Task manager should support Idle Wakeups on Windows

chrome/browser/task_management/sampling/shared_sampler_win.cc

+// Copyright 2016 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 "chrome/browser/task_management/sampling/shared_sampler.h"
+
+#include <windows.h>
+#include <winternl.h>
+
+#include <algorithm>
+
+#include "base/bind.h"
+#include "base/command_line.h"
+#include "base/path_service.h"
+#include "base/time/time.h"
+#include "chrome/browser/task_management/task_manager_observer.h"
+#include "chrome/common/chrome_constants.h"
+#include "content/public/browser/browser_thread.h"
+
+namespace task_management {
+
+namespace {
+
+// From <wdm.h>
+typedef LONG KPRIORITY;
+typedef LONG KWAIT_REASON;  // Full definition is in wdm.h
+
+// From ntddk.h
+typedef struct _VM_COUNTERS {
+  SIZE_T PeakVirtualSize;
+  SIZE_T VirtualSize;
+  ULONG PageFaultCount;
+  // Padding here in 64-bit
+  SIZE_T PeakWorkingSetSize;
+  SIZE_T WorkingSetSize;
+  SIZE_T QuotaPeakPagedPoolUsage;
+  SIZE_T QuotaPagedPoolUsage;
+  SIZE_T QuotaPeakNonPagedPoolUsage;
+  SIZE_T QuotaNonPagedPoolUsage;
+  SIZE_T PagefileUsage;
+  SIZE_T PeakPagefileUsage;
+} VM_COUNTERS;
+
+// Two possibilities available from here:
+// http://stackoverflow.com/questions/28858849/where-is-system-information-class-defined
+
+typedef enum _SYSTEM_INFORMATION_CLASS {
+  SystemProcessInformation = 5,  // This is the number that we need.
+} SYSTEM_INFORMATION_CLASS;
+
+// https://msdn.microsoft.com/en-us/library/gg750647.aspx?f=255&MSPPError=-2147217396
+typedef struct {
+  HANDLE UniqueProcess;  // Actually process ID
+  HANDLE UniqueThread;   // Actually thread ID
+} CLIENT_ID;
+
+// From http://alax.info/blog/1182, with corrections and modifications
+// Originally from
+// http://undocumented.ntinternals.net/index.html?page=UserMode%2FUndocumented%20Functions%2FSystem%20Information%2FStructures%2FSYSTEM_THREAD.html
+struct SYSTEM_THREAD_INFORMATION {
+  ULONGLONG KernelTime;
+  ULONGLONG UserTime;
+  ULONGLONG CreateTime;
+  ULONG WaitTime;
+  // Padding here in 64-bit
+  PVOID StartAddress;
+  CLIENT_ID ClientId;
+  KPRIORITY Priority;
+  LONG BasePriority;
+  ULONG ContextSwitchCount;
+  ULONG State;
+  KWAIT_REASON WaitReason;
+};
+#if _M_X64
+static_assert(sizeof(SYSTEM_THREAD_INFORMATION) == 80,
+              "Structure size mismatch");
+#else
+static_assert(sizeof(SYSTEM_THREAD_INFORMATION) == 64,
+              "Structure size mismatch");
+#endif
+
+// From http://alax.info/blog/1182, with corrections and modifications
+// Originally from
+// http://undocumented.ntinternals.net/index.html?page=UserMode%2FUndocumented%20Functions%2FSystem%20Information%2FStructures%2FSYSTEM_THREAD.html

[ncarter (slow), 2016/08/03 22:28:55]

Without objection, let's leave these definitions here, and trying to address
possible unification with crashpad as a subsequent CL. I think the review and
discussion will be much easier if we proceed that way, since it seems like
there's a range of considerations, and it's not 100% clear that sharing is
actually the best approach.

+struct SYSTEM_PROCESS_INFORMATION {

[brucedawson, 2016/08/03 00:36:55]

If this version isn't merged with other versions then it (and
SYSTEM_THREAD_INFORMATION, and any other types that might be used elsewhere)
should be renamed to avoid ODR violations. Perhaps append _CHROME to the end?

[ncarter (slow), 2016/08/03 22:28:55]

It's in an anonymous namespace, so I think there's no ODR worry. Right?

