trace_events: adds a new trace_events api

Removes the requirement to use `--trace-events-enabled` to enable trace events. Tracing is enabled automatically if there are any enabled categories. Adds a new `trace_events` module with an API for enabling/disabling trace events at runtime without a command line flag. ```js const trace_events = require('trace_events'); const categories = [ 'node.perf', 'node.async_hooks' ]; const tracing = trace_events.createTracing({ categories }); tracing.enable(); // do stuff tracing.disable(); ``` Multiple `Tracing` objects may exist and be enabled at any point in time. The enabled trace event categories is the union of all enabled `Tracing` objects and the `--trace-event-categories` flag. PR-URL: https://github.com/nodejs/node/pull/19803 Reviewed-By: Matteo Collina <matteo.collina@gmail.com> Reviewed-By: Ali Ijaz Sheikh <ofrobots@google.com> Reviewed-By: Franziska Hinkelmann <franziska.hinkelmann@gmail.com>
2018-04-03 18:05:33 -07:00 · 2018-04-03 18:05:33 -07:00 · 5c27e44488
commit 5c27e44488
parent 95fafc0254
25 changed files with 678 additions and 88 deletions
--- a/doc/api/_toc.md
+++ b/doc/api/_toc.md
@ -46,7 +46,7 @@
 * [String Decoder](string_decoder.html)
 * [Timers](timers.html)
 * [TLS/SSL](tls.html)
-* [Tracing](tracing.html)
+* [Trace Events](tracing.html)
 * [TTY](tty.html)
 * [UDP/Datagram](dgram.html)
 * [URL](url.html)
--- a/doc/api/errors.md
+++ b/doc/api/errors.md
@ -1550,6 +1550,18 @@ socket, which is only valid from a client.

 An attempt was made to renegotiate TLS on a socket instance with TLS disabled.

+<a id="ERR_TRACE_EVENTS_CATEGORY_REQUIRED"></a>
+### ERR_TRACE_EVENTS_CATEGORY_REQUIRED
+
+The `trace_events.createTracing()` method requires at least one trace event
+category.
+
+<a id="ERR_TRACE_EVENTS_UNAVAILABLE"></a>
+### ERR_TRACE_EVENTS_UNAVAILABLE
+
+The `trace_events` module could not be loaded because Node.js was compiled with
+the `--without-v8-platform` flag.
+
 <a id="ERR_TRANSFORM_ALREADY_TRANSFORMING"></a>
 ### ERR_TRANSFORM_ALREADY_TRANSFORMING

--- a/doc/api/tracing.md
+++ b/doc/api/tracing.md
@ -1,16 +1,15 @@
-# Tracing
+# Trace Events

 <!--introduced_in=v7.7.0-->

+> Stability: 1 - Experimental
+
 Trace Event provides a mechanism to centralize tracing information generated by
 V8, Node.js core, and userspace code.

-Tracing can be enabled by passing the `--trace-events-enabled` flag when
-starting a Node.js application.
-
-The set of categories for which traces are recorded can be specified using the
-`--trace-event-categories` flag followed by a list of comma separated category
-names.
+Tracing can be enabled with the `--trace-event-categories` command-line flag
+or by using the trace_events module. The `--trace-event-categories` flag accepts
+a list of comma-separated category names.

 The available categories are:

@ -27,7 +26,32 @@ The available categories are:
 By default the `node`, `node.async_hooks`, and `v8` categories are enabled.

 ```txt
-node --trace-events-enabled --trace-event-categories v8,node,node.async_hooks server.js
+node --trace-event-categories v8,node,node.async_hooks server.js
+```
+
+Prior versions of Node.js required the use of the `--trace-events-enabled`
+flag to enable trace events. This requirement has been removed. However, the
+`--trace-events-enabled` flag *may* still be used and will enable the
+`node`, `node.async_hooks`, and `v8` trace event categories by default.
+
+```txt
+node --trace-events-enabled
+
+// is equivalent to
+
+node --trace-event-categories v8,node,node.async_hooks
+```
+
+Alternatively, trace events may be enabled using the `trace_events` module:
+
+```js
+const trace_events = require('trace_events');
+const tracing = trace_events.createTracing({ categories: ['node.perf'] });
+tracing.enable();  // Enable trace event capture for the 'node.perf' category
+
+// do work
+
+tracing.disable();  // Disable trace event capture for the 'node.perf' category
 ```

 Running Node.js with tracing enabled will produce log files that can be opened
@ -40,7 +64,7 @@ be specified with `--trace-event-file-pattern` that accepts a template
 string that supports `${rotation}` and `${pid}`. For example:

 ```txt
-node --trace-events-enabled --trace-event-file-pattern '${pid}-${rotation}.log' server.js
+node --trace-event-categories v8 --trace-event-file-pattern '${pid}-${rotation}.log' server.js
 ```

 Starting with Node.js 10.0.0, the tracing system uses the same time source
@ -48,4 +72,124 @@ as the one used by `process.hrtime()`
 however the trace-event timestamps are expressed in microseconds,
 unlike `process.hrtime()` which returns nanoseconds.

