Support parallel captures from the StackSamplingProfiler.

base/profiler/stack_sampling_profiler.cc

Index: base/profiler/stack_sampling_profiler.cc
diff --git a/base/profiler/stack_sampling_profiler.cc b/base/profiler/stack_sampling_profiler.cc
index fa98bed116bb9b759ab610e5e0cd7ec77abc44a9..0263a9f81af95b8d605e09c685d9fd21e9489bcb 100644
--- a/base/profiler/stack_sampling_profiler.cc
+++ b/base/profiler/stack_sampling_profiler.cc
@@ -5,16 +5,23 @@
 #include "base/profiler/stack_sampling_profiler.h"
 
 #include <algorithm>
+#include <map>
 #include <utility>
 
+#include "base/atomic_sequence_num.h"
+#include "base/atomicops.h"
 #include "base/bind.h"
 #include "base/bind_helpers.h"
 #include "base/callback.h"
 #include "base/lazy_instance.h"
 #include "base/location.h"
 #include "base/macros.h"
+#include "base/memory/ptr_util.h"
+#include "base/memory/singleton.h"
 #include "base/profiler/native_stack_sampler.h"
 #include "base/synchronization/lock.h"
+#include "base/threading/thread.h"
+#include "base/threading/thread_restrictions.h"
 #include "base/threading/thread_task_runner_handle.h"
 #include "base/timer/elapsed_timer.h"
 
@@ -22,65 +29,9 @@ namespace base {
 
 namespace {
 
-// Used to ensure only one profiler is running at a time.
-LazyInstance<Lock>::Leaky concurrent_profiling_lock = LAZY_INSTANCE_INITIALIZER;
-
-// AsyncRunner ----------------------------------------------------------------
-
-// Helper class to allow a profiler to be run completely asynchronously from the
-// initiator, without being concerned with the profiler's lifetime.
-class AsyncRunner {
- public:
-  // Sets up a profiler and arranges for it to be deleted on its completed
-  // callback.
-  static void Run(PlatformThreadId thread_id,
-                  const StackSamplingProfiler::SamplingParams& params,
-                  const StackSamplingProfiler::CompletedCallback& callback);
-
- private:
-  AsyncRunner();
-
-  // Runs the callback and deletes the AsyncRunner instance. |profiles| is not
-  // const& because it must be passed with std::move.
-  static void RunCallbackAndDeleteInstance(
-      std::unique_ptr<AsyncRunner> object_to_be_deleted,
-      const StackSamplingProfiler::CompletedCallback& callback,
-      scoped_refptr<SingleThreadTaskRunner> task_runner,
-      StackSamplingProfiler::CallStackProfiles profiles);
-
-  std::unique_ptr<StackSamplingProfiler> profiler_;
-
-  DISALLOW_COPY_AND_ASSIGN(AsyncRunner);
-};
-
-// static
-void AsyncRunner::Run(
-    PlatformThreadId thread_id,
-    const StackSamplingProfiler::SamplingParams& params,
-    const StackSamplingProfiler::CompletedCallback &callback) {
-  std::unique_ptr<AsyncRunner> runner(new AsyncRunner);
-  AsyncRunner* temp_ptr = runner.get();
-  temp_ptr->profiler_.reset(
-      new StackSamplingProfiler(thread_id, params,
-                                Bind(&AsyncRunner::RunCallbackAndDeleteInstance,
-                                     Passed(&runner), callback,
-                                     ThreadTaskRunnerHandle::Get())));
-  // The callback won't be called until after Start(), so temp_ptr will still
-  // be valid here.
-  temp_ptr->profiler_->Start();
-}
-
-AsyncRunner::AsyncRunner() {}
-
-void AsyncRunner::RunCallbackAndDeleteInstance(
-    std::unique_ptr<AsyncRunner> object_to_be_deleted,
-    const StackSamplingProfiler::CompletedCallback& callback,
-    scoped_refptr<SingleThreadTaskRunner> task_runner,
-    StackSamplingProfiler::CallStackProfiles profiles) {
-  callback.Run(std::move(profiles));
-  // Delete the instance on the original calling thread.
-  task_runner->DeleteSoon(FROM_HERE, object_to_be_deleted.release());
-}
+// This value is used when there is no collection in progress and thus no ID
+// for referencing the active collection to the SamplingThread.
+const int NULL_COLLECTION_ID = -1;
 
 void ChangeAtomicFlags(subtle::Atomic32* flags,
                        subtle::Atomic32 set,
@@ -160,136 +111,647 @@ StackSamplingProfiler::CallStackProfile::CallStackProfile(
 
