V8 Sampling Profiler: Collect V8 sample trace events on Linux and MacOS

content/renderer/devtools/v8_sampling_profiler.cc

 // Copyright 2014 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include "content/renderer/devtools/v8_sampling_profiler.h"
 
+#if defined(OS_POSIX)
+#include <signal.h>

[yurys, 2015/03/18 12:49:03]

You'll likely need <ucontext.h> and <mach/mach.h> too.

[alph, 2015/03/18 13:37:49]

It compiles so far. I'll add these includes when it fails.

+#define USE_SIGNALS
+#endif
+
 #include "base/format_macros.h"
-#include "base/strings/string_util.h"
+#include "base/strings/stringprintf.h"
 #include "base/synchronization/cancellation_flag.h"
 #include "base/threading/platform_thread.h"
 #include "base/trace_event/trace_event.h"
 #include "base/trace_event/trace_event_argument.h"
 #include "content/renderer/render_thread_impl.h"
 #include "v8/include/v8.h"
 
+using base::trace_event::ConvertableToTraceFormat;
 using base::trace_event::TraceLog;
 using base::trace_event::TracedValue;
 using v8::Isolate;
 
 namespace content {
 
 namespace {
 
 std::string PtrToString(const void* value) {
-  char buffer[20];
-  base::snprintf(buffer, sizeof(buffer), "0x%" PRIx64,
-                 static_cast<uint64>(reinterpret_cast<intptr_t>(value)));
-  return buffer;
+  return base::StringPrintf(
+      "0x%" PRIx64, static_cast<uint64>(reinterpret_cast<intptr_t>(value)));
+}
+
+// Lock-free cache-friendly sampling circular queue for large
+// records. Intended for fast transfer of large records between a
+// single producer and a single consumer. If the queue is full,
+// StartEnqueue will return nullptr. The queue is designed with
+// a goal in mind to evade cache lines thrashing by preventing
+// simultaneous reads and writes to adjanced memory locations.
+template <typename T, unsigned Length>
+class SamplingCircularQueue {

[yurys, 2015/03/18 12:49:04]

Let's move it in a separate file.

[alph, 2015/03/18 13:37:49]

Done.