+## The `trace_events` module
+<!-- YAML
+added: REPLACEME
+-->
+
+### `Tracing` object
+<!-- YAML
+added: REPLACEME
+-->
+
+The `Tracing` object is used to enable or disable tracing for sets of
+categories. Instances are created using the `trace_events.createTracing()`
+method.
+
+When created, the `Tracing` object is disabled. Calling the
+`tracing.enable()` method adds the categories to the set of enabled trace event
+categories. Calling `tracing.disable()` will remove the categories from the
+set of enabled trace event categories.
+
+#### `tracing.categories`
+<!-- YAML
+added: REPLACEME
+-->
+
+* {string}
+
+A comma-separated list of the trace event categories covered by this
+`Tracing` object.
+
+#### `tracing.disable()`
+<!-- YAML
+added: REPLACEME
+-->
+
+Disables this `Tracing` object.
+
+Only trace event categories *not* covered by other enabled `Tracing` objects
+and *not* specified by the `--trace-event-categories` flag will be disabled.
+
+```js
+const trace_events = require('trace_events');
+const t1 = trace_events.createTracing({ categories: ['node', 'v8'] });
+const t2 = trace_events.createTracing({ categories: ['node.perf', 'node'] });
+t1.enable();
+t2.enable();
+
+// Prints 'node,node.perf,v8'
+console.log(trace_events.getEnabledCategories());
+
+t2.disable(); // will only disable emission of the 'node.perf' category
+
+// Prints 'node,v8'
+console.log(trace_events.getEnabledCategories());
+```
+
+#### `tracing.enable()`
+<!-- YAML
+added: REPLACEME
+-->
+
+Enables this `Tracing` object for the set of categories covered by the
+`Tracing` object.
+
+#### `tracing.enabled`
+<!-- YAML
+added: REPLACEME
+-->
+
+* {boolean} `true` only if the `Tracing` object has been enabled.
+
+### `trace_events.createTracing(options)`
+<!-- YAML
+added: REPLACEME
+-->
+
+* `options` {Object}
+  * `categories` {string[]} An array of trace category names. Values included
+    in the array are coerced to a string when possible. An error will be
+    thrown if the value cannot be coerced.
+* Returns: {Tracing}.
+
+Creates and returns a `Tracing` object for the given set of `categories`.
+
+```js
+const trace_events = require('trace_events');
+const categories = ['node.perf', 'node.async_hooks'];
+const tracing = trace_events.createTracing({ categories });
+tracing.enable();
+// do stuff
+tracing.disable();
+```
+
+### `trace_events.getEnabledCategories()`
+<!-- YAML
+added: REPLACEME
+-->
+
+* Returns: {string}
+
+Returns a comma-separated list of all currently-enabled trace event
+categories. The current set of enabled trace event categories is determined
+by the *union* of all currently-enabled `Tracing` objects and any categories
+enabled using the `--trace-event-categories` flag.
+
+Given the file `test.js` below, the command
+`node --trace-event-categories node.perf test.js` will print
+`'node.async_hooks,node.perf'` to the console.
+
+```js
+const trace_events = require('trace_events');
+const t1 = trace_events.createTracing({ categories: ['node.async_hooks'] });
+const t2 = trace_events.createTracing({ categories: ['node.perf'] });
+const t3 = trace_events.createTracing({ categories: ['v8'] });
+
+t1.enable();
+t2.enable();
+
+console.log(trace_events.getEnabledCategories());
+```
+
 [Performance API]: perf_hooks.html
--- a/lib/internal/errors.js
+++ b/lib/internal/errors.js
@ -984,6 +984,9 @@ E('ERR_TLS_REQUIRED_SERVER_NAME',
 E('ERR_TLS_SESSION_ATTACK', 'TLS session renegotiation attack detected', Error);
 E('ERR_TLS_SNI_FROM_SERVER',
  'Cannot issue SNI from a TLS server-side socket', Error);
+E('ERR_TRACE_EVENTS_CATEGORY_REQUIRED',
+  'At least one category is required', TypeError);
+E('ERR_TRACE_EVENTS_UNAVAILABLE', 'Trace events are unavailable', Error);
 E('ERR_TRANSFORM_ALREADY_TRANSFORMING',
  'Calling transform done when still transforming', Error);

--- a/lib/internal/modules/cjs/helpers.js
+++ b/lib/internal/modules/cjs/helpers.js
@ -101,7 +101,8 @@ const builtinLibs = [
  'assert', 'async_hooks', 'buffer', 'child_process', 'cluster', 'crypto',
  'dgram', 'dns', 'domain', 'events', 'fs', 'http', 'http2', 'https', 'net',
  'os', 'path', 'perf_hooks', 'punycode', 'querystring', 'readline', 'repl',
-  'stream', 'string_decoder', 'tls', 'tty', 'url', 'util', 'v8', 'vm', 'zlib'
+  'stream', 'string_decoder', 'tls', 'trace_events', 'tty', 'url', 'util',
+  'v8', 'vm', 'zlib'
 ];