 // StackSamplingProfiler::SamplingThread --------------------------------------
 
-StackSamplingProfiler::SamplingThread::SamplingThread(
-    std::unique_ptr<NativeStackSampler> native_sampler,
-    const SamplingParams& params,
-    const CompletedCallback& completed_callback)
-    : native_sampler_(std::move(native_sampler)),
-      params_(params),
-      stop_event_(WaitableEvent::ResetPolicy::AUTOMATIC,
-                  WaitableEvent::InitialState::NOT_SIGNALED),
-      completed_callback_(completed_callback) {}
+class StackSamplingProfiler::SamplingThread : public Thread {
+ public:
+  class TestAPI {
+   public:
+    // Reset the existing sampler. This will unfortunately create the object
+    // unnecessarily if it doesn't already exist but there's no way around that.
+    static void Reset();
+
+    // Disables inherent idle-shutdown behavior.
+    static void DisableIdleShutdown();
+
+    // Begins an idle shutdown as if the idle-timer had expired and wait for
+    // it to execute. Since the timer would have only been started at a time
+    // when the sampling thread actually was idle, this must be called only
+    // when it is known that there are no active sampling threads. If
+    // |simulate_intervening_add| is true then, when executed, the shutdown
+    // task will believe that a new collection has been added since it was
+    // posted.
+    static void ShutdownAssumingIdle(bool simulate_intervening_add);
+
+   private:
+    // Calls the sampling threads ShutdownTask and then signals an event.
+    static void ShutdownTaskAndSignalEvent(SamplingThread* sampler,
+                                           int add_events,
+                                           WaitableEvent* event);
+  };
+
+  struct CollectionContext {
+    CollectionContext(PlatformThreadId target,
+                      const SamplingParams& params,
+                      const CompletedCallback& callback,
+                      WaitableEvent* finished,
+                      std::unique_ptr<NativeStackSampler> sampler)
+        : collection_id(next_collection_id_.GetNext()),
+          target(target),
+          params(params),
+          callback(callback),
+          finished(finished),
+          native_sampler(std::move(sampler)) {}
+    ~CollectionContext() {}
+
+    // An identifier for this collection, used to uniquely identify it to
+    // outside interests.
+    const int collection_id;
+
+    const PlatformThreadId target;     // ID of The thread being sampled.
+    const SamplingParams params;       // Information about how to sample.
+    const CompletedCallback callback;  // Callback made when sampling complete.
+    WaitableEvent* const finished;     // Signaled when all sampling complete.
+
+    // Platform-specific module that does the actual sampling.
+    std::unique_ptr<NativeStackSampler> native_sampler;
+
+    // The absolute time for the next sample.
+    Time next_sample_time;
+
+    // The time that a profile was started, for calculating the total duration.
+    Time profile_start_time;
+
+    // Counters that indicate the current position along the acquisition.
+    int burst = 0;
+    int sample = 0;
+
+    // The collected stack samples. The active profile is always at the back().
+    CallStackProfiles profiles;
+
+   private:
+    static StaticAtomicSequenceNumber next_collection_id_;
+  };
+
+  // Gets the single instance of this class.
+  static SamplingThread* GetInstance();
+
+  // Adds a new CollectionContext to the thread. This can be called externally
+  // from any thread. This returns an ID that can later be used to stop
+  // the sampling.
+  int Add(std::unique_ptr<CollectionContext> collection);
+
+  // Removes an active collection based on its ID, forcing it to run its
+  // callback if any data has been collected. This can be called externally
+  // from any thread.
+  void Remove(int id);
 
