Support parallel captures from the StackSamplingProfiler.

base/profiler/stack_sampling_profiler_unittest.cc

 // Copyright 2015 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 <stddef.h>
 #include <stdint.h>
 
 #include <cstdlib>
 #include <memory>
 #include <utility>
 #include <vector>
 
 #include "base/bind.h"
 #include "base/compiler_specific.h"
 #include "base/macros.h"
 #include "base/memory/ptr_util.h"
-#include "base/message_loop/message_loop.h"
 #include "base/native_library.h"
 #include "base/path_service.h"
 #include "base/profiler/native_stack_sampler.h"
 #include "base/profiler/stack_sampling_profiler.h"
 #include "base/run_loop.h"
 #include "base/scoped_native_library.h"
 #include "base/strings/stringprintf.h"
 #include "base/strings/utf_string_conversions.h"
+#include "base/synchronization/lock.h"
 #include "base/synchronization/waitable_event.h"
 #include "base/threading/platform_thread.h"
+#include "base/threading/simple_thread.h"
 #include "base/time/time.h"
 #include "build/build_config.h"
 #include "testing/gtest/include/gtest/gtest.h"
 
 #if defined(OS_WIN)
 #include <intrin.h>
 #include <malloc.h>
 #include <windows.h>
 #else
 #include <alloca.h>
@@ skipping 306 matching lines @@
   target_thread.SignalThreadToFinish();
 
   PlatformThread::Join(target_thread_handle);
 }
 
 template <class Function>
 void WithTargetThread(Function function) {
   WithTargetThread(function, StackConfiguration(StackConfiguration::NORMAL));
 }
 
+// Waits for one of multiple samplings to complete. If |delegates| is provided,
+// the vector must be pre-populated and of the same size as |params|.
+void CreateProfilers(
+    PlatformThreadId target_thread_id,
+    const std::vector<SamplingParams>& params,
+    const std::vector<std::unique_ptr<NativeStackSamplerTestDelegate>>*
+        delegates,
+    std::vector<CallStackProfiles>* profiles,
+    std::vector<std::unique_ptr<StackSamplingProfiler>>* profilers,
+    std::vector<std::unique_ptr<WaitableEvent>>* completed) {
+  ASSERT_TRUE(!params.empty());
+  ASSERT_TRUE(profiles->empty());
+  ASSERT_TRUE(profilers->empty());
+  ASSERT_TRUE(completed->empty());
+  if (delegates)
+    ASSERT_EQ(params.size(), delegates->size());
+
+  // Vectors have to be appropriately sized in advance so that the addresses of
+  // values don't change.
+  const size_t count = params.size();
+  profiles->reserve(count);
+  profilers->reserve(count);
+  completed->reserve(count);
+
+  for (size_t i = 0; i < count; ++i) {
+    profiles->push_back(CallStackProfiles());
+    completed->push_back(
+        MakeUnique<WaitableEvent>(WaitableEvent::ResetPolicy::AUTOMATIC,
+                                  WaitableEvent::InitialState::NOT_SIGNALED));
+    const StackSamplingProfiler::CompletedCallback callback =
+        Bind(&SaveProfilesAndSignalEvent, Unretained(&profiles->back()),
+             Unretained(completed->back().get()));
+    profilers->push_back(MakeUnique<StackSamplingProfiler>(
+        target_thread_id, params[i], callback,
+        (delegates ? (*delegates)[i].get() : nullptr)));
+  }
+}
+
 // Captures profiles as specified by |params| on the TargetThread, and returns
 // them in |profiles|. Waits up to |profiler_wait_time| for the profiler to
 // complete.
 void CaptureProfiles(const SamplingParams& params, TimeDelta profiler_wait_time,
                      CallStackProfiles* profiles) {
   profiles->clear();
 
   WithTargetThread([&params, profiles,
                     profiler_wait_time](PlatformThreadId target_thread_id) {
     WaitableEvent sampling_thread_completed(
         WaitableEvent::ResetPolicy::MANUAL,
         WaitableEvent::InitialState::NOT_SIGNALED);
     const StackSamplingProfiler::CompletedCallback callback =
         Bind(&SaveProfilesAndSignalEvent, Unretained(profiles),
              Unretained(&sampling_thread_completed));
     StackSamplingProfiler profiler(target_thread_id, params, callback);
     profiler.Start();
     sampling_thread_completed.TimedWait(profiler_wait_time);
     profiler.Stop();
     sampling_thread_completed.Wait();
   });
 }
 
+// Waits for one of multiple samplings to complete.
+size_t WaitForSamplingComplete(
+    std::vector<std::unique_ptr<WaitableEvent>>* sampling_completed) {
+  // Map unique_ptrs to something that WaitMany can accept.
+  std::vector<WaitableEvent*> sampling_completed_rawptrs(
+      sampling_completed->size());
+  std::transform(
+      sampling_completed->begin(), sampling_completed->end(),
+      sampling_completed_rawptrs.begin(),
+      [](const std::unique_ptr<WaitableEvent>& elem) { return elem.get(); });
+  // Wait for one profiler to finish.
+  return WaitableEvent::WaitMany(sampling_completed_rawptrs.data(),
+                                 sampling_completed_rawptrs.size());
+}
+
 // If this executable was linked with /INCREMENTAL (the default for non-official
 // debug and release builds on Windows), function addresses do not correspond to
 // function code itself, but instead to instructions in the Incremental Link
 // Table that jump to the functions. Checks for a jump instruction and if
 // present does a little decompilation to find the function's actual starting
 // address.
 const void* MaybeFixupFunctionAddressForILT(const void* function_address) {
 #if defined(_WIN64)
   const unsigned char* opcode =
       reinterpret_cast<const unsigned char*>(function_address);
@@ skipping 50 matching lines @@
 // asynchronous library loading has completed before walking the stack. If
 // false, the unloading may still be occurring during the stack walk.
 void TestLibraryUnload(bool wait_until_unloaded) {
   // Test delegate that supports intervening between the copying of the stack
   // and the walking of the stack.
   class StackCopiedSignaler : public NativeStackSamplerTestDelegate {
    public:
     StackCopiedSignaler(WaitableEvent* stack_copied,
                         WaitableEvent* start_stack_walk,
                         bool wait_to_walk_stack)
-        : stack_copied_(stack_copied), start_stack_walk_(start_stack_walk),
-          wait_to_walk_stack_(wait_to_walk_stack) {
-    }
+        : stack_copied_(stack_copied),
+          start_stack_walk_(start_stack_walk),
+          wait_to_walk_stack_(wait_to_walk_stack) {}
 
     void OnPreStackWalk() override {
       stack_copied_->Signal();
       if (wait_to_walk_stack_)
         start_stack_walk_->Wait();
     }
 
    private:
     WaitableEvent* const stack_copied_;
     WaitableEvent* const start_stack_walk_;
@@ skipping 110 matching lines @@
     // TargetThread::SignalAndWaitUntilSignaled
     // TargetThread::OtherLibraryCallback
     // InvokeCallbackFunction (in other library)
     // TargetThread::CallThroughOtherLibrary
     EXPECT_EQ(3, other_library_frame - end_frame)
         << "Stack:\n"
         << FormatSampleForDiagnosticOutput(sample, profile.modules);
   }
 }
 
+class StackSamplingProfilerTest : public testing::Test {
+ public:
+  void SetUp() override {
+    StackSamplingProfiler::ResetAnnotationsForTesting();

[Mike Wittman, 2017/03/31 01:38:22]

Can you move this function into the TestAPI for consistency?