[stanisc, 2016/08/04 01:36:01]

I thought ODR would be avoided by placing this into anonymous namespace.

+  ULONG NextEntryOffset;
+  ULONG NumberOfThreads;
+  // http://processhacker.sourceforge.net/doc/struct___s_y_s_t_e_m___p_r_o_c_e_s_s___i_n_f_o_r_m_a_t_i_o_n.html
+  ULONGLONG WorkingSetPrivateSize;
+  ULONG HardFaultCount;
+  ULONG Reserved1;
+  ULONGLONG CycleTime;
+  ULONGLONG CreateTime;
+  ULONGLONG UserTime;
+  ULONGLONG KernelTime;
+  UNICODE_STRING ImageName;
+  KPRIORITY BasePriority;
+  HANDLE ProcessId;
+  HANDLE ParentProcessId;
+  ULONG HandleCount;
+  ULONG Reserved2[2];
+  // Padding here in 64-bit
+  VM_COUNTERS VirtualMemoryCounters;
+  size_t Reserved3;
+  IO_COUNTERS IoCounters;
+  SYSTEM_THREAD_INFORMATION Threads[1];
+};
+#if _M_X64
+static_assert(sizeof(SYSTEM_PROCESS_INFORMATION) == 336,
+              "Structure size mismatch");
+#else
+static_assert(sizeof(SYSTEM_PROCESS_INFORMATION) == 248,
+              "Structure size mismatch");
+#endif
+
+// ntstatus.h conflicts with windows.h so define this locally.
+#define STATUS_SUCCESS              ((NTSTATUS)0x00000000L)
+#define STATUS_BUFFER_TOO_SMALL     ((NTSTATUS)0xC0000023L)
+#define STATUS_INFO_LENGTH_MISMATCH ((NTSTATUS)0xC0000004L)
+
+// Wrapper for NtQuerySystemProcessInformation with buffer reallocation logic.
+bool QuerySystemProcessInformation(std::vector<BYTE>* data_buffer,
+                                   ULONG* data_size) {
+  typedef NTSTATUS(WINAPI * NTQUERYSYSTEMINFORMATION)(
+      SYSTEM_INFORMATION_CLASS SystemInformationClass, PVOID SystemInformation,
+      ULONG SystemInformationLength, PULONG ReturnLength);
+
+  HMODULE ntdll = ::GetModuleHandle(L"ntdll.dll");
+  if (!ntdll) {
+    NOTREACHED();
+    return false;
+  }
+
+  NTQUERYSYSTEMINFORMATION nt_query_system_information_ptr =
+      reinterpret_cast<NTQUERYSYSTEMINFORMATION>(
+          ::GetProcAddress(ntdll, "NtQuerySystemInformation"));
+  if (!nt_query_system_information_ptr) {
+    NOTREACHED();
+    return false;
+  }
+
+  ULONG buffer_size = data_buffer->size();
+
+  NTSTATUS result;
+
+  // There is a potential race condition between growing the buffer and new
+  // processes being creating. Try a few times before giving up.

[ncarter (slow), 2016/08/03 22:28:55]

"being creating" -> "being created"

[stanisc, 2016/08/04 01:36:01]

Done.

+  for (int i = 0; i < 10; i++) {
+    *data_size = 0;
+    result = nt_query_system_information_ptr(
+        SystemProcessInformation,
+        buffer_size > 0 ? data_buffer->data() : nullptr,
+        buffer_size, data_size);
+
+    if (result == STATUS_INFO_LENGTH_MISMATCH ||
+        result == STATUS_BUFFER_TOO_SMALL) {
+      // Insufficient buffer. Resize to the returned |data_size| plus 10% extra
+      // to avoid frequent reallocations and try again.
+      DCHECK_GT(*data_size, buffer_size);
+      buffer_size = static_cast<ULONG>(*data_size * 1.1);
+      data_buffer->resize(buffer_size);
+    } else {
+      // Either STATUS_SUCCESS or an error other than the two above.
+      break;
+    }
+  }
+
+  return result == STATUS_SUCCESS;
+}
+
+}  // namespace
+
+// Per-thread data extracted from SYSTEM_THREAD_INFORMATION
+// and stored in a snapshot.
+// This structure is accessed only on the worker thread.