-StackSamplingProfiler::SamplingThread::~SamplingThread() {}
+ private:
+  friend class TestAPI;
+  friend struct DefaultSingletonTraits<SamplingThread>;
+
+  // The different states in which the sampling-thread can be.
+  enum ThreadExecutionState {
+    // The thread is not running because it has never been started. It will be
+    // started when a sampling request is received.
+    NOT_STARTED,
+
+    // The thread is running and processing tasks. This is the state when any
+    // sampling requests are active and during the "idle" period afterward
+    // before the thread is stopped.
+    RUNNING,
+
+    // Once all sampling requests have finished and the "idle" period has
+    // expired, the thread will be set to this state and its shutdown
+    // initiated. A call to Stop() must be made to ensure the previous thread
+    // has completely exited before calling Start() and moving back to the
+    // RUNNING state.
+    EXITING,
+  };
+
+  SamplingThread();
+  ~SamplingThread() override;
+
+  // Get task runner that is usable from the outside.
+  scoped_refptr<SingleThreadTaskRunner> GetOrCreateTaskRunnerForAdd();
+  scoped_refptr<SingleThreadTaskRunner> GetTaskRunner(
+      ThreadExecutionState* out_state);
+
+  // Get task runner that is usable from the sampling thread itself.
+  scoped_refptr<SingleThreadTaskRunner> GetTaskRunnerOnSamplingThread();
+
+  // Finishes a collection and reports collected data via callback.
+  void FinishCollection(CollectionContext* collection);
+
+  // Records a single sample of a collection.
+  void RecordSample(CollectionContext* collection);
+
+  // Check if the sampling thread is idle and begin a shutdown if it is.
+  void ScheduleShutdownIfIdle();
+
+  // These methods are tasks that get posted to the internal message queue.
+  void AddCollectionTask(std::unique_ptr<CollectionContext> collection);
+  void RemoveCollectionTask(int id);
+  void PerformCollectionTask(int id);
+  void ShutdownTask(int add_events);
+
+  // Updates the |next_sample_time| time based on configured parameters.
+  // Returns true if there is a next sample or false if sampling is complete.
+  bool UpdateNextSampleTime(CollectionContext* collection);
+
+  // Thread:
+  void CleanUp() override;
+
+  // A map of IDs to collection contexts. Because this class is a singleton
+  // that is never destroyed, context objects will never be destructed except
+  // by explicit action. Thus, it's acceptable to pass unretained pointers
+  // to these objects when posting tasks.
+  std::map<int, std::unique_ptr<CollectionContext>> active_collections_;
+
+  // State maintained about the current execution (or non-execution) of
+  // the thread. This state must always be accessed while holding the
+  // lock. A copy of the task-runner is maintained here for use by any
+  // calling thread; this is necessary because Thread's accessor for it is
+  // not itself thread-safe. The lock is also used to order calls to the
+  // Thread API (Start, Stop, StopSoon, & DetachFromSequence) so that
+  // multiple threads may make those calls.
+  Lock thread_execution_state_lock_;  // Protects all thread_execution_state_*
+  ThreadExecutionState thread_execution_state_ = NOT_STARTED;
+  scoped_refptr<SingleThreadTaskRunner> thread_execution_state_task_runner_;
+  bool thread_execution_state_disable_idle_shutdown_for_testing_ = false;
+
+  // A counter that notes adds of new collection requests. It is incremented
+  // when changes occur so that delayed shutdown tasks are able to detect if
+  // samething new has happened while it was waiting. Like all "execution_state"
+  // vars, this must be accessed while holding |thread_execution_state_lock_|.
+  int thread_execution_state_add_events_ = 0;
+
+  DISALLOW_COPY_AND_ASSIGN(SamplingThread);
+};
 
-void StackSamplingProfiler::SamplingThread::ThreadMain() {
-  PlatformThread::SetName("Chrome_SamplingProfilerThread");
+// static
+void StackSamplingProfiler::SamplingThread::TestAPI::Reset() {
+  SamplingThread* sampler = SamplingThread::GetInstance();
+
+  ThreadExecutionState state;
+  {
+    AutoLock lock(sampler->thread_execution_state_lock_);
+    state = sampler->thread_execution_state_;
+    DCHECK(sampler->active_collections_.empty());
+  }
+
+  // Stop the thread and wait for it to exit. This has to be done through by
+  // the thread itself because it has taken ownership of its own lifetime.
+  if (state == RUNNING) {
+    ShutdownAssumingIdle(false);
+    state = EXITING;
+  }
+  // Make sure thread is cleaned up since state will be reset to NOT_STARTED.
+  if (state == EXITING)
+    sampler->Stop();
+
+  // Reset internal variables to the just-initialized state.
+  {
+    AutoLock lock(sampler->thread_execution_state_lock_);
+    sampler->thread_execution_state_ = NOT_STARTED;
+    sampler->thread_execution_state_task_runner_ = nullptr;
+    sampler->thread_execution_state_disable_idle_shutdown_for_testing_ = false;
+    sampler->thread_execution_state_add_events_ = 0;
+  }
+}
+
+// static
+void StackSamplingProfiler::SamplingThread::TestAPI::DisableIdleShutdown() {
+  SamplingThread* sampler = SamplingThread::GetInstance();
+
+  {
+    AutoLock lock(sampler->thread_execution_state_lock_);
+    sampler->thread_execution_state_disable_idle_shutdown_for_testing_ = true;
+  }
+}
+
+// static
+void StackSamplingProfiler::SamplingThread::TestAPI::ShutdownAssumingIdle(
+    bool simulate_intervening_add) {
+  SamplingThread* sampler = SamplingThread::GetInstance();
+
+  ThreadExecutionState state;
+  scoped_refptr<SingleThreadTaskRunner> task_runner =
+      sampler->GetTaskRunner(&state);
+  DCHECK_EQ(RUNNING, state);
+  DCHECK(task_runner);
+
+  int add_events;
+  {
+    AutoLock lock(sampler->thread_execution_state_lock_);
+    add_events = sampler->thread_execution_state_add_events_;
+    if (simulate_intervening_add)
+      ++sampler->thread_execution_state_add_events_;
+  }
 
