Wire up opt out mechanism for big span.

Currently, the feature itself is disabled by default. We will enable it in a follow-up cl.

PiperOrigin-RevId: 700080673
Change-Id: I3f54f08c23c793ef471828d0fbcb71111bc1398f

tcmalloc/BUILD

@@ -347,6 +347,16 @@ cc_library(
     alwayslink = 1,
 )
 
+cc_library(
+    # TODO(b/304135905):  Remove this opt-out.
+    name = "want_disable_tcmalloc_big_span",
+    srcs = ["want_disable_tcmalloc_big_span.cc"],
+    copts = ["-g0"] + TCMALLOC_DEFAULT_COPTS,
+    visibility = ["//visibility:public"],
+    deps = ["@com_google_absl//absl/base:core_headers"],
+    alwayslink = 1,
+)
+
 cc_library(
     # TODO(b/199203282, b/296281171):  Remove this opt-out.
     name = "want_disable_huge_region_more_often",

tcmalloc/static_vars.cc

@@ -35,6 +35,7 @@
 #include "tcmalloc/internal/atomic_stats_counter.h"
 #include "tcmalloc/internal/cache_topology.h"
 #include "tcmalloc/internal/config.h"
+#include "tcmalloc/internal/environment.h"
 #include "tcmalloc/internal/explicitly_constructed.h"
 #include "tcmalloc/internal/logging.h"
 #include "tcmalloc/internal/mincore.h"
@@ -60,6 +61,31 @@ GOOGLE_MALLOC_SECTION_BEGIN
 namespace tcmalloc {
 namespace tcmalloc_internal {
 
+ABSL_ATTRIBUTE_WEAK bool default_want_disable_tcmalloc_big_span();
+bool tcmalloc_big_span() {
+  // Disable 64B span if built against an opt-out.
+  if (default_want_disable_tcmalloc_big_span != nullptr) {
+    return false;
+  }
+
+  const char* e = thread_safe_getenv("TCMALLOC_DISABLE_BIG_SPAN");
+  if (e) {
+    switch (e[0]) {
+      case '0':
+        // TODO(b/304135905): Enable this.
+        return false;
+      case '1':
+        return false;
+      default:
+        TC_BUG("bad env var '%s'", e);
+        return false;
+    }
+  }
+
+  // TODO(b/304135905): Enable this by default.
+  return false;
+}
+
 // Cacheline-align our SizeMap and CpuCache.  They both have very hot arrays as
 // their first member variables, and aligning them reduces the number of cache
 // lines these arrays use.
@@ -177,7 +203,8 @@ ABSL_ATTRIBUTE_COLD ABSL_ATTRIBUTE_NOINLINE void Static::SlowInitIfNecessary() {
     peak_heap_tracker_.Init(&arena_);
 
     const bool large_span_experiment =
-        IsExperimentActive(Experiment::TEST_ONLY_TCMALLOC_BIG_SPAN);
+        IsExperimentActive(Experiment::TEST_ONLY_TCMALLOC_BIG_SPAN) ||
+        tcmalloc_big_span();
     Parameters::set_max_span_cache_size(
         large_span_experiment ? Span::kLargeCacheSize : Span::kCacheSize);
     Parameters::set_max_span_cache_array_size(

tcmalloc/static_vars.h

@@ -70,6 +70,8 @@ enum class SizeClassConfiguration {
   kReuse = 6,
 };
 
+bool tcmalloc_big_span();
+
 class Static final {
  public:
   constexpr Static() = default;

tcmalloc/testing/want_disable_tcmalloc_big_span_test_helper.cc

@@ -0,0 +1,33 @@
+// Copyright 2019 The TCMalloc Authors
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     https://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+
+#include <cstdio>
+#include <string>
+
+#include "absl/strings/match.h"
+#include "absl/strings/string_view.h"
+#include "tcmalloc/malloc_extension.h"
+
+int main(int argc, char** argv) {
+  std::string input = tcmalloc::MallocExtension::GetStats();
+
+  if (absl::StrContains(input, "PARAMETER max_span_cache_size 8")) {
+    printf("Active");
+  } else {
+    printf("Inactive");
+  }
+
+  return 0;
+}

tcmalloc/want_disable_tcmalloc_big_span.cc

@@ -0,0 +1,26 @@
+// Copyright 2022 The TCMalloc Authors
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     https://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+#include "absl/base/attributes.h"
+
+namespace tcmalloc {
+namespace tcmalloc_internal {
+
+// This - if linked into a binary - allows 64B span to be disabled.
+ABSL_ATTRIBUTE_UNUSED bool default_want_disable_tcmalloc_big_span() {
+  return true;
+}
+
+}  // namespace tcmalloc_internal
+}  // namespace tcmalloc