[ncarter (slow), 2016/08/03 22:28:55]

Reflow this comment paragraph. (if you happen to use Sublime Text, the "Wrap
Plus" package is really good at this).

[stanisc, 2016/08/04 01:36:01]

Done.

+struct ThreadData {
+  base::PlatformThreadId thread_id;
+  ULONG context_switches;
+};
+
+// Per-process data extracted from SYSTEM_PROCESS_INFORMATION
+// and stored in a snapshot.
+// This structure is accessed only on the worker thread.

[ncarter (slow), 2016/08/03 22:28:55]

Reflow this comment paragraph.

[stanisc, 2016/08/04 01:36:01]

Done.

+struct ProcessData {
+  ProcessData() = default;
+  ProcessData(ProcessData&&) = default;
+
+  std::vector<ThreadData> threads;
+
+ private:
+  DISALLOW_COPY_AND_ASSIGN(ProcessData);
+};
+
+typedef std::map<base::ProcessId, ProcessData> ProcessDataMap;
+
+// ProcessDataSnapshot gets created and accessed only on the worker thread.
+// This is used to calculate metrics like Idle Wakeups / sec that require
+// a delta between two snapshots.
+struct ProcessDataSnapshot {
+  ProcessDataMap processes;
+  base::TimeTicks timestamp;
+};
+
+ULONG CountContextSwitches(const ProcessData& process_data) {
+  ULONG context_switches = 0;
+  for (const auto& thread_data : process_data.threads) {
+    context_switches += thread_data.context_switches;
+  }
+
+  return context_switches;
+}
+
+ULONG CountContextSwitchesDelta(const ProcessData& prev_process_data,

[ncarter (slow), 2016/08/03 22:39:43]

The following definitions ought to be in an anonymous namespace if possible (to
prevent possible ODR conflicts):

ThreadData
ProcessData
ProcessDataMap
CountContextSwitchesDelta
CountContextSwitches
SeekInPreviousSnapshot

[stanisc, 2016/08/04 01:36:01]

OK. Moved everything to the anonymous namespace. Deleted CountContextSwitches.

+                                const ProcessData& new_process_data) {
+  // This one pass algorithm relies on the threads vectors to be
+  // ordered by thread_id.
+  ULONG delta = 0;
+  size_t prev_index = 0;
+
+  for (const auto& new_thread : new_process_data.threads) {
+    ULONG prev_thread_context_switches = 0;
+
+    // Iterate over the process threads from the previous snapshot skipping
+    // threads that don't exist anymore.

[brucedawson, 2016/08/03 00:36:55]

Maybe tweak this comment to clarify that you're not iterating over *all* of the
process threads from the previous snapshot - just up to the first one whose TID
is >= new_thread.thread_id. Otherwise it looks like an O(n^2) algorithm when
it's actually linear.

[stanisc, 2016/08/04 01:36:01]

Done.

