Introduce machinery to deterministically enable experiments based on hsotname.

tcmalloc/BUILD

@@ -57,6 +57,7 @@ cc_library(
         "@com_google_absl//absl/base",
         "@com_google_absl//absl/base:core_headers",
         "@com_google_absl//absl/base:nullability",
+        "@com_google_absl//absl/crc:crc32c",
         "@com_google_absl//absl/functional:function_ref",
         "@com_google_absl//absl/hash",
         "@com_google_absl//absl/strings",

tcmalloc/experiment.cc

@@ -14,7 +14,7 @@
 
 #include "tcmalloc/experiment.h"
 
-#include <string.h>
+#include <sys/utsname.h>
 
 #include <algorithm>
 #include <cassert>
@@ -27,10 +27,10 @@
 #include "absl/base/attributes.h"
 #include "absl/base/call_once.h"
 #include "absl/base/internal/cycleclock.h"
+#include "absl/crc/crc32c.h"
 #include "absl/functional/function_ref.h"
 #include "absl/hash/hash.h"
 #include "absl/strings/match.h"
-#include "absl/strings/str_cat.h"
 #include "absl/strings/string_view.h"
 #include "tcmalloc/experiment_config.h"
 #include "tcmalloc/internal/config.h"
@@ -75,11 +75,18 @@ const bool* GetSelectedExperiments() {
     const char* test_target = thread_safe_getenv("TEST_TARGET");
     const char* active_experiments = thread_safe_getenv(kExperiments);
     const char* disabled_experiments = thread_safe_getenv(kDisableExperiments);
+    struct utsname uts;
+    absl::string_view hostname;
+    if (const char* env = thread_safe_getenv("HOSTNAME")) {
+      hostname = env;
+    }
+
     SelectExperiments(
         by_id, test_target ? test_target : "",
         active_experiments ? active_experiments : "",
         disabled_experiments ? disabled_experiments : "",
-        active_experiments == nullptr && disabled_experiments == nullptr);
+        active_experiments == nullptr && disabled_experiments == nullptr,
+        hostname);
   });
   return by_id;
 }
@@ -104,16 +111,46 @@ void ParseExperiments(absl::string_view labels, F f) {
 
 }  // namespace
 
+uint16_t CalculateRolloutBucket(absl::string_view hostname,
+                                absl::string_view salt) {
+  uint32_t hash = static_cast<uint32_t>(
+      absl::ExtendCrc32c(absl::ComputeCrc32c(hostname), salt));
+  hash = hash ^ (hash >> 16);
+  return hash % 1024;
+}
+
+bool IsExperimentRolloutEnabled(const ExperimentConfig& config,
+                                absl::string_view hostname) {
+  if (hostname.empty()) {
+    return false;
+  }
+
+  // Ensure experiments are i.i.d. from one another by using their own names as
+  // a salt, unless explicitly requested.
+  absl::string_view salt =
+      config.rollout_salt.empty() ? config.name : config.rollout_salt;
+  uint16_t val = CalculateRolloutBucket(hostname, actual_salt);
+  return val >= config.rollout_lower_bound && val < config.rollout_upper_bound;
+}
+
 const bool* SelectExperiments(bool* buffer, absl::string_view test_target,
                               absl::string_view active,
                               absl::string_view disabled,
-                              bool unset) {
+                              bool unset, absl::string_view hostname) {
   memset(buffer, 0, sizeof(*buffer) * kNumExperiments);
 
   if (active == kEnableAll) {
     std::fill(buffer, buffer + kNumExperiments, true);
   }
 
+  for (const auto& config : experiments) {
+    if (config.rollout_upper_bound > 0) {
+      if (IsExperimentRolloutEnabled(config, hostname)) {
+        buffer[static_cast<int>(config.id)] = true;
+      }
+    }
+  }
+
   ParseExperiments(active, [buffer](absl::string_view token) {
     Experiment id;
     if (LookupExperimentID(token, &id)) {

tcmalloc/experiment.h

@@ -54,7 +54,13 @@ constexpr size_t kNumExperiments =
 const bool* absl_nonnull SelectExperiments(
     bool* absl_nonnull buffer, absl::string_view test_target,
     absl::string_view active, absl::string_view disabled,
-    bool unset);
+    bool unset, absl::string_view hostname);
+
+uint16_t CalculateRolloutBucket(absl::string_view hostname,
+                                absl::string_view salt);
+
+bool IsExperimentRolloutEnabled(const ExperimentConfig& config,
+                                absl::string_view hostname);
 
 }  // namespace tcmalloc_internal
 

tcmalloc/experiment_config.h

@@ -15,6 +15,8 @@
 #ifndef TCMALLOC_EXPERIMENT_CONFIG_H_
 #define TCMALLOC_EXPERIMENT_CONFIG_H_
 
+#include <cstdint>
+
 #include "absl/strings/string_view.h"
 
 // Autogenerated by experiments_proto_test --experiments_generate_config=true
@@ -45,6 +47,9 @@ struct ExperimentConfig {
   absl::string_view name;
   bool brittle = false;
   bool force_disable = false;
+  uint16_t rollout_lower_bound = 0;
+  uint16_t rollout_upper_bound = 0;
+  absl::string_view rollout_salt;
 };
 
 // clang-format off

tcmalloc/experiment_fuzz.cc

@@ -19,31 +19,39 @@
 #include "fuzztest/fuzztest.h"
 #include "absl/strings/string_view.h"
 #include "tcmalloc/experiment.h"
+#include "tcmalloc/experiment_config.h"
 
 namespace tcmalloc::tcmalloc_internal {
 namespace {
 
 void FuzzSelectExperiments(absl::string_view test_target,
                            absl::string_view active, absl::string_view disabled,
-                           bool unset) {
+                           bool unset, absl::string_view hostname) {
   if (unset && !test_target.empty() && (!active.empty() || !disabled.empty())) {
     return;
   }
 
   bool buffer[tcmalloc::tcmalloc_internal::kNumExperiments];
 
   SelectExperiments(buffer, test_target, active, disabled,
-                    unset);
+                    unset, hostname);
 }
 
 FUZZ_TEST(ExperimentTest, FuzzSelectExperiments);
 
+void FuzzRolloutEnabled(const ExperimentConfig& config,
+                        absl::string_view hostname) {
+  IsExperimentRolloutEnabled(config, hostname);
+}
+
+FUZZ_TEST(ExperimentTest, FuzzRolloutEnabled);
+
 TEST(ExperimentTest, FuzzSelectExperiments_b395212979) {
   FuzzSelectExperiments(
       "t_fuenchmark&"
       "vvvvvvvvvvvvvvvvvvVvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvv",
       "", "",
-      true);
+      true, "some_hostname");
 }
 
 }  // namespace