stats: Remember recent lookups and display them in an admin endpoint

docs/root/intro/version_history.rst

@@ -9,6 +9,8 @@ Version history
 * access log: reintroduce :ref:`filesystem <filesystem_stats>` stats and added the `write_failed` counter to track failed log writes
 * admin: added ability to configure listener :ref:`socket options <envoy_api_field_config.bootstrap.v2.Admin.socket_options>`.
 * admin: added config dump support for Secret Discovery Service :ref:`SecretConfigDump <envoy_api_msg_admin.v2alpha.SecretsConfigDump>`.
+* admin: added :http:get:`/stats/recentlookups`, :http:post:`/stats/recentlookups/clear`,
+   :http:post:`/stats/recentlookups/disable`, and :http:post:`/stats/recentlookups/enable` endpoints.
 * api: added ::ref:`set_node_on_first_message_only <envoy_api_field_core.ApiConfigSource.set_node_on_first_message_only>` option to omit the node identifier from the subsequent discovery requests on the same stream.
 * buffer filter: the buffer filter populates content-length header if not present, behavior can be disabled using the runtime feature `envoy.reloadable_features.buffer_filter_populate_content_length`.
 * config: added support for :ref:`delta xDS <arch_overview_dynamic_config_delta>` (including ADS) delivery

docs/root/operations/admin.rst

@@ -200,7 +200,7 @@ modify different aspects of the server:
 .. http:post:: /reset_counters
 
   Reset all counters to zero. This is useful along with :http:get:`/stats` during debugging. Note
-  that this does not drop any data sent to statsd. It just effects local output of the
+  that this does not drop any data sent to statsd. It just affects local output of the
   :http:get:`/stats` command.
 
 .. http:get:: /server_info
@@ -354,6 +354,55 @@ modify different aspects of the server:
   Envoy has updated (counters incremented at least once, gauges changed at least once,
   and histograms added to at least once)
 
+  .. http:get:: /stats/recentlookups
+
+  This endpoint helps Envoy developers debug potential contention
+  issues in the stats system. Initially, only the count of StatName
+  lookups is acumulated, not the specific names that are being looked
+  up. In order to see specific recent requests, you must enable the
+  feature by POSTing to `/stats/recentlookups/enable`. There may be
+  approximately 40-100 nanoseconds of added overhead per lookup.
+
+  When enabled, this endpoint emits a table of stat names that were
+  recently accessed as strings by Envoy. Ideally, strings should be
+  converted into StatNames, counters, gauges, and histograms by Envoy
+  code only during startup or when receiving a new configuration via
+  xDS. This is because when stats are looked up as strings they must
+  take a global symbol table lock. During startup this is acceptable,
+  but in response to user requests on high core-count machines, this
+  can cause performance issues due to mutex contention.
+
+  This option requires Envoy to be started with `use-fake-symbol-table 0`.
+
+  See :repo:`source/docs/stats.md` for more details.
+
+  Note also that actual mutex contention can be tracked via :http:get:`/contention`.
+
+  .. http:post:: /stats/recentlookups/enable
+
+  Turns on collection of recent lookup of stat-names, thus enabling
+  `/stats/recentlookups`.
+
+  See :repo:`source/docs/stats.md` for more details.
+
+  .. http:post:: /stats/recentlookups/disable
+
+  Turns off collection of recent lookup of stat-names, thus disabling
+  `/stats/recentlookups`. It also clears the list of lookups. However,
+  the total count, visible as stat `server.stats_recent_lookups`, is
+  not cleared, and continues to accumulate.
+
+  See :repo:`source/docs/stats.md` for more details.
+
+  .. http:post:: /stats/recentlookups/clear
+
+  Clears all outstanding lookups and counts. If called when recent lookup
+  collection is enabled, this clears all the, but collection
+  continues. If called when recent lookup collection is disabled,
+  there is no effect, as disabling collection clears the data.
+
+  See :repo:`source/docs/stats.md` for more details.
+
 .. _operations_admin_interface_runtime:
 
 .. http:get:: /runtime

include/envoy/stats/symbol_table.h

@@ -146,6 +146,32 @@ class SymbolTable {
   virtual void callWithStringView(StatName stat_name,
                                   const std::function<void(absl::string_view)>& fn) const PURE;
 