+ public:
+  // Executed on the application thread.
+  SamplingCircularQueue();
+  ~SamplingCircularQueue();
+
+  // StartEnqueue returns a pointer to a memory location for storing the next
+  // record or nullptr if all entries are full at the moment.
+  T* StartEnqueue();
+  // Notifies the queue that the producer has complete writing data into the
+  // memory returned by StartEnqueue and it can be passed to the consumer.
+  void FinishEnqueue();
+
+  // Executed on the consumer (analyzer) thread.
+  // Retrieves, but does not remove, the head of this queue, returning nullptr
+  // if this queue is empty. After the record had been read by a consumer,
+  // Remove must be called.
+  T* Peek();
+  void Remove();
+
+ private:
+  // Reserved values for the entry marker.
+  enum {
+    kEmpty,  // Marks clean (processed) entries.
+    kFull    // Marks entries already filled by the producer but not yet
+             // completely processed by the consumer.
+  };
+
+  struct /*V8_ALIGNED(PROCESSOR_CACHE_LINE_SIZE)*/ Entry {
+    Entry() : marker(kEmpty) {}
+    T record;
+    base::subtle::Atomic32 marker;
+  };
+
+  Entry* Next(Entry* entry);
+
+  Entry buffer_[Length];
+  /*V8_ALIGNED(PROCESSOR_CACHE_LINE_SIZE)*/ Entry* enqueue_pos_;
+  /*V8_ALIGNED(PROCESSOR_CACHE_LINE_SIZE)*/ Entry* dequeue_pos_;
+
+  DISALLOW_COPY_AND_ASSIGN(SamplingCircularQueue);
+};
+
+template <typename T, unsigned L>
+SamplingCircularQueue<T, L>::SamplingCircularQueue()
+    : enqueue_pos_(buffer_), dequeue_pos_(buffer_) {
+}
+
+template <typename T, unsigned L>
+SamplingCircularQueue<T, L>::~SamplingCircularQueue() {
+}
+
+template <typename T, unsigned L>
+T* SamplingCircularQueue<T, L>::Peek() {
+  base::subtle::MemoryBarrier();
+  if (base::subtle::Acquire_Load(&dequeue_pos_->marker) == kFull) {
+    return &dequeue_pos_->record;
+  }
+  return nullptr;
+}
+
+template <typename T, unsigned L>
+void SamplingCircularQueue<T, L>::Remove() {
+  base::subtle::Release_Store(&dequeue_pos_->marker, kEmpty);
+  dequeue_pos_ = Next(dequeue_pos_);
+}
+
+template <typename T, unsigned L>
+T* SamplingCircularQueue<T, L>::StartEnqueue() {
+  base::subtle::MemoryBarrier();
+  if (base::subtle::Acquire_Load(&enqueue_pos_->marker) == kEmpty) {
+    return &enqueue_pos_->record;
+  }
+  return nullptr;
+}
+
+template <typename T, unsigned L>
+void SamplingCircularQueue<T, L>::FinishEnqueue() {
+  base::subtle::Release_Store(&enqueue_pos_->marker, kFull);
+  enqueue_pos_ = Next(enqueue_pos_);
+}
+
+template <typename T, unsigned L>
+typename SamplingCircularQueue<T, L>::Entry* SamplingCircularQueue<T, L>::Next(
+    Entry* entry) {
+  Entry* next = entry + 1;
+  if (next == &buffer_[L])
+    return buffer_;
+  return next;
+}
+
+class SampleRecord {
+ public:
+  static const int kMaxFramesCountLog2 = 8;
+  static const unsigned kMaxFramesCount = (1u << kMaxFramesCountLog2) - 1;
+
+  SampleRecord() {}
+
+  base::TimeTicks timestamp() const { return timestamp_; }
+  void Collect(v8::Isolate* isolate,
+               base::TimeTicks timestamp,
+               const v8::RegisterState& state);
+  scoped_refptr<ConvertableToTraceFormat> ToTraceFormat() const;
+
+ private:
+  base::TimeTicks timestamp_;
+  unsigned vm_state_ : 4;
+  unsigned frames_count_ : kMaxFramesCountLog2;
+  const void* frames_[kMaxFramesCount];
+
+  DISALLOW_COPY_AND_ASSIGN(SampleRecord);
+};
+
+void SampleRecord::Collect(v8::Isolate* isolate,
+                           base::TimeTicks timestamp,
+                           const v8::RegisterState& state) {
+  v8::SampleInfo sample_info;
+  isolate->GetStackSample(state, (void**)frames_, kMaxFramesCount,
+                          &sample_info);
+  timestamp_ = timestamp;
+  frames_count_ = sample_info.frames_count;
+  vm_state_ = sample_info.vm_state;
+}
+
+scoped_refptr<ConvertableToTraceFormat> SampleRecord::ToTraceFormat() const {
+  scoped_refptr<TracedValue> data(new TracedValue());
+  const char* vm_state = nullptr;
+  switch (vm_state_) {
+    case v8::StateTag::JS:
+      vm_state = "js";
+      break;
+    case v8::StateTag::GC:
+      vm_state = "gc";
+      break;
+    case v8::StateTag::COMPILER:
+      vm_state = "compiler";
+      break;
+    case v8::StateTag::OTHER:
+      vm_state = "other";
+      break;
+    case v8::StateTag::EXTERNAL:
+      vm_state = "external";
+      break;
+    case v8::StateTag::IDLE:
+      vm_state = "idle";
+      break;
+    default:
+      NOTREACHED();
+  }
+  data->SetString("vm_state", vm_state);
+  data->BeginArray("stack");
+  for (unsigned i = 0; i < frames_count_; ++i) {
+    data->AppendString(PtrToString(frames_[i]));
+  }
+  data->EndArray();
+  return data;
 }
 
 }  // namespace
 
 // The class implements a sampler responsible for sampling a single thread.
 class Sampler {
  public:
-  explicit Sampler(Isolate* isolate) : isolate_(isolate) { DCHECK(isolate_); }
+  Sampler(base::PlatformThreadHandle thread_handle,
+          base::PlatformThreadId thread_id,
+          Isolate* isolate);
+  ~Sampler();
 
   static scoped_ptr<Sampler> CreateForCurrentThread();
+  static Sampler* GetInstance() { return tls_instance_.Pointer()->Get(); }
 
   // These methods are called from the sampling thread.
   void Start();
   void Stop();
   void Sample();
 