[Mike Wittman, 2017/03/31 01:38:22]

Also, this call should go in TearDown.

[bcwhite, 2017/03/31 13:57:56]

Done.  And it makes sense to call that as part of the more general Reset()
behavior.

[bcwhite, 2017/03/31 13:57:56]

Reset needs to happen in SetUp because it's possible that other tests (in other
files) could have set some values here.  But it should be in tear-down, too.

[Mike Wittman, 2017/03/31 18:12:33]

That's a very good point that we need to be concerned about other users of the
profiler. Unfortunately, I don't think running Reset() in SetUp() is sufficient
(or necessary, even) to solve the issue.

For the annotations case, non-profiler tests that exercise calls to
SetProcessMilestone() won't benefit from this fixture. They'll potentially set
the same milestone multiple times, hitting at least one DCHECK. We don't see
this problem now because none of the milestone setting code appears to be tested
outside of browser_tests and interactive_ui_tests which run one test per
process.

For the sampling thread case, I don't think anything needs to be done at SetUp()
to ensure correct behavior. The profiler tests are the only ones in the
base_unittests target that exercise the profiler, and should be the only ones in
that target that will ever do so, so running Reset() only in TearDown() is
sufficient. All the other unit test targets should be fine without doing Reset()
-- they'll get the normal thread restart behavior -- but it still would be good
practice to mock out the profiler in unit tests of new code using the profiler
to isolate that code from the profiler dependency.

Given these facts, I think the best course of action is:
 - only run DisableIdleShutdown() in SetUp()
 - only run Reset() in TearDown(), and add a comment on the fixture saying that
all profiler tests in base_unittests must use the fixture to ensure proper
clean-up
 - in a separate CL, update the milestone handling logic to accept milestones
being set multiple times and add a comment about why/how this happens

[bcwhite, 2017/04/03 20:18:12]

Done.

+    StackSamplingProfiler::TestAPI::DisableIdleShutdown();

[Mike Wittman, 2017/03/31 01:38:22]

It's worth commenting that we disable idle shutdown because it takes too long to
occur to be testable. The behavior is tested in some tests by artificially
triggering an idle shutdown.

[bcwhite, 2017/03/31 13:57:56]

Done.

+  }
+
+  void TearDown() override { StackSamplingProfiler::TestAPI::Reset(); }
+};
+
 }  // namespace
 
 // Checks that the basic expected information is present in a sampled call stack
 // profile.
 #if defined(STACK_SAMPLING_PROFILER_SUPPORTED)
 #define MAYBE_Basic Basic
 #else
 #define MAYBE_Basic DISABLED_Basic
 #endif
-TEST(StackSamplingProfilerTest, MAYBE_Basic) {
-  StackSamplingProfiler::ResetAnnotationsForTesting();
-
+TEST_F(StackSamplingProfilerTest, MAYBE_Basic) {
   SamplingParams params;
   params.sampling_interval = TimeDelta::FromMilliseconds(0);
   params.samples_per_burst = 1;
 
   std::vector<CallStackProfile> profiles;
   CaptureProfiles(params, AVeryLongTimeDelta(), &profiles);
 
   // Check that the profile and samples sizes are correct, and the module
   // indices are in range.
   ASSERT_EQ(1u, profiles.size());
@@ skipping 22 matching lines @@
   EXPECT_TRUE(PathService::Get(FILE_EXE, &executable_path));
   EXPECT_EQ(executable_path, profile.modules[loc->module_index].filename);
 }
 
 // Checks that annotations are recorded in samples.
 #if defined(STACK_SAMPLING_PROFILER_SUPPORTED)
 #define MAYBE_Annotations Annotations
 #else
 #define MAYBE_Annotations DISABLED_Annotations
 #endif
-TEST(StackSamplingProfilerTest, MAYBE_Annotations) {
-  StackSamplingProfiler::ResetAnnotationsForTesting();
-
+TEST_F(StackSamplingProfilerTest, MAYBE_Annotations) {
   SamplingParams params;
   params.sampling_interval = TimeDelta::FromMilliseconds(0);
   params.samples_per_burst = 1;
 
   // Check that a run picks up annotations.
   StackSamplingProfiler::SetProcessMilestone(1);
   std::vector<CallStackProfile> profiles1;
   CaptureProfiles(params, AVeryLongTimeDelta(), &profiles1);
   ASSERT_EQ(1u, profiles1.size());
   const CallStackProfile& profile1 = profiles1[0];
@@ skipping 13 matching lines @@
   EXPECT_EQ(sample1.process_milestones | (1u << 2), sample2.process_milestones);
 }
 
 // Checks that the profiler handles stacks containing dynamically-allocated
 // stack memory.
 #if defined(STACK_SAMPLING_PROFILER_SUPPORTED)
 #define MAYBE_Alloca Alloca
 #else
 #define MAYBE_Alloca DISABLED_Alloca
 #endif
-TEST(StackSamplingProfilerTest, MAYBE_Alloca) {
+TEST_F(StackSamplingProfilerTest, MAYBE_Alloca) {
   SamplingParams params;
   params.sampling_interval = TimeDelta::FromMilliseconds(0);
   params.samples_per_burst = 1;
 
   std::vector<CallStackProfile> profiles;
   WithTargetThread(
       [&params, &profiles](PlatformThreadId target_thread_id) {
         WaitableEvent sampling_thread_completed(
             WaitableEvent::ResetPolicy::MANUAL,
             WaitableEvent::InitialState::NOT_SIGNALED);
@@ skipping 32 matching lines @@
              reinterpret_cast<const void*>(&TargetThread::CallWithAlloca))
       << " was not found in stack:\n"
       << FormatSampleForDiagnosticOutput(sample, profile.modules);
 
   // These frames should be adjacent on the stack.
   EXPECT_EQ(1, alloca_frame - end_frame)
       << "Stack:\n"
       << FormatSampleForDiagnosticOutput(sample, profile.modules);
 }
 