+  using RecentLookupsFn = std::function<void(absl::string_view, uint64_t)>;
+
+  /**
+   * Calls the provided function with the name of the most recently looked-up
+   * symbols, including lookups on any StatNameSets, and with a count of
+   * the recent lookups on that symbol.
+   *
+   * @param iter the function to call for every recent item.
+   */
+  virtual uint64_t getRecentLookups(const RecentLookupsFn& iter) const PURE;
+
+  /**
+   * Clears the recent-lookups structures.
+   */
+  virtual void clearRecentLookups() PURE;
+
+  /**
+   * Sets the recent-lookup capacity.
+   */
+  virtual void setRecentLookupCapacity(uint64_t capacity) PURE;
+
+  /**
+   * @return The configured recent-lookup tracking capacity.
+   */
+  virtual uint64_t recentLookupCapacity() const PURE;
+
   /**
    * Creates a StatNameSet.
    *

source/common/stats/BUILD

@@ -156,6 +156,7 @@ envoy_cc_library(
     hdrs = ["symbol_table_impl.h"],
     external_deps = ["abseil_base"],
     deps = [
+        ":recent_lookups_lib",
         "//include/envoy/stats:symbol_table_interface",
         "//source/common/common:assert_lib",
         "//source/common/common:logger_lib",

source/common/stats/fake_symbol_table_impl.h

@@ -130,6 +130,10 @@ class FakeSymbolTableImpl : public SymbolTable {
     return StatNameSetPtr(new StatNameSet(*this, name));
   }
   void forgetSet(StatNameSet&) override {}
+  uint64_t getRecentLookups(const RecentLookupsFn&) const override { return 0; }
+  void clearRecentLookups() override {}
+  void setRecentLookupCapacity(uint64_t) override {}
+  uint64_t recentLookupCapacity() const override { return 0; }
 
 private:
   absl::string_view toStringView(const StatName& stat_name) const {

source/common/stats/symbol_table_impl.cc

@@ -126,6 +126,7 @@ void SymbolTableImpl::addTokensToEncoding(const absl::string_view name, Encoding
   // ref-counts in this.
   {
     Thread::LockGuard lock(lock_);
+    recent_lookups_.lookup(name);
     for (auto& token : tokens) {
       symbols.push_back(toSymbol(token));
     }
@@ -207,16 +208,94 @@ void SymbolTableImpl::free(const StatName& stat_name) {
   }
 }
 
-StatNameSetPtr SymbolTableImpl::makeSet(absl::string_view name) {
+uint64_t SymbolTableImpl::getRecentLookups(const RecentLookupsFn& iter) const {
+  uint64_t total = 0;
+  absl::flat_hash_map<std::string, uint64_t> name_count_map;
+
+  // We don't want to hold stat_name_set_mutex while calling the iterator, so
+  // buffer lookup_data.
+  {
+    Thread::LockGuard lock(stat_name_set_mutex_);
+    for (StatNameSet* stat_name_set : stat_name_sets_) {
+      total +=
+          stat_name_set->getRecentLookups([&name_count_map](absl::string_view str, uint64_t count) {
+            name_count_map[std::string(str)] += count;
+          });
+    }
+  }
+
+  // We also don't want to hold lock_ while calling the iterator, but we need it
+  // to access recent_lookups_.
+  {
+    Thread::LockGuard lock(lock_);
+    recent_lookups_.forEach(
+        [&name_count_map](absl::string_view str, uint64_t count)
+            NO_THREAD_SAFETY_ANALYSIS { name_count_map[std::string(str)] += count; });
+    total += recent_lookups_.total();
+  }
+
+  // Now we have the collated name-count map data: we need to vectorize and
+  // sort. We define the pair with the count first as std::pair::operator<
+  // prioritizes its first element over its second.
+  using LookupCount = std::pair<uint64_t, absl::string_view>;
+  std::vector<LookupCount> lookup_data;
+  lookup_data.reserve(name_count_map.size());
+  for (const auto& iter : name_count_map) {
+    lookup_data.emplace_back(LookupCount(iter.second, iter.first));
+  }
+  std::sort(lookup_data.begin(), lookup_data.end());
+  for (const LookupCount& lookup_count : lookup_data) {
+    iter(lookup_count.second, lookup_count.first);
+  }
+  return total;
+}
+
+void SymbolTableImpl::setRecentLookupCapacity(uint64_t capacity) {
+  {
+    Thread::LockGuard lock(stat_name_set_mutex_);
+    for (StatNameSet* stat_name_set : stat_name_sets_) {
+      stat_name_set->setRecentLookupCapacity(capacity);
+    }
+  }
+
+  {
+    Thread::LockGuard lock(lock_);
+    recent_lookups_.setCapacity(capacity);
+  }
+}
+
+void SymbolTableImpl::clearRecentLookups() {
+  {
+    Thread::LockGuard lock(stat_name_set_mutex_);
+    for (StatNameSet* stat_name_set : stat_name_sets_) {
+      stat_name_set->clearRecentLookups();
+    }
+  }
+  {
+    Thread::LockGuard lock(lock_);
+    recent_lookups_.clear();
+  }
+}
+
+uint64_t SymbolTableImpl::recentLookupCapacity() const {
   Thread::LockGuard lock(lock_);
-  // make_unique does not work with private ctor, even though FakeSymbolTableImpl is a friend.
+  return recent_lookups_.capacity();
+}
+
+StatNameSetPtr SymbolTableImpl::makeSet(absl::string_view name) {
+  const uint64_t capacity = recentLookupCapacity();
+  // make_unique does not work with private ctor, even though SymbolTableImpl is a friend.
   StatNameSetPtr stat_name_set(new StatNameSet(*this, name));
-  stat_name_sets_.insert(stat_name_set.get());
+  stat_name_set->setRecentLookupCapacity(capacity);
+  {
+    Thread::LockGuard lock(stat_name_set_mutex_);
+    stat_name_sets_.insert(stat_name_set.get());
+  }
   return stat_name_set;
 }
 
 void SymbolTableImpl::forgetSet(StatNameSet& stat_name_set) {
-  Thread::LockGuard lock(lock_);
+  Thread::LockGuard lock(stat_name_set_mutex_);
   stat_name_sets_.erase(&stat_name_set);
 }
 