-  // For now, just ignore any requests to profile while another profiler is
-  // working.
-  if (!concurrent_profiling_lock.Get().Try())
+  WaitableEvent executed(WaitableEvent::ResetPolicy::MANUAL,
+                         WaitableEvent::InitialState::NOT_SIGNALED);
+  // PostTaskAndReply won't work because thread and associated message-loop may
+  // be shut down.
+  task_runner->PostTask(FROM_HERE,
+                        Bind(&ShutdownTaskAndSignalEvent, Unretained(sampler),
+                             add_events, Unretained(&executed)));
+  executed.Wait();
+}
+
+// static
+void StackSamplingProfiler::SamplingThread::TestAPI::ShutdownTaskAndSignalEvent(
+    SamplingThread* sampler,
+    int add_events,
+    WaitableEvent* event) {
+  sampler->ShutdownTask(add_events);
+  event->Signal();
+}
+
+StaticAtomicSequenceNumber StackSamplingProfiler::SamplingThread::
+    CollectionContext::next_collection_id_;
+
+StackSamplingProfiler::SamplingThread::SamplingThread()
+    : Thread("StackSamplingProfiler") {}
+
+StackSamplingProfiler::SamplingThread::~SamplingThread() = default;
+
+StackSamplingProfiler::SamplingThread*
+StackSamplingProfiler::SamplingThread::GetInstance() {
+  return Singleton<SamplingThread, LeakySingletonTraits<SamplingThread>>::get();
+}
+
+int StackSamplingProfiler::SamplingThread::Add(
+    std::unique_ptr<CollectionContext> collection) {
+  // This is not to be run on the sampling thread.
+
+  int id = collection->collection_id;
+  scoped_refptr<SingleThreadTaskRunner> task_runner =
+      GetOrCreateTaskRunnerForAdd();
+
+  task_runner->PostTask(FROM_HERE, Bind(&SamplingThread::AddCollectionTask,
+                                        Unretained(this), Passed(&collection)));
+
+  return id;
+}
+
+void StackSamplingProfiler::SamplingThread::Remove(int id) {
+  // This is not to be run on the sampling thread.
+
+  ThreadExecutionState state;
+  scoped_refptr<SingleThreadTaskRunner> task_runner = GetTaskRunner(&state);
+  if (state != RUNNING)
     return;
+  DCHECK(task_runner);
+
+  // This can fail if the thread were to exit between acquisition of the task
+  // runner above and the call below. In that case, however, everything has
+  // stopped so there's no need to try to stop it.
+  task_runner->PostTask(FROM_HERE, Bind(&SamplingThread::RemoveCollectionTask,
+                                        Unretained(this), id));
+}
+
+scoped_refptr<SingleThreadTaskRunner>
+StackSamplingProfiler::SamplingThread::GetOrCreateTaskRunnerForAdd() {
+  AutoLock lock(thread_execution_state_lock_);
+
+  // The increment of the "add events" count is why this method is to be only
+  // called from "add".
+  ++thread_execution_state_add_events_;
+
+  if (thread_execution_state_ == RUNNING) {
+    DCHECK(thread_execution_state_task_runner_);
+    // This shouldn't be called from the sampling thread as it's inefficient.
+    // Use GetTaskRunnerOnSamplingThread() instead.
+    DCHECK_NE(GetThreadId(), PlatformThread::CurrentId());
+    return thread_execution_state_task_runner_;
+  }
+
+  if (thread_execution_state_ == EXITING) {
+    // The previous instance has only been partially cleaned up. It is necessary
+    // to call Stop() before Start().
+    Stop();
+  }
+
+  // The thread is not running. Start it and get associated runner. The task-
+  // runner has to be saved for future use because though it can be used from
+  // any thread, it can be acquired via task_runner() only on the created
+  // thread and the thread that creates it (i.e. this thread) for thread-safety
+  // reasons which are alleviated in SamplingThread by gating access to it with
+  // the |thread_execution_state_lock_|.
+  Start();
+  thread_execution_state_ = RUNNING;
+  thread_execution_state_task_runner_ = Thread::task_runner();
+
+  // Detach the sampling thread from the "sequence" (i.e. thread) that
+  // started it so that it can be self-managed or stopped by another thread.
+  DetachFromSequence();
+
+  return thread_execution_state_task_runner_;
+}
+
+scoped_refptr<SingleThreadTaskRunner>
+StackSamplingProfiler::SamplingThread::GetTaskRunner(
+    ThreadExecutionState* out_state) {
+  AutoLock lock(thread_execution_state_lock_);
+  if (out_state)
+    *out_state = thread_execution_state_;
+  if (thread_execution_state_ == RUNNING) {
+    // This shouldn't be called from the sampling thread as it's inefficient.
+    // Use GetTaskRunnerOnSamplingThread() instead.
+    DCHECK_NE(GetThreadId(), PlatformThread::CurrentId());
+    DCHECK(thread_execution_state_task_runner_);
+  } else {
+    DCHECK(!thread_execution_state_task_runner_);
+  }
+
+  return thread_execution_state_task_runner_;
+}
+
+scoped_refptr<SingleThreadTaskRunner>
+StackSamplingProfiler::SamplingThread::GetTaskRunnerOnSamplingThread() {
+  // This should be called only from the sampling thread as it has limited
+  // accessibility.
+  DCHECK_EQ(GetThreadId(), PlatformThread::CurrentId());
+
+  return Thread::task_runner();
+}
+
+void StackSamplingProfiler::SamplingThread::FinishCollection(
+    CollectionContext* collection) {
+  DCHECK_EQ(GetThreadId(), PlatformThread::CurrentId());
+
+  // If there is no duration for the final profile (because it was stopped),
+  // calculate it now.
+  if (!collection->profiles.empty() &&
+      collection->profiles.back().profile_duration == TimeDelta()) {
+    collection->profiles.back().profile_duration =
+        Time::Now() - collection->profile_start_time;
+  }
+
+  // Extract some information so callback and event-signalling can still be
+  // done after the collection has been removed from the list of "active" ones.
+  // This allows the the controlling object (and tests using it) to be confident
+  // that collection is fully finished when those things occur.
+  const CompletedCallback callback = collection->callback;
+  CallStackProfiles profiles = std::move(collection->profiles);
+  WaitableEvent* finished = collection->finished;
+
+  // Remove this collection from the map of known ones.  The |collection|
+  // parameter is invalid after this point.
+  size_t count = active_collections_.erase(collection->collection_id);
+  DCHECK_EQ(1U, count);
+
+  // Run the associated callback, passing the collected profiles.
+  callback.Run(std::move(profiles));
 