-// Checks that the fire-and-forget interface works.
-#if defined(STACK_SAMPLING_PROFILER_SUPPORTED)
-#define MAYBE_StartAndRunAsync StartAndRunAsync
-#else
-#define MAYBE_StartAndRunAsync DISABLED_StartAndRunAsync
-#endif
-TEST(StackSamplingProfilerTest, MAYBE_StartAndRunAsync) {
-  // StartAndRunAsync requires the caller to have a message loop.
-  MessageLoop message_loop;
-
-  SamplingParams params;
-  params.samples_per_burst = 1;
-
-  CallStackProfiles profiles;
-  WithTargetThread([&params, &profiles](PlatformThreadId target_thread_id) {
-    WaitableEvent sampling_thread_completed(
-        WaitableEvent::ResetPolicy::AUTOMATIC,
-        WaitableEvent::InitialState::NOT_SIGNALED);
-    const StackSamplingProfiler::CompletedCallback callback =
-        Bind(&SaveProfilesAndSignalEvent, Unretained(&profiles),
-             Unretained(&sampling_thread_completed));
-    StackSamplingProfiler::StartAndRunAsync(target_thread_id, params, callback);
-    RunLoop().RunUntilIdle();
-    sampling_thread_completed.Wait();
-  });
-
-  ASSERT_EQ(1u, profiles.size());
-}
-
 // Checks that the expected number of profiles and samples are present in the
 // call stack profiles produced.
 #if defined(STACK_SAMPLING_PROFILER_SUPPORTED)
 #define MAYBE_MultipleProfilesAndSamples MultipleProfilesAndSamples
 #else
 #define MAYBE_MultipleProfilesAndSamples DISABLED_MultipleProfilesAndSamples
 #endif
-TEST(StackSamplingProfilerTest, MAYBE_MultipleProfilesAndSamples) {
+TEST_F(StackSamplingProfilerTest, MAYBE_MultipleProfilesAndSamples) {
   SamplingParams params;
   params.burst_interval = params.sampling_interval =
       TimeDelta::FromMilliseconds(0);
   params.bursts = 2;
   params.samples_per_burst = 3;
 
   std::vector<CallStackProfile> profiles;
   CaptureProfiles(params, AVeryLongTimeDelta(), &profiles);
 
   ASSERT_EQ(2u, profiles.size());
   EXPECT_EQ(3u, profiles[0].samples.size());
   EXPECT_EQ(3u, profiles[1].samples.size());
 }
 
+// Checks that a profiler can stop/destruct without ever having started.
+#if defined(STACK_SAMPLING_PROFILER_SUPPORTED)
+#define MAYBE_StopWithoutStarting StopWithoutStarting
+#else
+#define MAYBE_StopWithoutStarting DISABLED_StopWithoutStarting
+#endif
+TEST_F(StackSamplingProfilerTest, MAYBE_StopWithoutStarting) {
+  WithTargetThread([](PlatformThreadId target_thread_id) {
+    SamplingParams params;
+    params.sampling_interval = TimeDelta::FromMilliseconds(0);
+    params.samples_per_burst = 1;
+
+    CallStackProfiles profiles;
+    WaitableEvent sampling_completed(WaitableEvent::ResetPolicy::MANUAL,
+                                     WaitableEvent::InitialState::NOT_SIGNALED);
+    const StackSamplingProfiler::CompletedCallback callback =
+        Bind(&SaveProfilesAndSignalEvent, Unretained(&profiles),
+             Unretained(&sampling_completed));
+    StackSamplingProfiler profiler(target_thread_id, params, callback);
+
+    profiler.Stop();  // Constructed but never started.
+    EXPECT_FALSE(sampling_completed.IsSignaled());
+  });
+}
+
+// Checks that its okay to stop a profiler before it finishes even when the
+// sampling thread continues to run.
+#if defined(STACK_SAMPLING_PROFILER_SUPPORTED)
+#define MAYBE_StopSafely StopSafely
+#else
+#define MAYBE_StopSafely DISABLED_StopSafely
+#endif
+TEST_F(StackSamplingProfilerTest, MAYBE_StopSafely) {
+  // Test delegate that counts samples.
+  class SampleRecordedCounter : public NativeStackSamplerTestDelegate {
+   public:
+    SampleRecordedCounter() {}
+
+    void OnPreStackWalk() override {
+      AutoLock lock(lock_);
+      ++count_;
+    }
+
+    size_t Get() {
+      AutoLock lock(lock_);
+      return count_;
+    }
+
+   private:
+    Lock lock_;
+    size_t count_ = 0;
+  };
+
+  WithTargetThread([](PlatformThreadId target_thread_id) {
+    std::vector<SamplingParams> params(2);
+
+    // Providing an initial delay makes it more likely that both will be
+    // scheduled before either starts to run. Once started, samples will
+    // run ordered by their scheduled, interleaved times regardless of
+    // whatever interval the thread wakes up.
+    params[0].initial_delay = TimeDelta::FromMilliseconds(10);
+    params[0].sampling_interval = TimeDelta::FromMilliseconds(1);
+    params[0].samples_per_burst = 100000;
+
+    params[1].initial_delay = TimeDelta::FromMilliseconds(10);
+    params[1].sampling_interval = TimeDelta::FromMilliseconds(1);
+    params[1].samples_per_burst = 100000;
+
+    std::vector<CallStackProfiles> profiles;
+    std::vector<std::unique_ptr<NativeStackSamplerTestDelegate>>
+        samples_recorded;
+    std::vector<std::unique_ptr<WaitableEvent>> sampling_completed;
+    std::vector<std::unique_ptr<StackSamplingProfiler>> profilers;
+    for (size_t i = 0; i < params.size(); ++i)
+      samples_recorded.push_back(MakeUnique<SampleRecordedCounter>());
+    CreateProfilers(target_thread_id, params, &samples_recorded, &profiles,
+                    &profilers, &sampling_completed);
+
+    profilers[0]->Start();
+    profilers[1]->Start();
+
+    // Wait for both to start accumulating samples. Using a WaitableEvent is
+    // possible but gets complicated later on because there's no way of knowing
+    // if 0 or 1 additional sample will be taken after Stop() and thus no way
+    // of knowing how many Wait() calls to make on it.
+    while (
+        static_cast<SampleRecordedCounter*>(samples_recorded[0].get())->Get() ==
+            0 ||
+        static_cast<SampleRecordedCounter*>(samples_recorded[1].get())->Get() ==
+            0)
+      PlatformThread::Sleep(TimeDelta::FromMilliseconds(1));
+
+    // Ensure that the first sampler can be safely stopped while the second
+    // continues to run. The stopped first profiler will still have a
+    // PerformCollectionTask pending that will do nothing when executed because
+    // the collection will have been removed by Stop().
+    profilers[0]->Stop();
+    sampling_completed[0]->Wait();
+    size_t count0 =
+        static_cast<SampleRecordedCounter*>(samples_recorded[0].get())->Get();
+    size_t count1 =
+        static_cast<SampleRecordedCounter*>(samples_recorded[1].get())->Get();
+
+    // Waiting for the second sampler to collect a couple samples ensures that
+    // the pending PerformCollectionTask for the first has executed because
+    // tasks are always ordered by their next scheduled time.
+    while (
+        static_cast<SampleRecordedCounter*>(samples_recorded[1].get())->Get() <
+        count1 + 2)
+      PlatformThread::Sleep(TimeDelta::FromMilliseconds(1));
+
+    // Ensure that the first profiler didn't do anything since it was stopped.
+    EXPECT_EQ(
+        count0,
+        static_cast<SampleRecordedCounter*>(samples_recorded[0].get())->Get());
+  });
+}
+
 // Checks that no call stack profiles are captured if the profiling is stopped
 // during the initial delay.
 #if defined(STACK_SAMPLING_PROFILER_SUPPORTED)
 #define MAYBE_StopDuringInitialDelay StopDuringInitialDelay
 #else
 #define MAYBE_StopDuringInitialDelay DISABLED_StopDuringInitialDelay
 #endif
-TEST(StackSamplingProfilerTest, MAYBE_StopDuringInitialDelay) {
+TEST_F(StackSamplingProfilerTest, MAYBE_StopDuringInitialDelay) {
   SamplingParams params;
   params.initial_delay = TimeDelta::FromSeconds(60);
 
   std::vector<CallStackProfile> profiles;
   CaptureProfiles(params, TimeDelta::FromMilliseconds(0), &profiles);
 