 if (typeof process.binding('inspector').open === 'function') {
--- a/lib/trace_events.js
+++ b/lib/trace_events.js
@ -0,0 +1,89 @@
+'use strict';
+
+const { hasTracing } = process.binding('config');
+const kHandle = Symbol('handle');
+const kEnabled = Symbol('enabled');
+const kCategories = Symbol('categories');
+
+const kMaxTracingCount = 10;
+
+const {
+  ERR_TRACE_EVENTS_CATEGORY_REQUIRED,
+  ERR_TRACE_EVENTS_UNAVAILABLE,
+  ERR_INVALID_ARG_TYPE
+} = require('internal/errors').codes;
+
+if (!hasTracing)
+  throw new ERR_TRACE_EVENTS_UNAVAILABLE();
+
+const { CategorySet, getEnabledCategories } = process.binding('trace_events');
+const { customInspectSymbol } = require('internal/util');
+const { format } = require('util');
+
+const enabledTracingObjects = new Set();
+
+class Tracing {
+  constructor(categories) {
+    this[kHandle] = new CategorySet(categories);
+    this[kCategories] = categories;
+    this[kEnabled] = false;
+  }
+
+  enable() {
+    if (!this[kEnabled]) {
+      this[kEnabled] = true;
+      this[kHandle].enable();
+      enabledTracingObjects.add(this);
+      if (enabledTracingObjects.size > kMaxTracingCount) {
+        process.emitWarning(
+          'Possible trace_events memory leak detected. There are more than ' +
+          `${kMaxTracingCount} enabled Tracing objects.`
+        );
+      }
+    }
+  }
+
+  disable() {
+    if (this[kEnabled]) {
+      this[kEnabled] = false;
+      this[kHandle].disable();
+      enabledTracingObjects.delete(this);
+    }
+  }
+
+  get enabled() {
+    return this[kEnabled];
+  }
+
+  get categories() {
+    return this[kCategories].join(',');
+  }
+
+  [customInspectSymbol](depth, opts) {
+    const obj = {
+      enabled: this.enabled,
+      categories: this.categories
+    };
+    return `Tracing ${format(obj)}`;
+  }
+}
+
+function createTracing(options) {
+  if (typeof options !== 'object' || options == null)
+    throw new ERR_INVALID_ARG_TYPE('options', 'object', options);
+
+  if (!Array.isArray(options.categories)) {
+    throw new ERR_INVALID_ARG_TYPE('options.categories', 'string[]',
+                                   options.categories);
+  }
+
+  if (options.categories.length <= 0)
+    throw new ERR_TRACE_EVENTS_CATEGORY_REQUIRED();
+
+  return new Tracing(options.categories);
+}
+
+module.exports = {
+  createTracing,
+  getEnabledCategories
+};
--- a/node.gyp
+++ b/node.gyp
@ -73,6 +73,7 @@
      'lib/tls.js',
      'lib/_tls_common.js',
      'lib/_tls_wrap.js',
+      'lib/trace_events.js',
      'lib/tty.js',
      'lib/url.js',
      'lib/util.js',
--- a/src/env-inl.h
+++ b/src/env-inl.h
@ -32,6 +32,7 @@
 #include "v8.h"
 #include "node_perf_common.h"
 #include "node_context_data.h"
+#include "tracing/agent.h"

 #include <stddef.h>
 #include <stdint.h>
@ -325,6 +326,10 @@ inline v8::Isolate* Environment::isolate() const {
  return isolate_;
 }

+inline tracing::Agent* Environment::tracing_agent() const {
+  return tracing_agent_;
+}
+
 inline Environment* Environment::from_immediate_check_handle(
    uv_check_t* handle) {
  return ContainerOf(&Environment::immediate_check_handle_, handle);
--- a/src/env.cc
+++ b/src/env.cc
@ -3,6 +3,7 @@
 #include "node_buffer.h"
 #include "node_platform.h"
 #include "node_file.h"
+#include "tracing/agent.h"

 #include <stdio.h>
 #include <algorithm>
@ -87,9 +88,11 @@ void InitThreadLocalOnce() {
 }

 Environment::Environment(IsolateData* isolate_data,
-                         Local<Context> context)
+                         Local<Context> context,
+                         tracing::Agent* tracing_agent)
    : isolate_(context->GetIsolate()),
      isolate_data_(isolate_data),
+      tracing_agent_(tracing_agent),
      immediate_info_(context->GetIsolate()),
      tick_info_(context->GetIsolate()),
      timer_base_(uv_now(isolate_data->event_loop())),
--- a/src/env.h
+++ b/src/env.h
@ -36,6 +36,7 @@
 #include "v8.h"
 #include "node.h"
 #include "node_http2_state.h"
+#include "tracing/agent.h"

 #include <list>
 #include <stdint.h>
@ -553,7 +554,9 @@ class Environment {
  static uv_key_t thread_local_env;
  static inline Environment* GetThreadLocalEnv();

-  Environment(IsolateData* isolate_data, v8::Local<v8::Context> context);
+  Environment(IsolateData* isolate_data,
+              v8::Local<v8::Context> context,
+              tracing::Agent* tracing_agent);
  ~Environment();

  void Start(int argc,
@ -585,6 +588,7 @@ class Environment {
  void StopProfilerIdleNotifier();

  inline v8::Isolate* isolate() const;
+  inline tracing::Agent* tracing_agent() const;
  inline uv_loop_t* event_loop() const;
  inline uint32_t watched_providers() const;