-  CallStackProfiles profiles;
-  CollectProfiles(&profiles);
-  concurrent_profiling_lock.Get().Release();
-  completed_callback_.Run(std::move(profiles));
-}
-
-// Depending on how long the sampling takes and the length of the sampling
-// interval, a burst of samples could take arbitrarily longer than
-// samples_per_burst * sampling_interval. In this case, we (somewhat
-// arbitrarily) honor the number of samples requested rather than strictly
-// adhering to the sampling intervals. Once we have established users for the
-// StackSamplingProfiler and the collected data to judge, we may go the other
-// way or make this behavior configurable.
-void StackSamplingProfiler::SamplingThread::CollectProfile(
-    CallStackProfile* profile,
-    TimeDelta* elapsed_time,
-    bool* was_stopped) {
-  ElapsedTimer profile_timer;
-  native_sampler_->ProfileRecordingStarting(&profile->modules);
-  profile->sampling_period = params_.sampling_interval;
-  *was_stopped = false;
-  TimeDelta previous_elapsed_sample_time;
-  for (int i = 0; i < params_.samples_per_burst; ++i) {
-    if (i != 0) {
-      // Always wait, even if for 0 seconds, so we can observe a signal on
-      // stop_event_.
-      if (stop_event_.TimedWait(
-              std::max(params_.sampling_interval - previous_elapsed_sample_time,
-                       TimeDelta()))) {
-        *was_stopped = true;
-        break;
-      }
-    }
-    ElapsedTimer sample_timer;
-    profile->samples.push_back(Sample());
-    native_sampler_->RecordStackSample(&profile->samples.back());
-    previous_elapsed_sample_time = sample_timer.Elapsed();
+  // Signal that this collection is finished.
+  finished->Signal();
+}
+
+void StackSamplingProfiler::SamplingThread::RecordSample(
+    CollectionContext* collection) {
+  DCHECK_EQ(GetThreadId(), PlatformThread::CurrentId());
+  DCHECK(collection->native_sampler);
+
+  // If this is the first sample of a burst, a new Profile needs to be created
+  // and filled.
+  if (collection->sample == 0) {
+    collection->profiles.push_back(CallStackProfile());
+    CallStackProfile& profile = collection->profiles.back();
+    profile.sampling_period = collection->params.sampling_interval;
+    collection->profile_start_time = Time::Now();
+    collection->native_sampler->ProfileRecordingStarting(&profile.modules);
   }
 