+  void DoSample(const v8::RegisterState& state);
+
+  bool EventsCollectedForTest() const {
+    return base::subtle::NoBarrier_Load(&code_added_events_count_) != 0 ||
+           base::subtle::NoBarrier_Load(&samples_count_) != 0;
+  }
+
  private:
   static void InstallJitCodeEventHandler(Isolate* isolate, void* data);
   static void HandleJitCodeEvent(const v8::JitCodeEvent* event);
-  static scoped_refptr<base::trace_event::ConvertableToTraceFormat>
-      JitCodeEventToTraceFormat(const v8::JitCodeEvent* event);
+  static scoped_refptr<ConvertableToTraceFormat> JitCodeEventToTraceFormat(
+      const v8::JitCodeEvent* event);
 
+  void InjectPendingEvents();
+
+  static const unsigned kNumberOfSamples = 10;
+  typedef SamplingCircularQueue<SampleRecord, kNumberOfSamples> SamplingQueue;
+
+  base::PlatformThreadId thread_id_;
+  base::PlatformThreadHandle thread_handle_;
   Isolate* isolate_;
+  scoped_ptr<SamplingQueue> samples_data_;
+  base::subtle::Atomic32 code_added_events_count_;
+  base::subtle::Atomic32 samples_count_;
+
+  static base::LazyInstance<base::ThreadLocalPointer<Sampler>>::Leaky
+      tls_instance_;
 };
 
+base::LazyInstance<base::ThreadLocalPointer<Sampler>>::Leaky
+    Sampler::tls_instance_ = LAZY_INSTANCE_INITIALIZER;
+
+Sampler::Sampler(base::PlatformThreadHandle thread_handle,
+                 base::PlatformThreadId thread_id,
+                 Isolate* isolate)
+    : thread_id_(thread_id),
+      thread_handle_(thread_handle),
+      isolate_(isolate),
+      code_added_events_count_(0),
+      samples_count_(0) {
+  DCHECK(isolate_);
+  DCHECK(!GetInstance());
+  tls_instance_.Pointer()->Set(this);
+}
+
+Sampler::~Sampler() {
+  DCHECK(GetInstance());
+  tls_instance_.Pointer()->Set(nullptr);

[yurys, 2015/03/18 12:49:03]

It is not safe to access TLS storage in signal handler. We should come up with
something smarter than this.

[alph, 2015/03/18 13:37:49]

It should be safe provided that:
- the tls variable is already allocated/initialized
- the signal handler only reads the variable.

+}
+
 // static
 scoped_ptr<Sampler> Sampler::CreateForCurrentThread() {
-  return scoped_ptr<Sampler>(new Sampler(Isolate::GetCurrent()));
+  return scoped_ptr<Sampler>(new Sampler(base::PlatformThread::CurrentHandle(),
+                                         base::PlatformThread::CurrentId(),
+                                         Isolate::GetCurrent()));
 }
 
 void Sampler::Start() {
+  samples_data_.reset(new SamplingQueue());
   v8::JitCodeEventHandler handler = &HandleJitCodeEvent;
   isolate_->RequestInterrupt(&InstallJitCodeEventHandler,
                              reinterpret_cast<void*>(handler));
 }
 
 void Sampler::Stop() {
   isolate_->RequestInterrupt(&InstallJitCodeEventHandler, nullptr);
+  samples_data_.reset();
 }
 
 void Sampler::Sample() {
+#if defined(USE_SIGNALS)
+  int error = pthread_kill(thread_handle_.platform_handle(), SIGPROF);
+  if (error) {
+    LOG(ERROR) << "pthread_kill failed with error " << error << " "
+               << strerror(error);
+  }
+  InjectPendingEvents();
+#endif
+}
+
+void Sampler::DoSample(const v8::RegisterState& state) {
+  // Called in the sampled thread signal handler.
+  // Because of that it is not allowed to do any memory allocation here.
+  base::TimeTicks timestamp = base::TimeTicks::NowFromSystemTraceTime();
+  SampleRecord* record = samples_data_->StartEnqueue();
+  if (!record) {
+    LOG(ERROR) << "No space left in the sampling buffer";

[yurys, 2015/03/18 12:49:04]

I'm pretty much sure it will allocate.

[alph, 2015/03/18 13:37:49]

Acknowledged.

