOC_REUSE_SIZE_CLASSES"},
     {Experiment::TCMALLOC_HUGE_REGION_DEMAND_BASED_RELEASE, "TCMALLOC_HUGE_REGION_DEMAND_BASED_RELEASE"},

tcmalloc/parameters.cc

@@ -121,6 +121,19 @@ static std::atomic<bool>& huge_region_demand_based_release_enabled() {
   return v;
 }
 
+// As set_metadata_hugepage is determined at runtime, we cannot require constant
+// initialization for the atomic.  This avoids an initialization order fiasco.
+static std::atomic<bool>& tag_metadata_hugepage_enabled() {
+  ABSL_CONST_INIT static absl::once_flag flag;
+  ABSL_CONST_INIT static std::atomic<bool> v{true};
+  absl::base_internal::LowLevelCallOnce(&flag, [&]() {
+    if (IsExperimentActive(Experiment::TCMALLOC_METADATA_HUGEPAGE)) {
+      v.store(false, std::memory_order_relaxed);
+    }
+  });
+  return v;
+}
+
 // As resize_size_class_max_capacity_enabled() is determined at runtime, we
 // cannot require constant initialization for the atomic.  This avoids an
 // initialization order fiasco.
@@ -291,6 +304,10 @@ bool Parameters::huge_region_demand_based_release() {
       std::memory_order_relaxed);
 }
 
+bool Parameters::tag_metadata_separately() {
+  return tag_metadata_hugepage_enabled().load(std::memory_order_relaxed);
+}
+
 bool Parameters::resize_size_class_max_capacity() {
   return resize_size_class_max_capacity_enabled().load(
       std::memory_order_relaxed);
@@ -504,7 +521,8 @@ void TCMalloc_Internal_SetReleasePagesFromHugeRegionEnabled(bool v) {
 }
 
 void TCMalloc_Internal_SetTagMetadataSeparatelyEnabled(bool v) {
-  Parameters::tag_metadata_separately_.store(v, std::memory_order_relaxed);
+  tcmalloc::tcmalloc_internal::tag_metadata_hugepage_enabled().store(
+      v, std::memory_order_relaxed);
 }
 
 void TCMalloc_Internal_SetResizeSizeClassMaxCapacityEnabled(bool v) {

tcmalloc/parameters.h

@@ -114,9 +114,7 @@ class Parameters {
     return release_pages_from_huge_region_.load(std::memory_order_relaxed);
   }
 
-  static bool tag_metadata_separately() {
-    return tag_metadata_separately_.load(std::memory_order_relaxed);
-  }
+  static bool tag_metadata_separately();
 
   static void set_tag_metadata_separately(bool value) {
     TCMalloc_Internal_SetTagMetadataSeparatelyEnabled(value);