@ -780,6 +784,7 @@ class Environment {

  v8::Isolate* const isolate_;
  IsolateData* const isolate_data_;
+  tracing::Agent* const tracing_agent_;
  uv_check_t immediate_check_handle_;
  uv_idle_t immediate_idle_handle_;
  uv_prepare_t idle_prepare_handle_;
--- a/src/node.cc
+++ b/src/node.cc
@ -191,7 +191,6 @@ static node_module* modlist_builtin;
 static node_module* modlist_internal;
 static node_module* modlist_linked;
 static node_module* modlist_addon;
-static bool trace_enabled = false;
 static std::string trace_enabled_categories;  // NOLINT(runtime/string)
 static std::string trace_file_pattern =  // NOLINT(runtime/string)
  "node_trace.${rotation}.log";
@ -277,20 +276,12 @@ DebugOptions debug_options;
 static struct {
 #if NODE_USE_V8_PLATFORM
  void Initialize(int thread_pool_size) {
-    if (trace_enabled) {
-      tracing_agent_.reset(new tracing::Agent(trace_file_pattern));
-      platform_ = new NodePlatform(thread_pool_size,
+    tracing_agent_.reset(new tracing::Agent(trace_file_pattern));
+    platform_ = new NodePlatform(thread_pool_size,
        tracing_agent_->GetTracingController());
-      V8::InitializePlatform(platform_);
-      tracing::TraceEventHelper::SetTracingController(
+    V8::InitializePlatform(platform_);
+    tracing::TraceEventHelper::SetTracingController(
        tracing_agent_->GetTracingController());
-    } else {
-      tracing_agent_.reset(nullptr);
-      platform_ = new NodePlatform(thread_pool_size, nullptr);
-      V8::InitializePlatform(platform_);
-      tracing::TraceEventHelper::SetTracingController(
-        new v8::TracingController());
-    }
  }

  void Dispose() {
@ -323,13 +314,17 @@ static struct {
 #endif  // HAVE_INSPECTOR

  void StartTracingAgent() {
-    tracing_agent_->Start(trace_enabled_categories);
+    tracing_agent_->Enable(trace_enabled_categories);
  }

  void StopTracingAgent() {
    tracing_agent_->Stop();
  }

+  tracing::Agent* GetTracingAgent() const {
+    return tracing_agent_.get();
+  }
+
  NodePlatform* Platform() {
    return platform_;
  }
@ -348,11 +343,15 @@ static struct {
  }

  void StartTracingAgent() {
-    fprintf(stderr, "Node compiled with NODE_USE_V8_PLATFORM=0, "
-                    "so event tracing is not available.\n");
+    if (!trace_enabled_categories.empty()) {
+      fprintf(stderr, "Node compiled with NODE_USE_V8_PLATFORM=0, "
+                      "so event tracing is not available.\n");
+    }
  }
  void StopTracingAgent() {}

+  tracing::Agent* GetTracingAgent() const { return nullptr; }
+
  NodePlatform* Platform() {
    return nullptr;
  }
@ -1990,9 +1989,7 @@ static void WaitForInspectorDisconnect(Environment* env) {

 static void Exit(const FunctionCallbackInfo<Value>& args) {
  WaitForInspectorDisconnect(Environment::GetCurrent(args));
-  if (trace_enabled) {
-    v8_platform.StopTracingAgent();
-  }
+  v8_platform.StopTracingAgent();
  exit(args[0]->Int32Value());
 }

@ -3287,9 +3284,7 @@ void SetupProcessObject(Environment* env,

 void SignalExit(int signo) {
  uv_tty_reset_mode();
-  if (trace_enabled) {
-    v8_platform.StopTracingAgent();
-  }
+  v8_platform.StopTracingAgent();
 #ifdef __FreeBSD__
  // FreeBSD has a nasty bug, see RegisterSignalHandler for details
  struct sigaction sa;
@ -3789,7 +3784,8 @@ static void ParseArgs(int* argc,
    } else if (strcmp(arg, "--no-force-async-hooks-checks") == 0) {
      no_force_async_hooks_checks = true;
    } else if (strcmp(arg, "--trace-events-enabled") == 0) {
-      trace_enabled = true;
+      if (trace_enabled_categories.empty())
+        trace_enabled_categories = "v8,node,node.async_hooks";
    } else if (strcmp(arg, "--trace-event-categories") == 0) {
      const char* categories = argv[index + 1];
      if (categories == nullptr) {
@ -4422,7 +4418,8 @@ Environment* CreateEnvironment(IsolateData* isolate_data,
  Isolate* isolate = context->GetIsolate();
  HandleScope handle_scope(isolate);
  Context::Scope context_scope(context);
-  auto env = new Environment(isolate_data, context);
+  auto env = new Environment(isolate_data, context,
+                             v8_platform.GetTracingAgent());
  env->Start(argc, argv, exec_argc, exec_argv, v8_is_profiling);
  return env;
 }
@ -4471,7 +4468,7 @@ inline int Start(Isolate* isolate, IsolateData* isolate_data,
  HandleScope handle_scope(isolate);
  Local<Context> context = NewContext(isolate);
  Context::Scope context_scope(context);
-  Environment env(isolate_data, context);
+  Environment env(isolate_data, context, v8_platform.GetTracingAgent());
  env.Start(argc, argv, exec_argc, exec_argv, v8_is_profiling);

  const char* path = argc > 1 ? argv[1] : nullptr;
@ -4628,19 +4625,13 @@ int Start(int argc, char** argv) {

  v8_platform.Initialize(v8_thread_pool_size);
  // Enable tracing when argv has --trace-events-enabled.
-  if (trace_enabled) {
-    fprintf(stderr, "Warning: Trace event is an experimental feature "
-            "and could change at any time.\n");
-    v8_platform.StartTracingAgent();
-  }
+  v8_platform.StartTracingAgent();
  V8::Initialize();
  performance::performance_v8_start = PERFORMANCE_NOW();
  v8_initialized = true;
  const int exit_code =
      Start(uv_default_loop(), argc, argv, exec_argc, exec_argv);
-  if (trace_enabled) {
-    v8_platform.StopTracingAgent();
-  }
+  v8_platform.StopTracingAgent();
  v8_initialized = false;
  V8::Dispose();

--- a/src/node_config.cc
+++ b/src/node_config.cc
@ -56,6 +56,10 @@ static void Initialize(Local<Object> target,
  READONLY_BOOLEAN_PROPERTY("hasSmallICU");
 #endif  // NODE_HAVE_SMALL_ICU