+    return;
+  }
+  record->Collect(isolate_, timestamp, state);
+  samples_data_->FinishEnqueue();
+  base::subtle::NoBarrier_AtomicIncrement(&samples_count_, 1);
+}
+
+void Sampler::InjectPendingEvents() {
+  SampleRecord* record = samples_data_->Peek();
+  while (record) {
+    TRACE_EVENT_SAMPLE_WITH_TID_AND_TIMESTAMP1(
+        TRACE_DISABLED_BY_DEFAULT("v8.cpu_profile"), "V8Sample", thread_id_,
+        (record->timestamp() - base::TimeTicks()).InMicroseconds(), "data",
+        record->ToTraceFormat());
+    samples_data_->Remove();
+    record = samples_data_->Peek();
+  }
 }
 
 // static
 void Sampler::InstallJitCodeEventHandler(Isolate* isolate, void* data) {
   // Called on the sampled V8 thread.
   TRACE_EVENT0(TRACE_DISABLED_BY_DEFAULT("v8.cpu_profile"),
                "Sampler::InstallJitCodeEventHandler");
   v8::JitCodeEventHandler handler =
       reinterpret_cast<v8::JitCodeEventHandler>(data);
   isolate->SetJitCodeEventHandler(
       v8::JitCodeEventOptions::kJitCodeEventEnumExisting, handler);
 }
 
 // static
 void Sampler::HandleJitCodeEvent(const v8::JitCodeEvent* event) {
   // Called on the sampled V8 thread.
+  Sampler* sampler = GetInstance();
+  // The sampler may have already been destroyed.
+  // That's fine, we're not interested in these events anymore.
+  if (!sampler)
+    return;
   switch (event->type) {
     case v8::JitCodeEvent::CODE_ADDED:
       TRACE_EVENT_INSTANT1(TRACE_DISABLED_BY_DEFAULT("v8.cpu_profile"),
                            "JitCodeAdded", TRACE_EVENT_SCOPE_THREAD, "data",
                            JitCodeEventToTraceFormat(event));
+      base::subtle::NoBarrier_AtomicIncrement(
+          &sampler->code_added_events_count_, 1);
       break;
 
     case v8::JitCodeEvent::CODE_MOVED:
       TRACE_EVENT_INSTANT1(TRACE_DISABLED_BY_DEFAULT("v8.cpu_profile"),
                            "JitCodeMoved", TRACE_EVENT_SCOPE_THREAD, "data",
                            JitCodeEventToTraceFormat(event));
       break;
 
     case v8::JitCodeEvent::CODE_REMOVED:
       TRACE_EVENT_INSTANT1(TRACE_DISABLED_BY_DEFAULT("v8.cpu_profile"),
                            "JitCodeRemoved", TRACE_EVENT_SCOPE_THREAD, "data",
                            JitCodeEventToTraceFormat(event));
       break;
 
     case v8::JitCodeEvent::CODE_ADD_LINE_POS_INFO:
     case v8::JitCodeEvent::CODE_START_LINE_INFO_RECORDING:
     case v8::JitCodeEvent::CODE_END_LINE_INFO_RECORDING:
       break;
   }
 }
 
 // static
-scoped_refptr<base::trace_event::ConvertableToTraceFormat>
-Sampler::JitCodeEventToTraceFormat(const v8::JitCodeEvent* event) {
-  // Called on the sampled thread.
+scoped_refptr<ConvertableToTraceFormat> Sampler::JitCodeEventToTraceFormat(
+    const v8::JitCodeEvent* event) {
   switch (event->type) {
     case v8::JitCodeEvent::CODE_ADDED: {
       scoped_refptr<TracedValue> data(new TracedValue());
       data->SetString("code_start", PtrToString(event->code_start));
       data->SetInteger("code_len", static_cast<unsigned>(event->code_len));
       data->SetString("name", std::string(event->name.str, event->name.len));
       return data;
     }
 
     case v8::JitCodeEvent::CODE_MOVED: {
@@ skipping 28 matching lines @@
 
   void Start();
   void Stop();
 
  private:
   void Sample();
   void InstallSamplers();
   void RemoveSamplers();
   void StartSamplers();
   void StopSamplers();
+
+  static void InstallSignalHandler();
+  static void RestoreSignalHandler();
+#ifdef USE_SIGNALS
+  static void HandleProfilerSignal(int signal, siginfo_t* info, void* context);
+#endif
+
   static void HandleJitCodeEvent(const v8::JitCodeEvent* event);
 
   Sampler* render_thread_sampler_;
   base::CancellationFlag cancellation_flag_;
   base::WaitableEvent* waitable_event_for_testing_;
   base::PlatformThreadHandle sampling_thread_handle_;
   std::vector<Sampler*> samplers_;
 
+  static bool signal_handler_installed_;

[yurys, 2015/03/18 12:49:03]

Should be behind #ifdef

[alph, 2015/03/18 13:37:49]

Done.

