[StatsPlugin] Use lock-free list for global stats plugins list (#38060)

Closes #38060

COPYBARA_INTEGRATE_REVIEW=https://github.com/grpc/grpc/pull/38060 from yashykt:MetricsLockFree d5bdaea2b5
PiperOrigin-RevId: 696320956

src/core/telemetry/metrics.cc

@@ -120,27 +120,30 @@ void GlobalStatsPluginRegistry::StatsPluginGroup::AddServerCallTracers(
   }
 }
 
-NoDestruct<Mutex> GlobalStatsPluginRegistry::mutex_;
-NoDestruct<std::vector<std::shared_ptr<StatsPlugin>>>
+std::atomic<GlobalStatsPluginRegistry::GlobalStatsPluginNode*>
     GlobalStatsPluginRegistry::plugins_;
 
 void GlobalStatsPluginRegistry::RegisterStatsPlugin(
     std::shared_ptr<StatsPlugin> plugin) {
-  MutexLock lock(&*mutex_);
-  plugins_->push_back(std::move(plugin));
+  GlobalStatsPluginNode* node = new GlobalStatsPluginNode();
+  node->plugin = std::move(plugin);
+  node->next = plugins_.load(std::memory_order_relaxed);
+  while (!plugins_.compare_exchange_weak(
+      node->next, node, std::memory_order_acq_rel, std::memory_order_relaxed)) {
+  }
 }
 
 GlobalStatsPluginRegistry::StatsPluginGroup
 GlobalStatsPluginRegistry::GetStatsPluginsForChannel(
     const experimental::StatsPluginChannelScope& scope) {
-  MutexLock lock(&*mutex_);
   StatsPluginGroup group;
-  for (const auto& plugin : *plugins_) {
+  for (GlobalStatsPluginNode* node = plugins_.load(std::memory_order_acquire);
+       node != nullptr; node = node->next) {
     bool is_enabled = false;
     std::shared_ptr<StatsPlugin::ScopeConfig> config;
-    std::tie(is_enabled, config) = plugin->IsEnabledForChannel(scope);
+    std::tie(is_enabled, config) = node->plugin->IsEnabledForChannel(scope);
     if (is_enabled) {
-      group.AddStatsPlugin(plugin, std::move(config));
+      group.AddStatsPlugin(node->plugin, std::move(config));
     }
   }
   return group;
@@ -148,14 +151,14 @@ GlobalStatsPluginRegistry::GetStatsPluginsForChannel(
 
 GlobalStatsPluginRegistry::StatsPluginGroup
 GlobalStatsPluginRegistry::GetStatsPluginsForServer(const ChannelArgs& args) {
-  MutexLock lock(&*mutex_);
   StatsPluginGroup group;
-  for (const auto& plugin : *plugins_) {
+  for (GlobalStatsPluginNode* node = plugins_.load(std::memory_order_acquire);
+       node != nullptr; node = node->next) {
     bool is_enabled = false;
     std::shared_ptr<StatsPlugin::ScopeConfig> config;
-    std::tie(is_enabled, config) = plugin->IsEnabledForServer(args);
+    std::tie(is_enabled, config) = node->plugin->IsEnabledForServer(args);
     if (is_enabled) {
-      group.AddStatsPlugin(plugin, std::move(config));
+      group.AddStatsPlugin(node->plugin, std::move(config));
     }
   }
   return group;

src/core/telemetry/metrics.h

@@ -446,6 +446,8 @@ class GlobalStatsPluginRegistry {
       return false;
     }
 
+    size_t size() const { return plugins_state_.size(); }
+
     // Registers a callback to be used to populate callback metrics.
     // The callback will update the specified metrics.  The callback
     // will be invoked no more often than min_interval.  Multiple callbacks may
@@ -508,13 +510,15 @@ class GlobalStatsPluginRegistry {
   static StatsPluginGroup GetStatsPluginsForServer(const ChannelArgs& args);
 
  private:
+  struct GlobalStatsPluginNode {
+    std::shared_ptr<StatsPlugin> plugin;
+    GlobalStatsPluginNode* next = nullptr;
+  };
   friend class GlobalStatsPluginRegistryTestPeer;
 
   GlobalStatsPluginRegistry() = default;
 
-  static NoDestruct<Mutex> mutex_;
-  static NoDestruct<std::vector<std::shared_ptr<StatsPlugin>>> plugins_
-      ABSL_GUARDED_BY(mutex_);
+  static std::atomic<GlobalStatsPluginNode*> plugins_;
 };
 
 // A metric callback that is registered with a stats plugin group.

test/core/telemetry/metrics_test.cc

@@ -15,6 +15,7 @@
 #include "src/core/telemetry/metrics.h"
 
 #include <memory>
+#include <thread>
 
 #include "absl/log/log.h"
 #include "gmock/gmock.h"
@@ -648,6 +649,38 @@ TEST_F(MetricsTest, FindInstrumentByName) {
                   ::testing::Eq(uint64_counter_handle.index))));
 }
 
+TEST_F(MetricsTest, ParallelStatsPluginRegistrationAndLookup) {
+  std::vector<std::thread> register_threads;
+  std::vector<std::thread> lookup_threads;
+  register_threads.reserve(100);
+  lookup_threads.reserve(100);
+  // 100 threads that register 100 stats plugins each
+  for (int i = 0; i < 100; ++i) {
+    register_threads.emplace_back([] {
+      for (int j = 0; j < 100; ++j) {
+        FakeStatsPluginBuilder().BuildAndRegister();
+      }
+    });
+  }
+  // 100 threads that keep looking up stats plugins till they see 10000 stats
+  // plugins
+  for (int i = 0; i < 100; ++i) {
+    lookup_threads.emplace_back([this] {
+      while (GlobalStatsPluginRegistry::GetStatsPluginsForChannel(
+                 StatsPluginChannelScope("", "", endpoint_config_))
+                 .size() < 10000) {
+      };
+    });
+  }
+  for (int i = 0; i < 100; ++i) {
+    register_threads[i].join();
+    lookup_threads[i].join();
+  }
+  EXPECT_THAT(GlobalStatsPluginRegistry::GetStatsPluginsForChannel(
+                  StatsPluginChannelScope("", "", endpoint_config_)),
+              ::testing::SizeIs(10000));
+}
+
 using MetricsDeathTest = MetricsTest;
 
 TEST_F(MetricsDeathTest, RegisterTheSameMetricNameWouldCrash) {

test/core/test_util/fake_stats_plugin.h

@@ -696,8 +696,14 @@ class GlobalInstrumentsRegistryTestPeer {
 class GlobalStatsPluginRegistryTestPeer {
  public:
   static void ResetGlobalStatsPluginRegistry() {
-    MutexLock lock(&*GlobalStatsPluginRegistry::mutex_);
-    GlobalStatsPluginRegistry::plugins_->clear();
+    GlobalStatsPluginRegistry::GlobalStatsPluginNode* node =
+        GlobalStatsPluginRegistry::plugins_.exchange(nullptr,
+                                                     std::memory_order_acq_rel);
+    while (node != nullptr) {
+      GlobalStatsPluginRegistry::GlobalStatsPluginNode* next = node->next;
+      delete node;
+      node = next;
+    }
   }
 };