+    for (; prev_index < prev_process_data.threads.size(); ++prev_index) {
+      const auto& prev_thread = prev_process_data.threads[prev_index];
+      if (prev_thread.thread_id == new_thread.thread_id) {
+        // Threads match between two snapshots. Use the previous snapshot
+        // thread's context_switches to subtract from the delta.
+        prev_thread_context_switches = prev_thread.context_switches;
+        ++prev_index;
+        break;
+      }
+
+      if (prev_thread.thread_id > new_thread.thread_id) {
+        // This is due to a new thread that didn't exist in the previous
+        // snapshot. Keep the zero value of |prev_thread_context_switches| which
+        // essentially means the entire number of context switches of the new
+        // thread will be added to the delta.
+        break;
+      }
+    }
+
+    delta += new_thread.context_switches - prev_thread_context_switches;
+  }
+
+  return delta;
+}
+
+SharedSampler::SharedSampler(
+    const scoped_refptr<base::SequencedTaskRunner>& blocking_pool_runner)
+    : refresh_flags_(0), previous_buffer_size_(0),
+      supported_image_names_(GetSupportedImageNames()),
+      blocking_pool_runner_(blocking_pool_runner) {
+  DCHECK(blocking_pool_runner.get());
+
+  // This object will be created on the UI thread, however the sequenced checker
+  // will be used to assert we're running the expensive operations on one of the
+  // blocking pool threads.
+  DCHECK_CURRENTLY_ON(content::BrowserThread::UI);
+  worker_pool_sequenced_checker_.DetachFromSequence();
+}
+
+SharedSampler::~SharedSampler() {}
+
+int64_t SharedSampler::SupportsFlags() const {
+  return REFRESH_TYPE_IDLE_WAKEUPS;
+}
+
+SharedSampler::Callbacks::Callbacks() {}
+
+SharedSampler::Callbacks::~Callbacks() {}
+
+SharedSampler::Callbacks::Callbacks(Callbacks&& other) {
+  on_idle_wakeups = std::move(other.on_idle_wakeups);
+}
+
+void SharedSampler::RegisterCallbacks(
+    base::ProcessId process_id,
+    const OnIdleWakeupsCallback& on_idle_wakeups) {
+  DCHECK_CURRENTLY_ON(content::BrowserThread::UI);
+
+  if (process_id == 0)
+    return;
+
+  Callbacks callbacks;
+  callbacks.on_idle_wakeups = on_idle_wakeups;
+  bool result = callbacks_map_.insert(
+      std::make_pair(process_id, std::move(callbacks))).second;
+  DCHECK(result);
+}
+
+void SharedSampler::UnregisterCallbacks(base::ProcessId process_id) {
+  DCHECK_CURRENTLY_ON(content::BrowserThread::UI);
+
+  if (process_id == 0)
+    return;
+
+  callbacks_map_.erase(process_id);
+
+  if (callbacks_map_.empty())
+    ClearState();
+}
+
+void SharedSampler::Refresh(base::ProcessId process_id, int64_t refresh_flags) {
+  DCHECK_CURRENTLY_ON(content::BrowserThread::UI);
+  DCHECK(callbacks_map_.find(process_id) != callbacks_map_.end());
+  DCHECK_NE(0, refresh_flags & SupportsFlags());
+
+  if (process_id == 0)
+    return;
+
+  if (refresh_flags_ == 0) {
+    base::PostTaskAndReplyWithResult(
+        blocking_pool_runner_.get(), FROM_HERE,
+        base::Bind(&SharedSampler::RefreshOnWorkerThread, this),
+        base::Bind(&SharedSampler::OnRefreshDone, this));
+  } else {
+    // A group of consecuitive Refresh calls should all specify the same

[brucedawson, 2016/08/03 00:36:55]

consecuitive -> consecutive

[stanisc, 2016/08/04 01:36:01]

Done.

+    // refresh flags.
+    DCHECK_EQ(refresh_flags, refresh_flags_);
+  }
+
+  refresh_flags_ |= refresh_flags;
+}
+
+void SharedSampler::ClearState() {
+  previous_snapshot_.reset();
+}
+
+std::unique_ptr<SharedSampler::RefreshResults>
+SharedSampler::RefreshOnWorkerThread() {
+  DCHECK(worker_pool_sequenced_checker_.CalledOnValidSequence());
+
+  std::unique_ptr<RefreshResults> results(new RefreshResults);
+
+  std::unique_ptr<ProcessDataSnapshot> snapshot = CaptureSnapshot();
+  if (snapshot) {
+    if (previous_snapshot_) {
+      MakeResultsFromTwoSnapshots(
+          *previous_snapshot_, *snapshot, results.get());
+    } else {
+      MakeResultsFromSnapshot(*snapshot, results.get());
+    }
+
+    previous_snapshot_ = std::move(snapshot);
+  } else {
+    // Failed to get snapshot. This is unlikely.
+    ClearState();
+  }
+
+  return results;
+}
+
+/* static */
+std::vector<base::FilePath> SharedSampler::GetSupportedImageNames() {
+  const wchar_t kNacl64Exe[] = L"nacl64.exe";
+
+  std::vector<base::FilePath> supported_names;
+
+  base::FilePath current_exe;
+  if (PathService::Get(base::FILE_EXE, &current_exe))
+    supported_names.push_back(current_exe.BaseName());
+
+  supported_names.push_back(
+      base::FilePath(chrome::kBrowserProcessExecutableName));
+  supported_names.push_back(base::FilePath(kNacl64Exe));
+
+  return supported_names;
+}
+
+bool SharedSampler::IsSupportedImageName(
+    base::FilePath::StringPieceType image_name) const {
+  for (const base::FilePath supported_name : supported_image_names_) {
+    if (base::FilePath::CompareEqualIgnoreCase(image_name,
+                                               supported_name.value()))
+      return true;
+  }
+
+  return false;
+}
+
+std::unique_ptr<ProcessDataSnapshot> SharedSampler::CaptureSnapshot() {
+  DCHECK(worker_pool_sequenced_checker_.CalledOnValidSequence());
+
+  ULONG data_size;
+
+  // Preallocate the buffer with the size determined on the previous call to
+  // QuerySystemProcessInformation. This should be sufficient most of the time.
+  // QuerySystemProcessInformation will grow the buffer if necessary.
+  std::vector<BYTE> data_buffer(previous_buffer_size_);