+#ifdef USE_SIGNALS
+  static struct sigaction old_signal_handler_;
+#endif
+
   DISALLOW_COPY_AND_ASSIGN(V8SamplingThread);
 };
 
+bool V8SamplingThread::signal_handler_installed_;
+#ifdef USE_SIGNALS
+struct sigaction V8SamplingThread::old_signal_handler_;
+#endif
+
 V8SamplingThread::V8SamplingThread(Sampler* render_thread_sampler,
                                    base::WaitableEvent* event)
     : render_thread_sampler_(render_thread_sampler),
       waitable_event_for_testing_(event) {
 }
 
 void V8SamplingThread::ThreadMain() {
   base::PlatformThread::SetName("V8SamplingProfilerThread");
   InstallSamplers();
   StartSamplers();
+  InstallSignalHandler();
   const int kSamplingFrequencyMicroseconds = 1000;
   while (!cancellation_flag_.IsSet()) {
     Sample();
-    if (waitable_event_for_testing_) {
+    if (waitable_event_for_testing_ &&
+        render_thread_sampler_->EventsCollectedForTest()) {
       waitable_event_for_testing_->Signal();
     }
+    // TODO(alph): make the samples firing interval not depend on the sample
+    // taking duration.
     base::PlatformThread::Sleep(
         base::TimeDelta::FromMicroseconds(kSamplingFrequencyMicroseconds));
   }
+  RestoreSignalHandler();
   StopSamplers();
   RemoveSamplers();
 }
 
 void V8SamplingThread::Sample() {
   for (Sampler* sampler : samplers_) {
     sampler->Sample();
   }
 }
 
@@ skipping 12 matching lines @@
     sampler->Start();
   }
 }
 
 void V8SamplingThread::StopSamplers() {
   for (Sampler* sampler : samplers_) {
     sampler->Stop();
   }
 }
 
+// static
+void V8SamplingThread::InstallSignalHandler() {
+#ifdef USE_SIGNALS
+  // There must be the only one!
+  DCHECK(!signal_handler_installed_);
+  struct sigaction sa;
+  sa.sa_sigaction = &HandleProfilerSignal;
+  sigemptyset(&sa.sa_mask);
+  sa.sa_flags = SA_RESTART | SA_SIGINFO;
+  signal_handler_installed_ =
+      (sigaction(SIGPROF, &sa, &old_signal_handler_) == 0);
+#endif
+}
+
+// static
+void V8SamplingThread::RestoreSignalHandler() {
+#ifdef USE_SIGNALS
+  if (!signal_handler_installed_)
+    return;
+  sigaction(SIGPROF, &old_signal_handler_, 0);
+  signal_handler_installed_ = false;
+#endif
+}
+
+#ifdef USE_SIGNALS
+// static
+void V8SamplingThread::HandleProfilerSignal(int signal,
+                                            siginfo_t* info,
+                                            void* context) {
+  if (signal != SIGPROF)
+    return;
+  ucontext_t* ucontext = reinterpret_cast<ucontext_t*>(context);
+  mcontext_t& mcontext = ucontext->uc_mcontext;
+  v8::RegisterState state;
+
+#if defined(OS_ANDROID)
+  // TODO(alph): Add support for Android
+  ALLOW_UNUSED_LOCAL(mcontext);
+
+#elif defined(OS_MACOSX)
+#if ARCH_CPU_64_BITS
+  state.pc = reinterpret_cast<void*>(mcontext->__ss.__rip);
+  state.sp = reinterpret_cast<void*>(mcontext->__ss.__rsp);
+  state.fp = reinterpret_cast<void*>(mcontext->__ss.__rbp);
+#elif ARCH_CPU_32_BITS
+  state.pc = reinterpret_cast<void*>(mcontext->__ss.__eip);
+  state.sp = reinterpret_cast<void*>(mcontext->__ss.__esp);
+  state.fp = reinterpret_cast<void*>(mcontext->__ss.__ebp);
+#endif  // ARCH_CPU_32_BITS
+
+#else
+#if ARCH_CPU_64_BITS
+  state.pc = reinterpret_cast<void*>(mcontext.gregs[REG_RIP]);
+  state.sp = reinterpret_cast<void*>(mcontext.gregs[REG_RSP]);
+  state.fp = reinterpret_cast<void*>(mcontext.gregs[REG_RBP]);
+#elif ARCH_CPU_32_BITS
+  state.pc = reinterpret_cast<void*>(mcontext.gregs[REG_EIP]);
+  state.sp = reinterpret_cast<void*>(mcontext.gregs[REG_ESP]);
+  state.fp = reinterpret_cast<void*>(mcontext.gregs[REG_EBP]);
+#endif  // ARCH_CPU_32_BITS
+#endif
+  Sampler::GetInstance()->DoSample(state);
+}
+#endif
+
 void V8SamplingThread::Start() {
   if (!base::PlatformThread::Create(0, this, &sampling_thread_handle_)) {
-    DCHECK(false) << "failed to create thread";
+    DCHECK(false) << "failed to create sampling thread";
   }
 }
 