+#if NODE_USE_V8_PLATFORM
+  READONLY_BOOLEAN_PROPERTY("hasTracing");
+#endif
+
  target->DefineOwnProperty(
      context,
      FIXED_ONE_BYTE_STRING(isolate, "icuDataDir"),
--- a/src/node_trace_events.cc
+++ b/src/node_trace_events.cc
@ -1,15 +1,98 @@
+#include "node.h"
 #include "node_internals.h"
 #include "tracing/agent.h"
+#include "env.h"
+#include "base_object-inl.h"
+
+#include <set>
+#include <string>

 namespace node {

+using v8::Array;
 using v8::Context;
 using v8::FunctionCallbackInfo;
+using v8::FunctionTemplate;
 using v8::Int32;
 using v8::Local;
 using v8::Object;
+using v8::String;
 using v8::Value;

+class NodeCategorySet : public BaseObject {
+ public:
+  ~NodeCategorySet() override {
+    // Verify that the thing was properly disabled before gc
+    CHECK_NE(enabled_, true);
+  }
+
+  static void New(const FunctionCallbackInfo<Value>& args);
+  static void Enable(const FunctionCallbackInfo<Value>& args);
+  static void Disable(const FunctionCallbackInfo<Value>& args);
+
+  const std::set<std::string>& GetCategories() { return categories_; }
+
+ private:
+  NodeCategorySet(Environment* env,
+                  Local<Object> wrap,
+                  std::set<std::string> categories) :
+        BaseObject(env, wrap), categories_(categories) {
+    MakeWeak<NodeCategorySet>(this);
+  }
+
+  bool enabled_ = false;
+  const std::set<std::string> categories_;
+};
+
+void NodeCategorySet::New(const FunctionCallbackInfo<Value>& args) {
+  Environment* env = Environment::GetCurrent(args);
+  std::set<std::string> categories;
+  CHECK(args[0]->IsArray());
+  Local<Array> cats = args[0].As<Array>();
+  for (size_t n = 0; n < cats->Length(); n++) {
+    Local<Value> category = cats->Get(env->context(), n).ToLocalChecked();
+    Utf8Value val(env->isolate(), category);
+    categories.emplace(*val);
+  }
+  CHECK_NE(env->tracing_agent(), nullptr);
+  new NodeCategorySet(env, args.This(), categories);
+}
+
+void NodeCategorySet::Enable(const FunctionCallbackInfo<Value>& args) {
+  Environment* env = Environment::GetCurrent(args);
+  NodeCategorySet* category_set;
+  ASSIGN_OR_RETURN_UNWRAP(&category_set, args.Holder());
+  CHECK_NE(category_set, nullptr);
+  auto categories = category_set->GetCategories();
+  if (!category_set->enabled_ && !categories.empty()) {
+    env->tracing_agent()->Enable(categories);
+    category_set->enabled_ = true;
+  }
+}
+
+void NodeCategorySet::Disable(const FunctionCallbackInfo<Value>& args) {
+  Environment* env = Environment::GetCurrent(args);
+  NodeCategorySet* category_set;
+  ASSIGN_OR_RETURN_UNWRAP(&category_set, args.Holder());
+  CHECK_NE(category_set, nullptr);
+  auto categories = category_set->GetCategories();
+  if (category_set->enabled_ && !categories.empty()) {
+    env->tracing_agent()->Disable(categories);
+    category_set->enabled_ = false;
+  }
+}
+
+void GetEnabledCategories(const FunctionCallbackInfo<Value>& args) {
+  Environment* env = Environment::GetCurrent(args);
+  std::string categories = env->tracing_agent()->GetEnabledCategories();
+  if (!categories.empty()) {
+    args.GetReturnValue().Set(
+      String::NewFromUtf8(env->isolate(),
+                          categories.c_str(),
+                          v8::NewStringType::kNormal).ToLocalChecked());
+  }
+}
+
 // The tracing APIs require category groups to be pointers to long-lived
 // strings. Those strings are stored here.
 static std::unordered_set<std::string> categoryGroups;
@ -140,6 +223,16 @@ void Initialize(Local<Object> target,

  env->SetMethod(target, "emit", Emit);
  env->SetMethod(target, "categoryGroupEnabled", CategoryGroupEnabled);
+  env->SetMethod(target, "getEnabledCategories", GetEnabledCategories);
+
+  Local<FunctionTemplate> category_set =
+      env->NewFunctionTemplate(NodeCategorySet::New);
+  category_set->InstanceTemplate()->SetInternalFieldCount(1);
+  env->SetProtoMethod(category_set, "enable", NodeCategorySet::Enable);
+  env->SetProtoMethod(category_set, "disable", NodeCategorySet::Disable);
+
+  target->Set(FIXED_ONE_BYTE_STRING(env->isolate(), "CategorySet"),
+              category_set->GetFunction());
 }

 }  // namespace node
--- a/src/tracing/agent.cc
+++ b/src/tracing/agent.cc
@ -5,55 +5,40 @@
 #include "tracing/node_trace_buffer.h"
 #include "tracing/node_trace_writer.h"

-#include "env-inl.h"
-
 namespace node {
 namespace tracing {

 using v8::platform::tracing::TraceConfig;
 using std::string;

