Add experiment flag that hugepages' cold heap.

PiperOrigin-RevId: 816942326
Change-Id: I432f870abe29a50f9de2a00bdba7692200e6a19e

Author: v-gogte

tcmalloc/experiment_config.h

@@ -29,6 +29,7 @@ enum class Experiment : int {
   TEST_ONLY_MM_VCPU,  // TODO: b/245776120 - Complete experiment.
   TEST_ONLY_TCMALLOC_EXTENDED_PRIORITY_LISTS_V1,  // TODO: b/437412600 - Complete experiment.
   TEST_ONLY_TCMALLOC_HUGE_CACHE_RELEASE_30S,  // TODO: b/319872040 - Complete experiment.
+  TEST_ONLY_TCMALLOC_MADV_COLD_HUGEPAGE,  // TODO: b/450368905 - Complete experiment.
   TEST_ONLY_TCMALLOC_POW2_SIZECLASS,
   TEST_ONLY_TCMALLOC_PREFERENTIAL_COLLAPSE,  // TODO: b/435257627 - Complete experiment.
   TEST_ONLY_TCMALLOC_REUSE_SIZE_CLASSES_V2,  // TODO: b/358126781 - Complete experiment.
@@ -52,6 +53,7 @@ inline constexpr ExperimentConfig experiments[] = {
     {Experiment::TEST_ONLY_MM_VCPU, "TEST_ONLY_MM_VCPU"},
     {Experiment::TEST_ONLY_TCMALLOC_EXTENDED_PRIORITY_LISTS_V1, "TEST_ONLY_TCMALLOC_EXTENDED_PRIORITY_LISTS_V1"},
     {Experiment::TEST_ONLY_TCMALLOC_HUGE_CACHE_RELEASE_30S, "TEST_ONLY_TCMALLOC_HUGE_CACHE_RELEASE_30S"},
+    {Experiment::TEST_ONLY_TCMALLOC_MADV_COLD_HUGEPAGE, "TEST_ONLY_TCMALLOC_MADV_COLD_HUGEPAGE"},
     {Experiment::TEST_ONLY_TCMALLOC_POW2_SIZECLASS, "TEST_ONLY_TCMALLOC_POW2_SIZECLASS"},
     {Experiment::TEST_ONLY_TCMALLOC_PREFERENTIAL_COLLAPSE, "TEST_ONLY_TCMALLOC_PREFERENTIAL_COLLAPSE"},
     {Experiment::TEST_ONLY_TCMALLOC_REUSE_SIZE_CLASSES_V2, "TEST_ONLY_TCMALLOC_REUSE_SIZE_CLASSES_V2"},

tcmalloc/internal/system_allocator.h

@@ -365,8 +365,10 @@ std::pair<void*, size_t> SystemAllocator<Topology>::MmapRegion::Alloc(
   // For cold regions (kInfrequentAccess) and sampled regions
   // (kInfrequentAllocation), we want as granular of access telemetry as
   // possible; this hint means we can get 4kiB granularity instead of 2MiB.
-  if (hint_ == AddressRegionFactory::UsageHint::kInfrequentAccess ||
-      hint_ == AddressRegionFactory::UsageHint::kInfrequentAllocation ||
+  if (((hint_ == AddressRegionFactory::UsageHint::kInfrequentAccess ||
+        hint_ == AddressRegionFactory::UsageHint::kInfrequentAllocation) &&
+       !IsExperimentActive(
+           Experiment::TEST_ONLY_TCMALLOC_MADV_COLD_HUGEPAGE)) ||
       system_allocator_internal::preferential_collapse()) {
     // This is only advisory, so ignore the error.
     ErrnoRestorer errno_restorer;