-  *elapsed_time = profile_timer.Elapsed();
-  profile->profile_duration = *elapsed_time;
-  native_sampler_->ProfileRecordingStopped();
+  // The currently active profile being captured.
+  CallStackProfile& profile = collection->profiles.back();
+
+  // Record a single sample.
+  profile.samples.push_back(Sample());
+  collection->native_sampler->RecordStackSample(&profile.samples.back());
+
+  // If this is the last sample of a burst, record the total time.
+  if (collection->sample == collection->params.samples_per_burst - 1) {
+    profile.profile_duration = Time::Now() - collection->profile_start_time;
+    collection->native_sampler->ProfileRecordingStopped();
+  }
 }
 
-// In an analogous manner to CollectProfile() and samples exceeding the expected
-// total sampling time, bursts may also exceed the burst_interval. We adopt the
-// same wait-and-see approach here.
-void StackSamplingProfiler::SamplingThread::CollectProfiles(
-    CallStackProfiles* profiles) {
-  if (stop_event_.TimedWait(params_.initial_delay))
+void StackSamplingProfiler::SamplingThread::ScheduleShutdownIfIdle() {
+  DCHECK_EQ(GetThreadId(), PlatformThread::CurrentId());
+
+  if (!active_collections_.empty())
     return;
 
-  TimeDelta previous_elapsed_profile_time;
-  for (int i = 0; i < params_.bursts; ++i) {
-    if (i != 0) {
-      // Always wait, even if for 0 seconds, so we can observe a signal on
-      // stop_event_.
-      if (stop_event_.TimedWait(
-              std::max(params_.burst_interval - previous_elapsed_profile_time,
-                       TimeDelta())))
-        return;
-    }
-
-    CallStackProfile profile;
-    bool was_stopped = false;
-    CollectProfile(&profile, &previous_elapsed_profile_time, &was_stopped);
-    if (!profile.samples.empty())
-      profiles->push_back(std::move(profile));
-
-    if (was_stopped)
+  int add_events;
+  {
+    AutoLock lock(thread_execution_state_lock_);
+    if (thread_execution_state_disable_idle_shutdown_for_testing_)
       return;
+    add_events = thread_execution_state_add_events_;
   }
+
+  GetTaskRunnerOnSamplingThread()->PostDelayedTask(
+      FROM_HERE,
+      Bind(&SamplingThread::ShutdownTask, Unretained(this), add_events),
+      TimeDelta::FromSeconds(60));
 }
 