[brucedawson, 2016/08/03 00:36:55]

This may be premature optimization, but std::vector, while very convenient, is
not actually ideal.

std::vector zeroes the array of bytes in its constructor, which is unnecessary
in this one case.
std::vector copies the existing data when you resize, which is unnecessary in
this one case.

It would be easy enough to write a little class that would allocate an
uninitialized array of bytes, and on resize would just free it and reallocate
another array of bytes (instead of copying). I'm not sure if it's worth it. The
redundant zeroing will happen every time but isn't *that* expensive, and the
unnecessary copying would be rare. But, just wanted to throw it out there. I got
curious as to how much code it would actually take and the (untested!) results
are below. FWIW.

class NotVector {
  NotVector(size_t size) {
    size_ = size;
    buffer_ = new BYTE[size];
  }
  ~NotVector() {
    delete [] buffer_;
  }

  void resize(size_t size) {
    delete [] buffer_;
    size_ = size;
    buffer_ = new BYTE[size];
  }

  size_t size() {
    return size_;
  }

  BYTE* data() {
    return buffer_;
  }

  BYTE& operator[](size_t index) {
    return buffer_[index];
  }

  DISALLOW_COPY_AND_ASSIGN(NotVector);

 private:
  BYTE* buffer_;
  size_t size_;
};

[ncarter (slow), 2016/08/03 22:28:55]

Use unique_ptr<BYTE[]> -- possibly wrapped in a class like bruce suggests -- if
vector doesn't cut it. I'm okay with vector for now; let's let any optimizations
we do be guided by profiling data. 

We could probably learn a lot by adding TRACE_EVENT macros to all the
worker-thread refresh operations in the task manager. We do a lot of operations
already each refresh that are O(#processes) and I doubt that an extra memzero
will be noticable; I would wager that changing map<pid, ?>'s to hash maps, or
preserving buffers across refresh cycles, would result in more measurable wins.

[stanisc, 2016/08/04 01:36:01]

OK. I've implemented something similar. I liked the idea because it allowed me
to encapsulate the data size inside this class in addition to the data buffer
and the buffer size.