   EXPECT_TRUE(profiles.empty());
 }
 
 // Checks that the single completed call stack profile is captured if the
 // profiling is stopped between bursts.
 #if defined(STACK_SAMPLING_PROFILER_SUPPORTED)
 #define MAYBE_StopDuringInterBurstInterval StopDuringInterBurstInterval
 #else
 #define MAYBE_StopDuringInterBurstInterval DISABLED_StopDuringInterBurstInterval
 #endif
-TEST(StackSamplingProfilerTest, MAYBE_StopDuringInterBurstInterval) {
+TEST_F(StackSamplingProfilerTest, MAYBE_StopDuringInterBurstInterval) {
   SamplingParams params;
   params.sampling_interval = TimeDelta::FromMilliseconds(0);
   params.burst_interval = TimeDelta::FromSeconds(60);
   params.bursts = 2;
   params.samples_per_burst = 1;
 
   std::vector<CallStackProfile> profiles;
   CaptureProfiles(params, TimeDelta::FromMilliseconds(50), &profiles);
 
   ASSERT_EQ(1u, profiles.size());
   EXPECT_EQ(1u, profiles[0].samples.size());
 }
 
-// Checks that incomplete call stack profiles are captured.
+// Checks that tasks can be stopped before completion and incomplete call stack
+// profiles are captured.
 #if defined(STACK_SAMPLING_PROFILER_SUPPORTED)
 #define MAYBE_StopDuringInterSampleInterval StopDuringInterSampleInterval
 #else
 #define MAYBE_StopDuringInterSampleInterval \
   DISABLED_StopDuringInterSampleInterval
 #endif
-TEST(StackSamplingProfilerTest, MAYBE_StopDuringInterSampleInterval) {
-  SamplingParams params;
-  params.sampling_interval = TimeDelta::FromSeconds(60);
-  params.samples_per_burst = 2;
+TEST_F(StackSamplingProfilerTest, MAYBE_StopDuringInterSampleInterval) {
+  // Test delegate that counts samples.
+  class SampleRecordedCounter : public NativeStackSamplerTestDelegate {
+   public:
+    SampleRecordedCounter() {}
 
-  std::vector<CallStackProfile> profiles;
-  CaptureProfiles(params, TimeDelta::FromMilliseconds(50), &profiles);
+    void OnPreStackWalk() override {
+      AutoLock lock(lock_);
+      ++count_;
+    }
 
-  ASSERT_EQ(1u, profiles.size());
-  EXPECT_EQ(1u, profiles[0].samples.size());
+    size_t Get() {
+      AutoLock lock(lock_);
+      return count_;
+    }
+
+   private:
+    Lock lock_;
+    size_t count_ = 0;
+  };
+
+  WithTargetThread([](PlatformThreadId target_thread_id) {
+    std::vector<SamplingParams> params(1);
+
+    params[0].sampling_interval = TimeDelta::FromMilliseconds(1);
+    params[0].samples_per_burst = 100000;
+
+    std::vector<CallStackProfiles> profiles;
+    std::vector<std::unique_ptr<NativeStackSamplerTestDelegate>>
+        samples_recorded;
+    std::vector<std::unique_ptr<WaitableEvent>> sampling_completed;
+    std::vector<std::unique_ptr<StackSamplingProfiler>> profilers;
+    for (size_t i = 0; i < params.size(); ++i)
+      samples_recorded.push_back(MakeUnique<SampleRecordedCounter>());
+    CreateProfilers(target_thread_id, params, &samples_recorded, &profiles,
+                    &profilers, &sampling_completed);
+
+    profilers[0]->Start();
+
+    // Wait for profiler to start accumulating samples.
+    while (

[Mike Wittman, 2017/03/31 01:38:22]

This test would be simpler and easier to understand using a WaitableEvent that
gets signaled in the test delegate and waited on here, along with two samples
per burst and a long sampling interval.

[bcwhite, 2017/03/31 13:57:56]

Done.

+        static_cast<SampleRecordedCounter*>(samples_recorded[0].get())->Get() ==
+        0)
+      PlatformThread::Sleep(TimeDelta::FromMilliseconds(1));
+
+    // Ensure that it can stop safely.
+    profilers[0]->Stop();
+    sampling_completed[0]->Wait();
+
+    ASSERT_EQ(1u, profiles[0].size());
+    EXPECT_EQ(
+        static_cast<SampleRecordedCounter*>(samples_recorded[0].get())->Get(),
+        profiles[0][0].samples.size());
+  });
 }
 
 // Checks that we can destroy the profiler while profiling.
 #if defined(STACK_SAMPLING_PROFILER_SUPPORTED)
 #define MAYBE_DestroyProfilerWhileProfiling DestroyProfilerWhileProfiling
 #else
 #define MAYBE_DestroyProfilerWhileProfiling \
   DISABLED_DestroyProfilerWhileProfiling
 #endif
-TEST(StackSamplingProfilerTest, MAYBE_DestroyProfilerWhileProfiling) {
+TEST_F(StackSamplingProfilerTest, MAYBE_DestroyProfilerWhileProfiling) {
   SamplingParams params;
   params.sampling_interval = TimeDelta::FromMilliseconds(10);
 
   CallStackProfiles profiles;
   WithTargetThread([&params, &profiles](PlatformThreadId target_thread_id) {
     std::unique_ptr<StackSamplingProfiler> profiler;
     profiler.reset(new StackSamplingProfiler(
         target_thread_id, params, Bind(&SaveProfiles, Unretained(&profiles))));
     profiler->Start();
     profiler.reset();
 
     // Wait longer than a sample interval to catch any use-after-free actions by
     // the profiler thread.
     PlatformThread::Sleep(TimeDelta::FromMilliseconds(50));
   });
 }
 