-void StackSamplingProfiler::SamplingThread::Stop() {
-  stop_event_.Signal();
+void StackSamplingProfiler::SamplingThread::AddCollectionTask(
+    std::unique_ptr<CollectionContext> collection) {
+  DCHECK_EQ(GetThreadId(), PlatformThread::CurrentId());
+
+  const int collection_id = collection->collection_id;
+  const TimeDelta initial_delay = collection->params.initial_delay;
+
+  active_collections_.insert(
+      std::make_pair(collection_id, std::move(collection)));
+
+  GetTaskRunnerOnSamplingThread()->PostDelayedTask(
+      FROM_HERE,
+      Bind(&SamplingThread::PerformCollectionTask, Unretained(this),
+           collection_id),
+      initial_delay);
+
+  // Another increment of "add events" serves to invalidate any pending
+  // shutdown tasks that may have been initiated between the Add() and this
+  // task running.
+  {
+    AutoLock lock(thread_execution_state_lock_);
+    ++thread_execution_state_add_events_;
+  }
+}
+
+void StackSamplingProfiler::SamplingThread::RemoveCollectionTask(int id) {
+  DCHECK_EQ(GetThreadId(), PlatformThread::CurrentId());
+
+  auto found = active_collections_.find(id);
+  if (found == active_collections_.end())
+    return;
+
+  FinishCollection(found->second.get());
+  ScheduleShutdownIfIdle();
+}
+
+void StackSamplingProfiler::SamplingThread::PerformCollectionTask(int id) {
+  DCHECK_EQ(GetThreadId(), PlatformThread::CurrentId());
+
+  auto found = active_collections_.find(id);
+
+  // The task won't be found if it has been stopped.
+  if (found == active_collections_.end())
+    return;
+
+  CollectionContext* collection = found->second.get();
+
+  // Handle first-run with no "next time".
+  if (collection->next_sample_time == Time())
+    collection->next_sample_time = Time::Now();
+
+  // Do the collection of a single sample.
+  RecordSample(collection);
+
+  // Update the time of the next sample recording.
+  if (UpdateNextSampleTime(collection)) {
+    bool success = GetTaskRunnerOnSamplingThread()->PostDelayedTask(
+        FROM_HERE,
+        Bind(&SamplingThread::PerformCollectionTask, Unretained(this), id),
+        std::max(collection->next_sample_time - Time::Now(), TimeDelta()));
+    DCHECK(success);
+  } else {
+    // All capturing has completed so finish the collection. By not re-adding
+    // it to the task queue, the collection will "expire" (i.e. no further work
+    // will be done). The |collection| variable will be invalid after this call.
+    FinishCollection(collection);
+    ScheduleShutdownIfIdle();
+  }
+}
+
+void StackSamplingProfiler::SamplingThread::ShutdownTask(int add_events) {
+  DCHECK_EQ(GetThreadId(), PlatformThread::CurrentId());
+
+  // Holding this lock ensures that any attempt to start another job will
+  // get postponed until |thread_execution_state_| is updated, thus eliminating
+  // the race in starting a new thread while the previous one is exiting.
+  AutoLock lock(thread_execution_state_lock_);
+
+  // If the current count of creation requests doesn't match the passed count
+  // then other tasks have been created since this was posted. Abort shutdown.
+  if (thread_execution_state_add_events_ != add_events)
+    return;
+
+  // There can be no new AddCollectionTasks at this point because creating
+  // those always increments "add events". There may be other requests, like
+  // Remove, but it's okay to schedule the thread to stop once they've been
+  // executed (i.e. "soon").
+  DCHECK(active_collections_.empty());
+  StopSoon();
+
+  // StopSoon will have set the owning sequence (again) so it must be detached
+  // (again) in order for Stop/Start to be called (again) should more work
+  // come in. Holding the |thread_execution_state_lock_| ensures the necessary
+  // happens-after with regard to this detach and future Thread API calls.
+  DetachFromSequence();
+
+  // Set the thread_state variable so the thread will be restarted when new
+  // work comes in. Remove the |thread_execution_state_task_runner_| to avoid
+  // confusion.
+  thread_execution_state_ = EXITING;
+  thread_execution_state_task_runner_ = nullptr;
+}
+
+bool StackSamplingProfiler::SamplingThread::UpdateNextSampleTime(
+    CollectionContext* collection) {
+  // This will keep a consistent average interval between samples but will
+  // result in constant series of acquisitions, thus nearly locking out the
+  // target thread, if the interval is smaller than the time it takes to
+  // actually acquire the sample. Anything sampling that quickly is going
+  // to be a problem anyway so don't worry about it.
+  if (++collection->sample < collection->params.samples_per_burst) {
+    collection->next_sample_time += collection->params.sampling_interval;
+    return true;
+  }
+
+  if (++collection->burst < collection->params.bursts) {
+    collection->sample = 0;
+    collection->next_sample_time += collection->params.burst_interval;
+    return true;
+  }
+
+  return false;
+}
+
+void StackSamplingProfiler::SamplingThread::CleanUp() {
+  DCHECK_EQ(GetThreadId(), PlatformThread::CurrentId());
+
+  // There should be no collections remaining when the thread stops.
+  DCHECK(active_collections_.empty());
+
+  // Let the parent clean up.
+  Thread::CleanUp();
 }
 
 // StackSamplingProfiler ------------------------------------------------------
 
+// static
+void StackSamplingProfiler::TestAPI::Reset() {
+  SamplingThread::TestAPI::Reset();
+  ResetAnnotations();
+}
+
+// static
+void StackSamplingProfiler::TestAPI::ResetAnnotations() {
+  subtle::NoBarrier_Store(&process_milestones_, 0u);
+}
+
+// static
+bool StackSamplingProfiler::TestAPI::IsSamplingThreadRunning() {
+  return SamplingThread::GetInstance()->IsRunning();
+}
+
+// static
+void StackSamplingProfiler::TestAPI::DisableIdleShutdown() {
+  SamplingThread::TestAPI::DisableIdleShutdown();
+}
+
+// static
+void StackSamplingProfiler::TestAPI::PerformSamplingThreadIdleShutdown(
+    bool simulate_intervening_start) {
+  SamplingThread::TestAPI::ShutdownAssumingIdle(simulate_intervening_start);
+}
+
 subtle::Atomic32 StackSamplingProfiler::process_milestones_ = 0;
 