-Agent::Agent(const std::string& log_file_pattern) {
-  int err = uv_loop_init(&tracing_loop_);
-  CHECK_EQ(err, 0);
-
-  NodeTraceWriter* trace_writer = new NodeTraceWriter(
-      log_file_pattern, &tracing_loop_);
-  TraceBuffer* trace_buffer = new NodeTraceBuffer(
-      NodeTraceBuffer::kBufferChunks, trace_writer, &tracing_loop_);
+Agent::Agent(const std::string& log_file_pattern)
+    : log_file_pattern_(log_file_pattern) {
  tracing_controller_ = new TracingController();
-  tracing_controller_->Initialize(trace_buffer);
+  tracing_controller_->Initialize(nullptr);
 }

-void Agent::Start(const string& enabled_categories) {
-  TraceConfig* trace_config = new TraceConfig();
-  if (!enabled_categories.empty()) {
-    std::stringstream category_list(enabled_categories);
-    while (category_list.good()) {
-      std::string category;
-      getline(category_list, category, ',');
-      trace_config->AddIncludedCategory(category.c_str());
-    }
-  } else {
-    trace_config->AddIncludedCategory("v8");
-    trace_config->AddIncludedCategory("node");
-    trace_config->AddIncludedCategory("node.async_hooks");
-  }
+void Agent::Start() {
+  if (started_)
+    return;
+
+  CHECK_EQ(uv_loop_init(&tracing_loop_), 0);
+
+  NodeTraceWriter* trace_writer =
+      new NodeTraceWriter(log_file_pattern_, &tracing_loop_);
+  TraceBuffer* trace_buffer = new NodeTraceBuffer(
+      NodeTraceBuffer::kBufferChunks, trace_writer, &tracing_loop_);
+  tracing_controller_->Initialize(trace_buffer);

  // This thread should be created *after* async handles are created
  // (within NodeTraceWriter and NodeTraceBuffer constructors).
  // Otherwise the thread could shut down prematurely.
-  int err = uv_thread_create(&thread_, ThreadCb, this);
-  CHECK_EQ(err, 0);
-
-  tracing_controller_->StartTracing(trace_config);
+  CHECK_EQ(0, uv_thread_create(&thread_, ThreadCb, this));
  started_ = true;
 }

 void Agent::Stop() {
-  if (!started_) {
+  if (!started_)
    return;
-  }
  // Perform final Flush on TraceBuffer. We don't want the tracing controller
  // to flush the buffer again on destruction of the V8::Platform.
  tracing_controller_->StopTracing();
@ -70,5 +55,73 @@ void Agent::ThreadCb(void* arg) {
  uv_run(&agent->tracing_loop_, UV_RUN_DEFAULT);
 }

+void Agent::Enable(const std::string& categories) {
+  if (!categories.empty()) {
+    std::stringstream category_list(categories);
+    while (category_list.good()) {
+      std::string category;
+      getline(category_list, category, ',');
+      categories_.insert(category.c_str());
+    }
+    RestartTracing();
+  }
+}
+
+void Agent::Enable(const std::set<std::string>& categories) {
+  if (!categories.empty()) {
+    categories_.insert(categories.begin(), categories.end());
+    RestartTracing();
+  }
+}
+
+void Agent::Disable(const std::set<std::string>& categories) {
+  if (!categories.empty()) {
+    for (auto category : categories) {
+      auto pos = categories_.lower_bound(category);
+      if (pos != categories_.end())
+        categories_.erase(pos);
+    }
+    RestartTracing();
+  }
+}
+
+void Agent::RestartTracing() {
+  static bool warned;
+  if (!warned) {
+    warned = true;
+    fprintf(stderr, "Warning: Trace event is an experimental feature "
+            "and could change at any time.\n");
+  }
+  Start();  // Start the agent if it hasn't already been started
+  tracing_controller_->StopTracing();
+  auto config = CreateTraceConfig();
+  if (config != nullptr)
+    tracing_controller_->StartTracing(config);
+}
+
+TraceConfig* Agent::CreateTraceConfig() {
+  if (categories_.empty())
+    return nullptr;
+  TraceConfig* trace_config = new TraceConfig();
+  for (auto category = categories_.begin();
+       category != categories_.end();
+       category = categories_.upper_bound(*category)) {
+    trace_config->AddIncludedCategory(category->c_str());
+  }
+  return trace_config;
+}
+
+std::string Agent::GetEnabledCategories() {
+  std::string categories;
+  for (auto category = categories_.begin();
+       category != categories_.end();
+       category = categories_.upper_bound(*category)) {
+    if (!categories.empty())
+      categories += ',';
+    categories += *category;
+  }
+  return categories;
+}
+
 }  // namespace tracing
 }  // namespace node
--- a/src/tracing/agent.h
+++ b/src/tracing/agent.h
@ -5,9 +5,14 @@
 #include "uv.h"
 #include "v8.h"

+#include <set>
+#include <string>
+
 namespace node {
 namespace tracing {

+using v8::platform::tracing::TraceConfig;
+
 class TracingController : public v8::platform::tracing::TracingController {
 public:
  TracingController() : v8::platform::tracing::TracingController() {}
@ -20,17 +25,29 @@ class TracingController : public v8::platform::tracing::TracingController {
 class Agent {
 public:
  explicit Agent(const std::string& log_file_pattern);
-  void Start(const std::string& enabled_categories);
  void Stop();

  TracingController* GetTracingController() { return tracing_controller_; }

+  void Enable(const std::string& categories);
+  void Enable(const std::set<std::string>& categories);
+  void Disable(const std::set<std::string>& categories);
+  std::string GetEnabledCategories();
+
 private:
  static void ThreadCb(void* arg);