 // Checks that the same profiler may be run multiple times.
 #if defined(STACK_SAMPLING_PROFILER_SUPPORTED)
 #define MAYBE_CanRunMultipleTimes CanRunMultipleTimes
 #else
 #define MAYBE_CanRunMultipleTimes DISABLED_CanRunMultipleTimes
 #endif
-TEST(StackSamplingProfilerTest, MAYBE_CanRunMultipleTimes) {
+TEST_F(StackSamplingProfilerTest, MAYBE_CanRunMultipleTimes) {
+  WithTargetThread([](PlatformThreadId target_thread_id) {
+    SamplingParams params;
+    params.sampling_interval = TimeDelta::FromMilliseconds(0);
+    params.samples_per_burst = 1;
+
+    CallStackProfiles profiles;
+    WaitableEvent sampling_completed(WaitableEvent::ResetPolicy::MANUAL,
+                                     WaitableEvent::InitialState::NOT_SIGNALED);
+    const StackSamplingProfiler::CompletedCallback callback =
+        Bind(&SaveProfilesAndSignalEvent, Unretained(&profiles),
+             Unretained(&sampling_completed));
+    StackSamplingProfiler profiler(target_thread_id, params, callback);
+
+    // Just start and stop to execute code paths.
+    profiler.Start();
+    profiler.Stop();
+    sampling_completed.Wait();
+
+    // Ensure a second request will run and not block.
+    sampling_completed.Reset();
+    profiles.clear();
+    profiler.Start();
+    sampling_completed.Wait();
+    profiler.Stop();
+    ASSERT_EQ(1u, profiles.size());
+  });
+}
+
+// Checks that the different profilers may be run.
+#if defined(STACK_SAMPLING_PROFILER_SUPPORTED)
+#define MAYBE_CanRunMultipleProfilers CanRunMultipleProfilers
+#else
+#define MAYBE_CanRunMultipleProfilers DISABLED_CanRunMultipleProfilers
+#endif
+TEST_F(StackSamplingProfilerTest, MAYBE_CanRunMultipleProfilers) {
   SamplingParams params;
   params.sampling_interval = TimeDelta::FromMilliseconds(0);
   params.samples_per_burst = 1;
 
   std::vector<CallStackProfile> profiles;
   CaptureProfiles(params, AVeryLongTimeDelta(), &profiles);
   ASSERT_EQ(1u, profiles.size());
 
   profiles.clear();
   CaptureProfiles(params, AVeryLongTimeDelta(), &profiles);
   ASSERT_EQ(1u, profiles.size());
 }
 