 StackSamplingProfiler::StackSamplingProfiler(
-    PlatformThreadId thread_id,
     const SamplingParams& params,
-    const CompletedCallback& callback)
-    : StackSamplingProfiler(thread_id, params, callback, nullptr) {}
+    const CompletedCallback& callback,
+    NativeStackSamplerTestDelegate* test_delegate)
+    : StackSamplingProfiler(base::PlatformThread::CurrentId(),
+                            params,
+                            callback,
+                            test_delegate) {}
 
 StackSamplingProfiler::StackSamplingProfiler(
     PlatformThreadId thread_id,
     const SamplingParams& params,
     const CompletedCallback& callback,
     NativeStackSamplerTestDelegate* test_delegate)
-    : thread_id_(thread_id), params_(params), completed_callback_(callback),
-      test_delegate_(test_delegate) {
-}
+    : thread_id_(thread_id),
+      params_(params),
+      completed_callback_(callback),
+      // The event starts "signaled" so code knows it's safe to start thread
+      // and "manual" so that it can be waited in multiple places.
+      profiling_inactive_(WaitableEvent::ResetPolicy::MANUAL,
+                          WaitableEvent::InitialState::SIGNALED),
+      collection_id_(NULL_COLLECTION_ID),
+      test_delegate_(test_delegate) {}
 
 StackSamplingProfiler::~StackSamplingProfiler() {
+  // Stop returns immediately but the shutdown runs asynchronously. There is a
+  // non-zero probability that one more sample will be taken after this call
+  // returns.
   Stop();
-  if (!sampling_thread_handle_.is_null())
-    PlatformThread::Join(sampling_thread_handle_);
-}
 
-// static
-void StackSamplingProfiler::StartAndRunAsync(
-    PlatformThreadId thread_id,
-    const SamplingParams& params,
-    const CompletedCallback& callback) {
-  CHECK(ThreadTaskRunnerHandle::Get());
-  AsyncRunner::Run(thread_id, params, callback);
+  // The behavior of sampling a thread that has exited is undefined and could
+  // cause Bad Things(tm) to occur. The safety model provided by this class is
+  // that an instance of this object is expected to live at least as long as
+  // the thread it is sampling. However, because the sampling is performed
+  // asynchronously by the SamplingThread, there is no way to guarantee this
+  // is true without waiting for it to signal that it has finished.
+  //
+  // The wait time should, at most, be only as long as it takes to collect one
+  // sample (~200us) or none at all if sampling has already completed.
+  ThreadRestrictions::ScopedAllowWait allow_wait;
+  profiling_inactive_.Wait();
 }
 
 void StackSamplingProfiler::Start() {
@@ -299,19 +761,25 @@ void StackSamplingProfiler::Start() {
   std::unique_ptr<NativeStackSampler> native_sampler =
       NativeStackSampler::Create(thread_id_, &RecordAnnotations,
                                  test_delegate_);
+
   if (!native_sampler)
     return;
 
-  sampling_thread_.reset(new SamplingThread(std::move(native_sampler), params_,
-                                            completed_callback_));
-  if (!PlatformThread::Create(0, sampling_thread_.get(),
-                              &sampling_thread_handle_))
-    sampling_thread_.reset();
+  // Wait for profiling to be "inactive", then reset it for the upcoming run.
+  profiling_inactive_.Wait();
+  profiling_inactive_.Reset();
+
+  DCHECK_EQ(NULL_COLLECTION_ID, collection_id_);
+  collection_id_ = SamplingThread::GetInstance()->Add(
+      MakeUnique<SamplingThread::CollectionContext>(
+          thread_id_, params_, completed_callback_, &profiling_inactive_,
+          std::move(native_sampler)));
+  DCHECK_NE(NULL_COLLECTION_ID, collection_id_);
 }
 
 void StackSamplingProfiler::Stop() {
-  if (sampling_thread_)
-    sampling_thread_->Stop();
+  SamplingThread::GetInstance()->Remove(collection_id_);
+  collection_id_ = NULL_COLLECTION_ID;
 }
 
 // static
@@ -323,11 +791,6 @@ void StackSamplingProfiler::SetProcessMilestone(int milestone) {
 }
 
 // static
-void StackSamplingProfiler::ResetAnnotationsForTesting() {
-  subtle::NoBarrier_Store(&process_milestones_, 0u);
-}
-
-// static
 void StackSamplingProfiler::RecordAnnotations(Sample* sample) {
   // The code inside this method must not do anything that could acquire a
   // mutex, including allocating memory (which includes LOG messages) because