@@ -484,10 +563,27 @@ StatName StatNameSet::getDynamic(absl::string_view token) {
     Stats::StatName& stat_name_ref = dynamic_stat_names_[token];
     if (stat_name_ref.empty()) { // Note that builtin_stat_names_ already has one for "".
       stat_name_ref = pool_.add(token);
+      recent_lookups_.lookup(token);
     }
     return stat_name_ref;
   }
 }
 
+uint64_t StatNameSet::getRecentLookups(const RecentLookups::IterFn& iter) const {
+  absl::MutexLock lock(&mutex_);
+  recent_lookups_.forEach(iter);
+  return recent_lookups_.total();
+}
+
+void StatNameSet::clearRecentLookups() {
+  absl::MutexLock lock(&mutex_);
+  recent_lookups_.clear();
+}
+
+void StatNameSet::setRecentLookupCapacity(uint64_t capacity) {
+  absl::MutexLock lock(&mutex_);
+  recent_lookups_.setCapacity(capacity);
+}
+
 } // namespace Stats
 } // namespace Envoy

source/common/stats/symbol_table_impl.h

@@ -18,6 +18,7 @@
 #include "common/common/stack_array.h"
 #include "common/common/thread.h"
 #include "common/common/utility.h"
+#include "common/stats/recent_lookups.h"
 
 #include "absl/container/flat_hash_map.h"
 #include "absl/strings/str_join.h"
@@ -161,6 +162,10 @@ class SymbolTableImpl : public SymbolTable {
 
   StatNameSetPtr makeSet(absl::string_view name) override;
   void forgetSet(StatNameSet& stat_name_set) override;
+  uint64_t getRecentLookups(const RecentLookupsFn&) const override;
+  void clearRecentLookups() override;
+  void setRecentLookupCapacity(uint64_t capacity) override;
+  uint64_t recentLookupCapacity() const override;
 
 private:
   friend class StatName;
@@ -176,6 +181,9 @@ class SymbolTableImpl : public SymbolTable {
   // This must be held during both encode() and free().
   mutable Thread::MutexBasicLockable lock_;
 
+  // This must be held while updating stat_name_sets_.
+  mutable Thread::MutexBasicLockable stat_name_set_mutex_;
+
   /**
    * Decodes a vector of symbols back into its period-delimited stat name. If
    * decoding fails on any part of the symbol_vec, we release_assert and crash
@@ -241,7 +249,9 @@ class SymbolTableImpl : public SymbolTable {
   // TODO(ambuc): There might be an optimization here relating to storing ranges of freed symbols
   // using an Envoy::IntervalSet.
   std::stack<Symbol> pool_ GUARDED_BY(lock_);
-  absl::flat_hash_set<StatNameSet*> stat_name_sets_ GUARDED_BY(lock_);
+  RecentLookups recent_lookups_ GUARDED_BY(lock_);
+
+  absl::flat_hash_set<StatNameSet*> stat_name_sets_ GUARDED_BY(stat_name_set_mutex_);
 };
 
 /**
@@ -709,19 +719,33 @@ class StatNameSet {
     return pool_.add(str);
   }
 
+  /**
+   * Clears recent lookups.
+   */
+  void clearRecentLookups();
+
+  /**
+   * Sets the number of names recorded in the recent-lookups set.
+   *
+   * @param capacity the capacity to configure.
+   */
+  void setRecentLookupCapacity(uint64_t capacity);
+
 private:
   friend class FakeSymbolTableImpl;
   friend class SymbolTableImpl;
 
   StatNameSet(SymbolTable& symbol_table, absl::string_view name);
+  uint64_t getRecentLookups(const RecentLookups::IterFn& iter) const;
 
   const std::string name_;
   Stats::SymbolTable& symbol_table_;
   Stats::StatNamePool pool_ GUARDED_BY(mutex_);
-  absl::Mutex mutex_;
+  mutable absl::Mutex mutex_;
   using StringStatNameMap = absl::flat_hash_map<std::string, Stats::StatName>;
   StringStatNameMap builtin_stat_names_;
   StringStatNameMap dynamic_stat_names_ GUARDED_BY(mutex_);
+  RecentLookups recent_lookups_ GUARDED_BY(mutex_);
 };
 
 } // namespace Stats