+  void Start();
+  void RestartTracing();
+
+  TraceConfig* CreateTraceConfig();
+
+  const std::string& log_file_pattern_;
  uv_thread_t thread_;
  uv_loop_t tracing_loop_;
  bool started_ = false;
+
+  std::multiset<std::string> categories_;
  TracingController* tracing_controller_ = nullptr;
 };

--- a/test/common/index.js
+++ b/test/common/index.js
@ -30,6 +30,7 @@ const { exec, execSync, spawn, spawnSync } = require('child_process');
 const stream = require('stream');
 const util = require('util');
 const Timer = process.binding('timer_wrap').Timer;
+const { hasTracing } = process.binding('config');
 const { fixturesDir } = require('./fixtures');
 const tmpdir = require('./tmpdir');

@ -201,6 +202,12 @@ Object.defineProperty(exports, 'hasCrypto', {
  }
 });

+Object.defineProperty(exports, 'hasTracing', {
+  get: function() {
+    return Boolean(hasTracing);
+  }
+});
+
 Object.defineProperty(exports, 'hasFipsCrypto', {
  get: function() {
    return exports.hasCrypto && require('crypto').fips;
--- a/test/parallel/test-module-cjs-helpers.js
+++ b/test/parallel/test-module-cjs-helpers.js
@ -7,5 +7,5 @@ const { builtinLibs } = require('internal/modules/cjs/helpers');

 const hasInspector = process.config.variables.v8_enable_inspector === 1;

-const expectedLibs = hasInspector ? 32 : 31;
+const expectedLibs = hasInspector ? 33 : 32;
 assert.strictEqual(builtinLibs.length, expectedLibs);
--- a/test/parallel/test-trace-events-api.js
+++ b/test/parallel/test-trace-events-api.js
@ -0,0 +1,164 @@
+// Flags: --expose-gc --no-warnings
+'use strict';
+
+const common = require('../common');
+
+if (!common.hasTracing)
+  common.skip('missing trace events');
+
+const assert = require('assert');
+const cp = require('child_process');
+const path = require('path');
+const fs = require('fs');
+const tmpdir = require('../common/tmpdir');
+const {
+  createTracing,
+  getEnabledCategories
+} = require('trace_events');
+
+const isChild = process.argv[2] === 'child';
+const enabledCategories = isChild ? 'foo' : undefined;
+
+assert.strictEqual(getEnabledCategories(), enabledCategories);
+[1, 'foo', true, false, null, undefined].forEach((i) => {
+  common.expectsError(() => createTracing(i), {
+    code: 'ERR_INVALID_ARG_TYPE',
+    type: TypeError
+  });
+  common.expectsError(() => createTracing({ categories: i }), {
+    code: 'ERR_INVALID_ARG_TYPE',
+    type: TypeError
+  });
+});
+
+common.expectsError(
+  () => createTracing({ categories: [] }),
+  {
+    code: 'ERR_TRACE_EVENTS_CATEGORY_REQUIRED',
+    type: TypeError
+  }
+);
+
+const tracing = createTracing({ categories: [ 'node.perf' ] });
+
+assert.strictEqual(tracing.categories, 'node.perf');
+assert.strictEqual(tracing.enabled, false);
+
+assert.strictEqual(getEnabledCategories(), enabledCategories);
+tracing.enable();
+tracing.enable();  // purposefully enable twice to test calling twice
+assert.strictEqual(tracing.enabled, true);
+
+assert.strictEqual(getEnabledCategories(),
+                   isChild ? 'foo,node.perf' : 'node.perf');
+
+tracing.disable();
+assert.strictEqual(tracing.enabled, false);
+
+const tracing2 = createTracing({ categories: [ 'foo' ] });
+tracing2.enable();
+assert.strictEqual(getEnabledCategories(), 'foo');
+
+tracing2.disable();
+tracing2.disable();  // purposefully disable twice to test calling twice
+assert.strictEqual(getEnabledCategories(), enabledCategories);
+
+if (isChild) {
+  tracing.enable();
+  const { performance } = require('perf_hooks');
+
+  // Will emit mark and measure trace events
+  performance.mark('A');
+  setTimeout(() => {
+    performance.mark('B');
+    performance.measure('A to B', 'A', 'B');
+  }, 1);
+
+  // Intentional non-op, part of the test
+  function f() {}
+  const ff = performance.timerify(f);
+  ff();  // Will emit a timerify trace event
+} else {
+
+  // Test that enabled tracing references do not get garbage collected
+  // until after they are disabled.
+  {
+    {
+      let tracing3 = createTracing({ categories: [ 'abc' ] });
+      tracing3.enable();
+      assert.strictEqual(getEnabledCategories(), 'abc');
+      tracing3 = undefined;
+    }
+    global.gc();
+    assert.strictEqual(getEnabledCategories(), 'abc');
+    // Not able to disable the thing after this point, however.
+  }
+
+  {
+    common.expectWarning(
+      'Warning',
+      'Possible trace_events memory leak detected. There are more than ' +
+      '10 enabled Tracing objects.',
+      common.noWarnCode);
+    for (let n = 0; n < 10; n++) {
+      const tracing = createTracing({ categories: [ `a${n}` ] });
+      tracing.enable();
+    }
+  }
+
+  tmpdir.refresh();
+  process.chdir(tmpdir.path);
+
+  const expectedMarks = ['A', 'B'];
+  const expectedBegins = [
+    { cat: 'node,node.perf,node.perf.timerify', name: 'f' },
+    { cat: 'node,node.perf,node.perf.usertiming', name: 'A to B' }
+  ];
+  const expectedEnds = [
+    { cat: 'node,node.perf,node.perf.timerify', name: 'f' },
+    { cat: 'node,node.perf,node.perf.usertiming', name: 'A to B' }
+  ];
+
+  const proc = cp.fork(__filename,
+                       ['child'],
+                       { execArgv: [ '--expose-gc',
+                                     '--trace-event-categories',
+                                     'foo' ] });
+
+  proc.once('exit', common.mustCall(() => {
+    const file = path.join(tmpdir.path, 'node_trace.1.log');
+
+    assert(common.fileExists(file));
+    fs.readFile(file, common.mustCall((err, data) => {
+      const traces = JSON.parse(data.toString()).traceEvents;
+      assert.strictEqual(traces.length,
+                         expectedMarks.length +
+                         expectedBegins.length +
+                         expectedEnds.length);
+
+      traces.forEach((trace) => {
+        assert.strictEqual(trace.pid, proc.pid);
+        switch (trace.ph) {
+          case 'R':
+            assert.strictEqual(trace.cat,
+                               'node,node.perf,node.perf.usertiming');
+            assert.strictEqual(trace.name,
+                               expectedMarks.shift());
+            break;
+          case 'b':
+            const expectedBegin = expectedBegins.shift();
+            assert.strictEqual(trace.cat, expectedBegin.cat);
+            assert.strictEqual(trace.name, expectedBegin.name);
+            break;
+          case 'e':
+            const expectedEnd = expectedEnds.shift();
+            assert.strictEqual(trace.cat, expectedEnd.cat);
+            assert.strictEqual(trace.name, expectedEnd.name);
+            break;
+          default:
+            assert.fail('Unexpected trace event phase');
+        }
+      });
+    }));
+  }));
+}
--- a/test/parallel/test-trace-events-async-hooks.js
+++ b/test/parallel/test-trace-events-async-hooks.js
@ -13,8 +13,7 @@ tmpdir.refresh();
 process.chdir(tmpdir.path);