 void V8SamplingThread::Stop() {
   cancellation_flag_.Set();
   base::PlatformThread::Join(sampling_thread_handle_);
 }
 
 V8SamplingProfiler::V8SamplingProfiler(bool underTest)
     : sampling_thread_(nullptr),
-      render_thread_sampler_(Sampler::CreateForCurrentThread()) {
+      render_thread_sampler_(Sampler::CreateForCurrentThread()),
+      message_loop_proxy_(base::MessageLoopProxy::current()) {
   DCHECK(underTest || RenderThreadImpl::current());
   // Force the "v8.cpu_profile" category to show up in the trace viewer.
   TraceLog::GetCategoryGroupEnabled(
       TRACE_DISABLED_BY_DEFAULT("v8.cpu_profile"));
   TraceLog::GetInstance()->AddEnabledStateObserver(this);
 }
 
 V8SamplingProfiler::~V8SamplingProfiler() {
   TraceLog::GetInstance()->RemoveEnabledStateObserver(this);
   DCHECK(!sampling_thread_.get());
 }
 
+void V8SamplingProfiler::StartSamplingThread() {
+  DCHECK(!sampling_thread_.get());
+  sampling_thread_.reset(new V8SamplingThread(
+      render_thread_sampler_.get(), waitable_event_for_testing_.get()));
+  sampling_thread_->Start();
+}
+
 void V8SamplingProfiler::OnTraceLogEnabled() {
   bool enabled;
   TRACE_EVENT_CATEGORY_GROUP_ENABLED(
       TRACE_DISABLED_BY_DEFAULT("v8.cpu_profile"), &enabled);
   if (!enabled)
     return;
 
   // Do not enable sampling profiler in continuous mode, as losing
   // Jit code events may not be afforded.
+  // TODO(alph): add support of infinite recording of meta trace events.
   base::trace_event::TraceRecordMode record_mode =
       TraceLog::GetInstance()->GetCurrentTraceOptions().record_mode;
   if (record_mode == base::trace_event::TraceRecordMode::RECORD_CONTINUOUSLY)
     return;
 
-  DCHECK(!sampling_thread_.get());
-  sampling_thread_.reset(new V8SamplingThread(
-      render_thread_sampler_.get(), waitable_event_for_testing_.get()));
-  sampling_thread_->Start();
+  message_loop_proxy_->PostTask(
+      FROM_HERE, base::Bind(&V8SamplingProfiler::StartSamplingThread,
+                            base::Unretained(this)));
 }
 
 void V8SamplingProfiler::OnTraceLogDisabled() {
   if (!sampling_thread_.get())
     return;
   sampling_thread_->Stop();
   sampling_thread_.reset();
 }
 
 void V8SamplingProfiler::EnableSamplingEventForTesting() {
   waitable_event_for_testing_.reset(new base::WaitableEvent(false, false));
 }
 
 void V8SamplingProfiler::WaitSamplingEventForTesting() {
   waitable_event_for_testing_->Wait();
 }
 
 }  // namespace blink