+
+  if (!QuerySystemProcessInformation(&data_buffer, &data_size))
+    return std::unique_ptr<ProcessDataSnapshot>();
+
+  previous_buffer_size_ = data_buffer.size();
+
+  std::unique_ptr<ProcessDataSnapshot> snapshot(new ProcessDataSnapshot);
+  snapshot->timestamp = base::TimeTicks::Now();
+
+  for (ULONG offset = 0; offset < data_size; ) {
+    auto pi = reinterpret_cast<const SYSTEM_PROCESS_INFORMATION*>(
+        &data_buffer[offset]);
+
+    // Validate that the offset is valid and all needed data is within
+    // the buffer boundary.
+    if (offset + sizeof(SYSTEM_PROCESS_INFORMATION) > data_size)
+      break;
+    if (offset + sizeof(SYSTEM_PROCESS_INFORMATION) +
+            (pi->NumberOfThreads - 1) * sizeof(SYSTEM_THREAD_INFORMATION) >
+        data_size)
+      break;
+
+    if (pi->ImageName.Buffer) {
+      // Validate that the image name is within the buffer boundary.
+      // ImageName.Length seems to be in bytes rather than characters.
+      ULONG image_name_offset =
+          reinterpret_cast<BYTE*>(pi->ImageName.Buffer) - data_buffer.data();
+      if (image_name_offset + pi->ImageName.Length > data_size)
+        break;
+
+      // Check if this is a chrome process. Ignore all other processes.
+      if (IsSupportedImageName(pi->ImageName.Buffer)) {
+        // Collect enough data to be able to do a diff between two snapshots.
+        // Some threads might stop or new threads might be created between two
+        // snapshots. If a thread with a large number of context switches gets
+        // terminated the total number of context switches for the process might
+        // go down and the delta would be negative.
+        // To avoid that we need to compare thread IDs between two snapshots and
+        // not count context switches for threads that are missing in the most
+        // recent snapshot.
+        ProcessData process_data;
+
+        // Iterate over threads and store each thread's ID and number of context
+        // switches.
+        for (ULONG thread_index = 0; thread_index < pi->NumberOfThreads;
+             ++thread_index) {
+          const SYSTEM_THREAD_INFORMATION* ti = &pi->Threads[thread_index];
+          if (ti->ClientId.UniqueProcess != pi->ProcessId)
+            continue;
+
+          ThreadData thread_data;
+          thread_data.thread_id = static_cast<base::PlatformThreadId>(
+              reinterpret_cast<uintptr_t>(ti->ClientId.UniqueThread));
+          thread_data.context_switches = ti->ContextSwitchCount;
+          process_data.threads.push_back(thread_data);
+        }
+
+        // Order thread data by thread ID to help diff two snapshots.
+        std::sort(process_data.threads.begin(), process_data.threads.end(),
+            [](const ThreadData& l, const ThreadData r) {
+              return l.thread_id < r.thread_id;
+            });
+
+        base::ProcessId process_id = static_cast<base::ProcessId>(
+            reinterpret_cast<uintptr_t>(pi->ProcessId));
+        bool inserted = snapshot->processes.insert(
+            std::make_pair(process_id, std::move(process_data))).second;
+        DCHECK(inserted);

[brucedawson, 2016/08/03 00:36:55]

Consider this syntax instead?

snapshot->processes[process_id] = std::move(process_data);

Ditto for the other use of .insert.

[stanisc, 2016/08/04 01:36:01]

The operator[] version doesn't seem to work with DISALLOW_COPY_AND_ASSIGN. The
compiler complained that it couldn't access the private assignment operator. I
think I should keep insert even though I agree it is less readable.

+      }
+    }
+
+    // Check for end of the list.
+    if (!pi->NextEntryOffset)
+      break;
+
+    // Jump to the next entry.
+    offset += pi->NextEntryOffset;
+  }
+
+  return snapshot;
+}
+
+// Seeks a matching ProcessData by Process ID in a previous snapshot.
+// This uses the fact that ProcessDataMap entries are ordered by Process ID.
+const ProcessData* SeekInPreviousSnapshot(
+    base::ProcessId process_id, ProcessDataMap::const_iterator* iter_to_advance,
+    const ProcessDataMap::const_iterator& range_end) {
+  for (; *iter_to_advance != range_end ; ++(*iter_to_advance)) {
+    if ((*iter_to_advance)->first == process_id) {
+      return &((*iter_to_advance)++)->second;
+    }
+    if ((*iter_to_advance)->first > process_id)
+      break;
+  }
+
+  return nullptr;
+}
+
+void SharedSampler::MakeResultsFromTwoSnapshots(
+    const ProcessDataSnapshot& prev_snapshot,
+    const ProcessDataSnapshot& snapshot,
+    RefreshResults* results) {
+  // Time delta in seconds.
+  double time_delta = (snapshot.timestamp - prev_snapshot.timestamp)
+        .InMillisecondsF() / 1000;

[ncarter (slow), 2016/08/03 22:28:55]

You can do .InSecondsF() instead of .InMillisecondsF() / 1000. Or you can just
leave it as a TimeDelta, and do the computation in fixed point, like this:

base::TimeDelta time_delta = snapshot.timestamp - prev_snapshot.timestamp;
result.idle_wakeups_per_second =
    idle_wakeups_delta * base::TimeDelta::FromSeconds(1) / time_delta;

[stanisc, 2016/08/04 01:36:01]

Somehow I didn't see InSecondsF() when I looked at TimeDelta methods. Fixed.

