Create a field trial to test if we can detect good QPC implementations.

chrome/browser/metrics/time_ticks_experiment_win.cc

+// Copyright (c) 2013 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/metrics/time_ticks_experiment_win.h"
+
+#if defined(OS_WIN)
+
+#include "base/cpu.h"
+#include "base/metrics/histogram.h"
+#include "base/win/windows_version.h"
+
+#include <windows.h>
+
+namespace chrome_browser_metrics {
+
+namespace {
+
+const int kNumIterations = 1000;
+
+}  // anonymous namespace
+
+void CollectTimeTicksStats() {
+  // This bit is supposed to indicate that rdtsc is safe across cores. If so, we
+  // can use QPC as long as it uses rdtsc.
+  // TODO(simonjam): We should look for other signals that QPC might be safe and
+  // test them out here.
+  base::CPU cpu;
+  UMA_HISTOGRAM_BOOLEAN("WinTimeTicks.NonStopTsc",
+                        cpu.has_non_stop_time_stamp_counter());
+  if (!cpu.has_non_stop_time_stamp_counter()) {
+    return;
+  }
+
+  SYSTEM_INFO sys_info;
+  GetSystemInfo(&sys_info);
+  DWORD num_cores = sys_info.dwNumberOfProcessors;
+  DWORD current_core = 0;
+
+  typedef DWORD (WINAPI *GetCurrentProcessorFunction)();
+  GetCurrentProcessorFunction get_current_processor_func = NULL;
+
+  if (num_cores > 1) {
+    // Windows XP doesn't have a function to tell which core we're currently
+    // running on. Here, we determine if the function is available and use it if
+    // so. If the function is not available, we bail.
+    HMODULE module = GetModuleHandle(TEXT("kernel32.dll"));
+    if (!module) {
+      return;
+    }
+    get_current_processor_func =
+        reinterpret_cast<GetCurrentProcessorFunction>(
+            GetProcAddress(module, "GetCurrentProcessorNumber"));
+    if (!get_current_processor_func) {

[Sigurður Ásgeirsson, 2013/04/17 13:36:16]

since this function is unavailable on Windows XP, you'll get no metrics at all
for XP.
If you're good with that, then this is fine, but somehow I don't think that was
your intent?

+      return;
+    }
+    current_core = get_current_processor_func();
+  }
+
+  base::win::OSInfo* info = base::win::OSInfo::GetInstance();
+  UMA_HISTOGRAM_ENUMERATION("WinTimeTicks.VersionTotal", info->version(),
+                            base::win::VERSION_WIN_LAST);
+
+  DWORD starting_core = get_current_processor_func();
+  bool did_change_cores = false;
+
+  LARGE_INTEGER qpc_frequency;
+  QueryPerformanceFrequency(&qpc_frequency);
+
+  int min_delta = 1e9;
+  LARGE_INTEGER qpc_last;
+  QueryPerformanceCounter(&qpc_last);
+  for (int i = 0; i < kNumIterations; ++i) {
+    LARGE_INTEGER qpc_now;
+    QueryPerformanceCounter(&qpc_now);
+    int delta = static_cast<int>(qpc_now.QuadPart - qpc_last.QuadPart);
+    if (delta != 0) {
+      min_delta = std::min(min_delta, delta);
+    }
+    qpc_last = qpc_now;
+
+    if (num_cores > 1 && (i % 100) == 0) {
+      ++current_core;
+      if (current_core > num_cores) {
+        current_core = 0;
+      }
+      SetThreadAffinityMask(GetCurrentThread(), 1 << current_core);

[Sigurður Ásgeirsson, 2013/04/17 13:36:16]

This function can fail and a failure in this function is significant to your
code, check the return value. See also below.

+      if (!did_change_cores &&

[Sigurður Ásgeirsson, 2013/04/17 13:36:16]

My earlier comment on this code was to the effect that I believe it's an
inviolate post-condition of SetThreadAffinityMask, that upon *successful* return
of the function, you'll be running on one of the cores you specified. 

Witness:
http://msdn.microsoft.com/en-ca/library/windows/desktop/ms686247(v=vs.85).aspx
"If the new thread affinity mask does not specify the processor that is
currently running the thread, the thread is rescheduled on one of the allowable
processors."

So, instead of checking the current core explicitly, you may assume that you're
running on the one you specified. Since it's a "the world's coming to an end"
condition if you're not (a system API has violated it's post-conditions), it
might be appropriate to CHECK or DCHECK here if the function pointer is not
NULL.

+          get_current_processor_func() != starting_core) {
+        did_change_cores = true;
+      }
+    }
+  }
+
+  if (num_cores > 1) {
+    if (did_change_cores) {
+      UMA_HISTOGRAM_ENUMERATION("WinTimeTicks.ChangedCores", info->version(),
+                                base::win::VERSION_WIN_LAST);
+    } else {
+      UMA_HISTOGRAM_ENUMERATION("WinTimeTicks.FailedToChangeCores",
+                                info->version(), base::win::VERSION_WIN_LAST);
+    }
+  }
+
+  if (min_delta >= 0) {
+    UMA_HISTOGRAM_ENUMERATION("WinTimeTicks.NonDecreasing", info->version(),
+                              base::win::VERSION_WIN_LAST);
+  } else {
+    return;
+  }
+
+  int min_delta_ns = static_cast<int>(
+      min_delta * (1e9 / qpc_frequency.QuadPart));
+  UMA_HISTOGRAM_CUSTOM_COUNTS("WinTimeTicks.MinResolutionNanoseconds",
+                              min_delta_ns, 1, 1000000, 50);
+
+  bool success = min_delta_ns <= 10000;
+  if (success) {
+    UMA_HISTOGRAM_ENUMERATION("WinTimeTicks.VersionSuccessful",
+                              info->version(), base::win::VERSION_WIN_LAST);
+  }
+}
+
+}  // namespace chrome_browser_net
+
+#endif  // defined(OS_WIN)