-// Checks that requests to start profiling while another profile is taking place
-// are ignored.
-#if defined(STACK_SAMPLING_PROFILER_SUPPORTED)
-#define MAYBE_ConcurrentProfiling ConcurrentProfiling
-#else
-#define MAYBE_ConcurrentProfiling DISABLED_ConcurrentProfiling
-#endif
-TEST(StackSamplingProfilerTest, MAYBE_ConcurrentProfiling) {
-  WithTargetThread([](PlatformThreadId target_thread_id) {
-    SamplingParams params[2];
+// Checks that a sampler can be started while another is running.
+#if defined(STACK_SAMPLING_PROFILER_SUPPORTED)
+#define MAYBE_MultipleStart MultipleStart
+#else
+#define MAYBE_MultipleStart DISABLED_MultipleStart
+#endif
+TEST_F(StackSamplingProfilerTest, MAYBE_MultipleStart) {
+  WithTargetThread([](PlatformThreadId target_thread_id) {
+    std::vector<SamplingParams> params(2);
+
+    params[0].initial_delay = AVeryLongTimeDelta();
+    params[0].samples_per_burst = 1;
+
+    params[1].sampling_interval = TimeDelta::FromMilliseconds(1);
+    params[1].samples_per_burst = 1;
+
+    std::vector<CallStackProfiles> profiles;
+    std::vector<std::unique_ptr<WaitableEvent>> sampling_completed;
+    std::vector<std::unique_ptr<StackSamplingProfiler>> profilers;
+    CreateProfilers(target_thread_id, params, nullptr, &profiles, &profilers,
+                    &sampling_completed);
+
+    profilers[0]->Start();
+    profilers[1]->Start();
+    sampling_completed[1]->Wait();
+    EXPECT_EQ(1u, profiles[1].size());
+  });
+}
+
+// Checks that the sampling thread can shut down.
+#if defined(STACK_SAMPLING_PROFILER_SUPPORTED)
+#define MAYBE_SamplerIdleShutdown SamplerIdleShutdown
+#else
+#define MAYBE_SamplerIdleShutdown DISABLED_SamplerIdleShutdown
+#endif
+TEST_F(StackSamplingProfilerTest, MAYBE_SamplerIdleShutdown) {
+  SamplingParams params;
+  params.sampling_interval = TimeDelta::FromMilliseconds(0);
+  params.samples_per_burst = 1;
+
+  std::vector<CallStackProfile> profiles;
+  CaptureProfiles(params, AVeryLongTimeDelta(), &profiles);
+  ASSERT_EQ(1u, profiles.size());
+
+  // Capture thread should still be running at this point.
+  ASSERT_TRUE(StackSamplingProfiler::TestAPI::IsSamplingThreadRunning());
+
+  // Initiate an "idle" shutdown and ensure it happens. Idle-shutdown was
+  // disabled above so the test will fail due to a timeout if it does not
+  // exit.
+  StackSamplingProfiler::TestAPI::PerformSamplingThreadIdleShutdown(false);
+
+  // While the shutdown has been initiated, the actual exit of the thread still
+  // happens asynchronously. Watch until the thread actually exits. This test
+  // will time-out in the case of failure.
+  while (StackSamplingProfiler::TestAPI::IsSamplingThreadRunning())
+    PlatformThread::Sleep(base::TimeDelta::FromMilliseconds(1));
+}
+
+// Checks that additional requests will restart a stopped profiler.
+#if defined(STACK_SAMPLING_PROFILER_SUPPORTED)
+#define MAYBE_WillRestartSamplerAfterIdleShutdown \
+  WillRestartSamplerAfterIdleShutdown
+#else
+#define MAYBE_WillRestartSamplerAfterIdleShutdown \
+  DISABLED_WillRestartSamplerAfterIdleShutdown
+#endif
+TEST_F(StackSamplingProfilerTest, MAYBE_WillRestartSamplerAfterIdleShutdown) {
+  SamplingParams params;
+  params.sampling_interval = TimeDelta::FromMilliseconds(0);
+  params.samples_per_burst = 1;
+
+  std::vector<CallStackProfile> profiles;
+  CaptureProfiles(params, AVeryLongTimeDelta(), &profiles);
+  ASSERT_EQ(1u, profiles.size());
+
+  // Capture thread should still be running at this point.
+  ASSERT_TRUE(StackSamplingProfiler::TestAPI::IsSamplingThreadRunning());
+
+  // Post a ShutdownTask on the sampling thread which, when executed, will
+  // mark the thread as EXITING and begin shut down of the thread.
+  StackSamplingProfiler::TestAPI::PerformSamplingThreadIdleShutdown(false);
+
+  // Ensure another capture will start the sampling thread and run.
+  profiles.clear();
+  CaptureProfiles(params, AVeryLongTimeDelta(), &profiles);
+  ASSERT_EQ(1u, profiles.size());
+  EXPECT_TRUE(StackSamplingProfiler::TestAPI::IsSamplingThreadRunning());
+}
+
+// Checks that it's safe to stop a task after it's completed and the sampling
+// thread has shut-down for being idle.
+#if defined(STACK_SAMPLING_PROFILER_SUPPORTED)
+#define MAYBE_StopAfterIdleShutdown StopAfterIdleShutdown
+#else
+#define MAYBE_StopAfterIdleShutdown DISABLED_StopAfterIdleShutdown
+#endif
+TEST_F(StackSamplingProfilerTest, MAYBE_StopAfterIdleShutdown) {
+  WithTargetThread([](PlatformThreadId target_thread_id) {
+    std::vector<SamplingParams> params(1);
+
+    params[0].sampling_interval = TimeDelta::FromMilliseconds(1);
+    params[0].samples_per_burst = 1;
+
+    std::vector<CallStackProfiles> profiles;
+    std::vector<std::unique_ptr<WaitableEvent>> sampling_completed;
+    std::vector<std::unique_ptr<StackSamplingProfiler>> profilers;
+    CreateProfilers(target_thread_id, params, nullptr, &profiles, &profilers,
+                    &sampling_completed);
+
+    profilers[0]->Start();
+    sampling_completed[0]->Wait();
+
+    // Capture thread should still be running at this point.
+    ASSERT_TRUE(StackSamplingProfiler::TestAPI::IsSamplingThreadRunning());
+
+    // Perform an idle shutdown.
+    StackSamplingProfiler::TestAPI::PerformSamplingThreadIdleShutdown(false);
+
+    // Stop should be safe though its impossible to know at this moment if the
+    // sampling thread has completely exited or will just "stop soon".
+    profilers[0]->Stop();
+  });
+}
+
+// Checks that profilers can run both before and after the sampling thread has
+// started.
+#if defined(STACK_SAMPLING_PROFILER_SUPPORTED)
+#define MAYBE_ProfileBeforeAndAfterSamplingThreadRunning \
+  ProfileBeforeAndAfterSamplingThreadRunning
+#else
+#define MAYBE_ProfileBeforeAndAfterSamplingThreadRunning \
+  DISABLED_ProfileBeforeAndAfterSamplingThreadRunning
+#endif
+TEST_F(StackSamplingProfilerTest,
+       MAYBE_ProfileBeforeAndAfterSamplingThreadRunning) {
+  WithTargetThread([](PlatformThreadId target_thread_id) {
+    std::vector<SamplingParams> params(2);
+
+    params[0].initial_delay = AVeryLongTimeDelta();
+    params[0].sampling_interval = TimeDelta::FromMilliseconds(1);
+    params[0].samples_per_burst = 1;
+
+    params[1].initial_delay = TimeDelta::FromMilliseconds(0);
+    params[1].sampling_interval = TimeDelta::FromMilliseconds(1);
+    params[1].samples_per_burst = 1;
+
+    std::vector<CallStackProfiles> profiles;
+    std::vector<std::unique_ptr<WaitableEvent>> sampling_completed;
+    std::vector<std::unique_ptr<StackSamplingProfiler>> profilers;
+    CreateProfilers(target_thread_id, params, nullptr, &profiles, &profilers,
+                    &sampling_completed);
+
+    // First profiler is started when there has never been a sampling thread.
+    EXPECT_FALSE(StackSamplingProfiler::TestAPI::IsSamplingThreadRunning());
+    profilers[0]->Start();
+    // Second profiler is started when sampling thread is already running.
+    EXPECT_TRUE(StackSamplingProfiler::TestAPI::IsSamplingThreadRunning());
+    profilers[1]->Start();
+
+    // Only the second profiler should finish before test times out.
+    size_t completed_profiler = WaitForSamplingComplete(&sampling_completed);
+    EXPECT_EQ(1U, completed_profiler);
+  });
+}
+
+// Checks that an idle-shutdown task will abort if a new profiler starts
+// between when it was posted and when it runs.
+#if defined(STACK_SAMPLING_PROFILER_SUPPORTED)
+#define MAYBE_IdleShutdownAbort IdleShutdownAbort
+#else
+#define MAYBE_IdleShutdownAbort DISABLED_IdleShutdownAbort
+#endif
+TEST_F(StackSamplingProfilerTest, MAYBE_IdleShutdownAbort) {
+  WithTargetThread([](PlatformThreadId target_thread_id) {
+    std::vector<SamplingParams> params(1);
+
+    params[0].sampling_interval = TimeDelta::FromMilliseconds(1);
+    params[0].samples_per_burst = 1;
+
+    std::vector<CallStackProfiles> profiles;
+    std::vector<std::unique_ptr<WaitableEvent>> sampling_completed;
+    std::vector<std::unique_ptr<StackSamplingProfiler>> profilers;
+    CreateProfilers(target_thread_id, params, nullptr, &profiles, &profilers,
+                    &sampling_completed);
+
+    profilers[0]->Start();
+    sampling_completed[0]->Wait();
+
+    // Perform an idle shutdown but simulate that a new capture is started
+    // before it can actually run.
+    StackSamplingProfiler::TestAPI::PerformSamplingThreadIdleShutdown(true);
+
+    // Though the shutdown-task has been executed, any actual exit of the
+    // thread is asynchronous so there is no way to detect that *didn't* exit
+    // except to wait a reasonable amount of time and then check. Since the
+    // thread was just running ("perform" blocked until it was), it should
+    // finish almost immediately and without any waiting for tasks or events.
+    PlatformThread::Sleep(base::TimeDelta::FromMilliseconds(200));
+    EXPECT_TRUE(StackSamplingProfiler::TestAPI::IsSamplingThreadRunning());
+  });
+}
+
+// Checks that synchronized multiple sampling requests execute in parallel.
+#if defined(STACK_SAMPLING_PROFILER_SUPPORTED)
+#define MAYBE_ConcurrentProfiling_InSync ConcurrentProfiling_InSync
+#else
+#define MAYBE_ConcurrentProfiling_InSync DISABLED_ConcurrentProfiling_InSync
+#endif
+TEST_F(StackSamplingProfilerTest, MAYBE_ConcurrentProfiling_InSync) {
+  WithTargetThread([](PlatformThreadId target_thread_id) {
+    std::vector<SamplingParams> params(2);
+
+    // Providing an initial delay makes it more likely that both will be
+    // scheduled before either starts to run. Once started, samples will
+    // run ordered by their scheduled, interleaved times regardless of
+    // whatever interval the thread wakes up. Thus, total execution time
+    // will be 10ms (delay) + 10x1ms (sampling) + 1/2 timer minimum interval.
     params[0].initial_delay = TimeDelta::FromMilliseconds(10);
-    params[0].sampling_interval = TimeDelta::FromMilliseconds(0);
-    params[0].samples_per_burst = 1;
-
-    params[1].sampling_interval = TimeDelta::FromMilliseconds(0);
-    params[1].samples_per_burst = 1;
-
-    CallStackProfiles profiles[2];
-    std::vector<std::unique_ptr<WaitableEvent>> sampling_completed(2);
-    std::vector<std::unique_ptr<StackSamplingProfiler>> profiler(2);
-    for (int i = 0; i < 2; ++i) {
-      sampling_completed[i] =
-          MakeUnique<WaitableEvent>(WaitableEvent::ResetPolicy::AUTOMATIC,
-                                    WaitableEvent::InitialState::NOT_SIGNALED);
-      const StackSamplingProfiler::CompletedCallback callback =
-          Bind(&SaveProfilesAndSignalEvent, Unretained(&profiles[i]),
-               Unretained(sampling_completed[i].get()));
-      profiler[i] = MakeUnique<StackSamplingProfiler>(target_thread_id,
-                                                      params[i], callback);
-    }
-
-    profiler[0]->Start();
-    profiler[1]->Start();
-
-    std::vector<WaitableEvent*> sampling_completed_rawptrs(
-        sampling_completed.size());
-    std::transform(
-        sampling_completed.begin(), sampling_completed.end(),
-        sampling_completed_rawptrs.begin(),
-        [](const std::unique_ptr<WaitableEvent>& elem) { return elem.get(); });
+    params[0].sampling_interval = TimeDelta::FromMilliseconds(1);
+    params[0].samples_per_burst = 9;
+
+    params[1].initial_delay = TimeDelta::FromMilliseconds(11);
+    params[1].sampling_interval = TimeDelta::FromMilliseconds(1);
+    params[1].samples_per_burst = 8;
+
+    std::vector<CallStackProfiles> profiles;
+    std::vector<std::unique_ptr<WaitableEvent>> sampling_completed;
+    std::vector<std::unique_ptr<StackSamplingProfiler>> profilers;
+    CreateProfilers(target_thread_id, params, nullptr, &profiles, &profilers,
+                    &sampling_completed);
+
+    profilers[0]->Start();
+    profilers[1]->Start();
+
     // Wait for one profiler to finish.
-    size_t completed_profiler =
-        WaitableEvent::WaitMany(sampling_completed_rawptrs.data(), 2);
+    size_t completed_profiler = WaitForSamplingComplete(&sampling_completed);
+    ASSERT_EQ(1u, profiles[completed_profiler].size());
+
+    size_t other_profiler = 1 - completed_profiler;
+    // Wait for the other profiler to finish.
+    sampling_completed[other_profiler]->Wait();
+    ASSERT_EQ(1u, profiles[other_profiler].size());
+
+    // Ensure each got the correct number of samples.
+    EXPECT_EQ(9u, profiles[0][0].samples.size());
+    EXPECT_EQ(8u, profiles[1][0].samples.size());
+  });
+}
+
+// Checks that several mixed sampling requests execute in parallel.
+#if defined(STACK_SAMPLING_PROFILER_SUPPORTED)
+#define MAYBE_ConcurrentProfiling_Mixed ConcurrentProfiling_Mixed
+#else
+#define MAYBE_ConcurrentProfiling_Mixed DISABLED_ConcurrentProfiling_Mixed
+#endif
+TEST_F(StackSamplingProfilerTest, MAYBE_ConcurrentProfiling_Mixed) {
+  WithTargetThread([](PlatformThreadId target_thread_id) {
+    std::vector<SamplingParams> params(3);
+
+    params[0].initial_delay = TimeDelta::FromMilliseconds(8);
+    params[0].sampling_interval = TimeDelta::FromMilliseconds(4);
+    params[0].samples_per_burst = 10;
+
+    params[1].initial_delay = TimeDelta::FromMilliseconds(9);
+    params[1].sampling_interval = TimeDelta::FromMilliseconds(3);
+    params[1].samples_per_burst = 10;
+
+    params[2].initial_delay = TimeDelta::FromMilliseconds(10);
+    params[2].sampling_interval = TimeDelta::FromMilliseconds(2);
+    params[2].samples_per_burst = 10;
+
+    std::vector<CallStackProfiles> profiles;
+    std::vector<std::unique_ptr<WaitableEvent>> sampling_completed;
+    std::vector<std::unique_ptr<StackSamplingProfiler>> profilers;
+    CreateProfilers(target_thread_id, params, nullptr, &profiles, &profilers,
+                    &sampling_completed);
+
+    for (size_t i = 0; i < profilers.size(); ++i)
+      profilers[i]->Start();
+
+    // Wait for one profiler to finish.
+    size_t completed_profiler = WaitForSamplingComplete(&sampling_completed);
     EXPECT_EQ(1u, profiles[completed_profiler].size());
-
-    size_t other_profiler = 1 - completed_profiler;
-    // Give the other profiler a chance to run and observe that it hasn't.
-    EXPECT_FALSE(sampling_completed[other_profiler]->TimedWait(
-        TimeDelta::FromMilliseconds(25)));
-
-    // Start the other profiler again and it should run.
-    profiler[other_profiler]->Start();
-    sampling_completed[other_profiler]->Wait();
-    EXPECT_EQ(1u, profiles[other_profiler].size());
-  });
-}
-
+    // Stop and destroy all profilers, always in the same order. Don't crash.
+    for (size_t i = 0; i < profilers.size(); ++i)
+      profilers[i]->Stop();
+    for (size_t i = 0; i < profilers.size(); ++i)
+      profilers[i].reset();
+  });
+}
+
 // Checks that a stack that runs through another library produces a stack with
 // the expected functions.
 #if defined(STACK_SAMPLING_PROFILER_SUPPORTED)
 #define MAYBE_OtherLibrary OtherLibrary
 #else
 #define MAYBE_OtherLibrary DISABLED_OtherLibrary
 #endif
-TEST(StackSamplingProfilerTest, MAYBE_OtherLibrary) {
+TEST_F(StackSamplingProfilerTest, MAYBE_OtherLibrary) {
   SamplingParams params;
   params.sampling_interval = TimeDelta::FromMilliseconds(0);
   params.samples_per_burst = 1;
 