+
+  // Iterate over processes in both snapshots in parallel. This algorithm relies
+  // on map entries being ordered by Process ID.
+  ProcessDataMap::const_iterator prev_iter = prev_snapshot.processes.begin();
+
+  for (const auto& current_entry : snapshot.processes) {
+    base::ProcessId process_id = current_entry.first;
+    const ProcessData& process = current_entry.second;
+
+    const ProcessData* prev_snapshot_process = SeekInPreviousSnapshot(
+        process_id, &prev_iter, prev_snapshot.processes.end());
+
+    // Delta between the old snapshot and the new snapshot.
+    int idle_wakeups_delta;
+
+    if (prev_snapshot_process) {
+      // Processes match between two snapshots. Diff context switches.
+      idle_wakeups_delta =
+          CountContextSwitchesDelta(*prev_snapshot_process, process);
+    } else {
+      // Process is missing in the previous snapshot.
+      // Use entire number of context switches of the current process.
+      idle_wakeups_delta = CountContextSwitches(process);

[ncarter (slow), 2016/08/03 22:28:55]

CountContextSwitches(x) 

  seems equivalent to

CountContextSwitchesDelta(ProcessData(), x)

If you want to, this could let us get rid of CountContextSwitches.

[stanisc, 2016/08/04 01:36:01]

Indeed. Done.

+    }
+
+    RefreshResult result;
+    result.process_id = process_id;
+    result.idle_wakeups_per_second =
+        static_cast<int>(round(idle_wakeups_delta / time_delta));
+    results->push_back(result);
+  }
+}
+
+void SharedSampler::MakeResultsFromSnapshot(const ProcessDataSnapshot& snapshot,
+                                            RefreshResults* results) {
+  for (const auto& pair : snapshot.processes) {
+    RefreshResult result;
+    result.process_id = pair.first;
+    // Use 0 for Idle Wakeups / sec in this case. This is consistent with
+    // ProcessMetrics::CalculateIdleWakeupsPerSecond implementation.
+    result.idle_wakeups_per_second = 0;
+    results->push_back(result);
+  }
+}
+
+void SharedSampler::OnRefreshDone(
+    std::unique_ptr<RefreshResults> refresh_results) {
+  DCHECK_CURRENTLY_ON(content::BrowserThread::UI);
+  DCHECK_NE(0, refresh_flags_);
+
+  size_t result_index = 0;
+
+  for (const auto& callback_entry : callbacks_map_) {
+    base::ProcessId process_id = callback_entry.first;
+    // A sentinel value of -1 is used when the result isn't available.
+    // Task manager will use this to display 'N/A'.
+    int idle_wakeups_per_second = -1;
+
+    // Match refresh result by |process_id|.
+    // This relies on refresh results being ordered by Process ID.
+    // Please note that |refresh_results| might contain some extra entries that
+    // don't exist in |callbacks_map_| if there is more than one instance of
+    // Chrome. It might be missing some entries too if there is a race condition
+    // between getting process information on the worker thread and adding a
+    // corresponding TaskGroup to the task manager.
+    for (; result_index < refresh_results->size(); ++result_index) {
+      const auto& result = (*refresh_results)[result_index];
+      if (result.process_id == process_id) {
+        // Data matched in |refresh_results|.
+        idle_wakeups_per_second = result.idle_wakeups_per_second;
+        ++result_index;
+        break;
+      }
+
+      if (result.process_id > process_id) {
+        // |refresh_results| contains an extra entry for a process that

[stanisc, 2016/08/01 22:34:27]

This comment is wrong. It is the opposite - process_id is missing in
|refresh_results|.
I'll fix this in the next patch set.

[stanisc, 2016/08/04 01:36:01]

Done.

+        // isn't tracked by |callbacks_map_|.
+        break;
+      }
+    }
+
+    if (TaskManagerObserver::IsResourceRefreshEnabled(REFRESH_TYPE_IDLE_WAKEUPS,
+                                                      refresh_flags_)) {
+      callback_entry.second.on_idle_wakeups.Run(idle_wakeups_per_second);
+    }
+  }
+
+  // Reset refresh_results_ to trigger RefreshOnWorkerThread next time Refresh
+  // is called.
+  refresh_flags_ = 0;
+}
+
+}  // namespace task_management