 const proc = cp.spawn(process.execPath,
-                      [ '--trace-events-enabled',
-                        '--trace-event-categories', 'node.async_hooks',
+                      [ '--trace-event-categories', 'node.async_hooks',
                        '-e', CODE ]);

 proc.once('exit', common.mustCall(() => {
--- a/test/parallel/test-trace-events-binding.js
+++ b/test/parallel/test-trace-events-binding.js
@ -25,8 +25,7 @@ tmpdir.refresh();
 process.chdir(tmpdir.path);

 const proc = cp.spawn(process.execPath,
-                      [ '--trace-events-enabled',
-                        '--trace-event-categories', 'custom',
+                      [ '--trace-event-categories', 'custom',
                        '-e', CODE ]);

 proc.once('exit', common.mustCall(() => {
--- a/test/parallel/test-trace-events-bootstrap.js
+++ b/test/parallel/test-trace-events-bootstrap.js
@ -32,7 +32,6 @@ if (process.argv[2] === 'child') {
  const proc = cp.fork(__filename,
                       [ 'child' ], {
                         execArgv: [
-                           '--trace-events-enabled',
                           '--trace-event-categories',
                           'node.bootstrap'
                         ]
--- a/test/parallel/test-trace-events-category-used.js
+++ b/test/parallel/test-trace-events-category-used.js
@ -13,7 +13,7 @@ process.chdir(tmpdir.path);

 const procEnabled = cp.spawn(
  process.execPath,
-  [ '--trace-events-enabled', '--trace-event-categories', 'custom', '-e', CODE ]
+  [ '--trace-event-categories', 'custom', '-e', CODE ]
 );
 let procEnabledOutput = '';

@ -25,7 +25,7 @@ procEnabled.once('exit', common.mustCall(() => {

 const procDisabled = cp.spawn(
  process.execPath,
-  [ '--trace-events-enabled', '--trace-event-categories', 'other', '-e', CODE ]
+  [ '--trace-event-categories', 'other', '-e', CODE ]
 );
 let procDisabledOutput = '';

--- a/test/parallel/test-trace-events-none.js
+++ b/test/parallel/test-trace-events-none.js
@ -13,7 +13,7 @@ process.chdir(tmpdir.path);

 const proc_no_categories = cp.spawn(
  process.execPath,
-  [ '--trace-events-enabled', '--trace-event-categories', '""', '-e', CODE ]
+  [ '--trace-event-categories', '""', '-e', CODE ]
 );

 proc_no_categories.once('exit', common.mustCall(() => {
--- a/test/parallel/test-trace-events-perf.js
+++ b/test/parallel/test-trace-events-perf.js
@ -39,7 +39,6 @@ if (process.argv[2] === 'child') {
                         'child'
                       ], {
                         execArgv: [
-                           '--trace-events-enabled',
                           '--trace-event-categories',
                           'node.perf'
                         ]
--- a/tools/doc/type-parser.js
+++ b/tools/doc/type-parser.js
@ -114,6 +114,8 @@ const customTypesMap = {
  'tls.Server': 'tls.html#tls_class_tls_server',
  'tls.TLSSocket': 'tls.html#tls_class_tls_tlssocket',

+  'Tracing': 'tracing.html#tracing_tracing_object',
+
  'URL': 'url.html#url_the_whatwg_url_api',
  'URLSearchParams': 'url.html#url_class_urlsearchparams'
 };