   std::vector<CallStackProfile> profiles;
   {
     ScopedNativeLibrary other_library(LoadOtherLibrary());
     WithTargetThread(
         [&params, &profiles](PlatformThreadId target_thread_id) {
           WaitableEvent sampling_thread_completed(
@@ skipping 50 matching lines @@
       << "Stack:\n" << FormatSampleForDiagnosticOutput(sample, profile.modules);
 }
 
 // Checks that a stack that runs through a library that is unloading produces a
 // stack, and doesn't crash.
 #if defined(STACK_SAMPLING_PROFILER_SUPPORTED)
 #define MAYBE_UnloadingLibrary UnloadingLibrary
 #else
 #define MAYBE_UnloadingLibrary DISABLED_UnloadingLibrary
 #endif
-TEST(StackSamplingProfilerTest, MAYBE_UnloadingLibrary) {
+TEST_F(StackSamplingProfilerTest, MAYBE_UnloadingLibrary) {
   TestLibraryUnload(false);
 }
 
 // Checks that a stack that runs through a library that has been unloaded
 // produces a stack, and doesn't crash.
 #if defined(STACK_SAMPLING_PROFILER_SUPPORTED)
 #define MAYBE_UnloadedLibrary UnloadedLibrary
 #else
 #define MAYBE_UnloadedLibrary DISABLED_UnloadedLibrary
 #endif
-TEST(StackSamplingProfilerTest, MAYBE_UnloadedLibrary) {
+TEST_F(StackSamplingProfilerTest, MAYBE_UnloadedLibrary) {
   TestLibraryUnload(true);
 }
 
+// Checks that different threads can be sampled in parallel.
+#if defined(STACK_SAMPLING_PROFILER_SUPPORTED)
+#define MAYBE_MultipleSampledThreads MultipleSampledThreads
+#else
+#define MAYBE_MultipleSampledThreads DISABLED_MultipleSampledThreads
+#endif
+TEST_F(StackSamplingProfilerTest, MAYBE_MultipleSampledThreads) {
+  // Create target threads. The extra parethesis around the StackConfiguration
+  // call are to avoid the most-vexing-parse problem.
+  TargetThread target_thread1((StackConfiguration(StackConfiguration::NORMAL)));
+  TargetThread target_thread2((StackConfiguration(StackConfiguration::NORMAL)));
+  PlatformThreadHandle target_thread_handle1, target_thread_handle2;
+  EXPECT_TRUE(
+      PlatformThread::Create(0, &target_thread1, &target_thread_handle1));
+  EXPECT_TRUE(
+      PlatformThread::Create(0, &target_thread2, &target_thread_handle2));
+  target_thread1.WaitForThreadStart();
+  target_thread2.WaitForThreadStart();
+
+  // Providing an initial delay makes it more likely that both will be
+  // scheduled before either starts to run. Once started, samples will
+  // run ordered by their scheduled, interleaved times regardless of
+  // whatever interval the thread wakes up.
+  SamplingParams params1, params2;
+  params1.initial_delay = TimeDelta::FromMilliseconds(10);
+  params1.sampling_interval = TimeDelta::FromMilliseconds(1);
+  params1.samples_per_burst = 9;
+  params2.initial_delay = TimeDelta::FromMilliseconds(10);
+  params2.sampling_interval = TimeDelta::FromMilliseconds(1);
+  params2.samples_per_burst = 8;
+
+  std::vector<CallStackProfile> profiles1, profiles2;
+
+  WaitableEvent sampling_thread_completed1(
+      WaitableEvent::ResetPolicy::MANUAL,
+      WaitableEvent::InitialState::NOT_SIGNALED);
+  const StackSamplingProfiler::CompletedCallback callback1 =
+      Bind(&SaveProfilesAndSignalEvent, Unretained(&profiles1),
+           Unretained(&sampling_thread_completed1));
+  StackSamplingProfiler profiler1(target_thread1.id(), params1, callback1);
+
+  WaitableEvent sampling_thread_completed2(
+      WaitableEvent::ResetPolicy::MANUAL,
+      WaitableEvent::InitialState::NOT_SIGNALED);
+  const StackSamplingProfiler::CompletedCallback callback2 =
+      Bind(&SaveProfilesAndSignalEvent, Unretained(&profiles2),
+           Unretained(&sampling_thread_completed2));
+  StackSamplingProfiler profiler2(target_thread2.id(), params2, callback2);
+
+  // Finally the real work.
+  profiler1.Start();
+  profiler2.Start();
+  sampling_thread_completed1.Wait();
+  sampling_thread_completed2.Wait();
+  ASSERT_EQ(1u, profiles1.size());
+  EXPECT_EQ(9u, profiles1[0].samples.size());
+  ASSERT_EQ(1u, profiles2.size());
+  EXPECT_EQ(8u, profiles2[0].samples.size());
+
+  target_thread1.SignalThreadToFinish();
+  target_thread2.SignalThreadToFinish();
+  PlatformThread::Join(target_thread_handle1);
+  PlatformThread::Join(target_thread_handle2);
+}
+
+// A simple thread that runs a profiler on another thread.
+class ProfilerThread : public SimpleThread {
+ public:
+  ProfilerThread(const std::string& name,
+                 PlatformThreadId thread_id,
+                 const SamplingParams& params)
+      : SimpleThread(name, Options()),
+        run_(WaitableEvent::ResetPolicy::MANUAL,
+             WaitableEvent::InitialState::NOT_SIGNALED),
+        completed_(WaitableEvent::ResetPolicy::MANUAL,
+                   WaitableEvent::InitialState::NOT_SIGNALED),
+        profiler_(thread_id,
+                  params,
+                  Bind(&SaveProfilesAndSignalEvent,
+                       Unretained(&profiles_),
+                       Unretained(&completed_))) {}
+
+  void Run() override {
+    run_.Wait();
+    profiler_.Start();
+  }
+
+  void Go() { run_.Signal(); }
+
+  void Wait() { completed_.Wait(); }
+
+  CallStackProfiles& profiles() { return profiles_; }
+
+ private:
+  WaitableEvent run_;
+
+  CallStackProfiles profiles_;
+  WaitableEvent completed_;
+  StackSamplingProfiler profiler_;
+};
+
+// Checks that different threads can run samplers in parallel.
+#if defined(STACK_SAMPLING_PROFILER_SUPPORTED)
+#define MAYBE_MultipleProfilerThreads MultipleProfilerThreads
+#else
+#define MAYBE_MultipleProfilerThreads DISABLED_MultipleProfilerThreads
+#endif
+TEST_F(StackSamplingProfilerTest, MAYBE_MultipleProfilerThreads) {
+  WithTargetThread([](PlatformThreadId target_thread_id) {
+    // Providing an initial delay makes it more likely that both will be
+    // scheduled before either starts to run. Once started, samples will
+    // run ordered by their scheduled, interleaved times regardless of
+    // whatever interval the thread wakes up.
+    SamplingParams params1, params2;
+    params1.initial_delay = TimeDelta::FromMilliseconds(10);
+    params1.sampling_interval = TimeDelta::FromMilliseconds(1);
+    params1.samples_per_burst = 9;
+    params2.initial_delay = TimeDelta::FromMilliseconds(10);
+    params2.sampling_interval = TimeDelta::FromMilliseconds(1);
+    params2.samples_per_burst = 8;
+
+    // Start the profiler threads and give them a moment to get going.
+    ProfilerThread profiler1("profiler_thread1", target_thread_id, params1);
+    ProfilerThread profiler2("profiler_thread2", target_thread_id, params2);
+    profiler1.Start();
+    profiler2.Start();
+    PlatformThread::Sleep(base::TimeDelta::FromMilliseconds(10));
+
+    // This will (approximately) synchronize the two threads.
+    profiler1.Go();
+    profiler2.Go();
+
+    // Wait for them both to finish and validate collection.
+    profiler1.Wait();
+    profiler2.Wait();
+    ASSERT_EQ(1u, profiler1.profiles().size());
+    EXPECT_EQ(9u, profiler1.profiles()[0].samples.size());
+    ASSERT_EQ(1u, profiler2.profiles().size());
+    EXPECT_EQ(8u, profiler2.profiles()[0].samples.size());
+
+    profiler1.Join();
+    profiler2.Join();
+  });
+}
+
 }  // namespace base