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diff --git a/xpcom/threads/CPUUsageWatcher.h b/xpcom/threads/CPUUsageWatcher.h
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+/* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
+/* vim: set ts=8 sts=2 et sw=2 tw=80: */
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+#ifndef mozilla_CPUUsageWatcher_h
+#define mozilla_CPUUsageWatcher_h
+
+#include <stdint.h>
+
+#include "mozilla/HangAnnotations.h"
+#include "mozilla/Result.h"
+
+// We only support OSX and Windows, because on Linux we're forced to read
+// from /proc/stat in order to get global CPU values. We would prefer to not
+// eat that cost for this.
+#if defined(NIGHTLY_BUILD) && (defined(XP_WIN) || defined(XP_MACOSX))
+#  define CPU_USAGE_WATCHER_ACTIVE
+#endif
+
+namespace mozilla {
+
+// Start error values at 1 to allow using the UnusedZero Result
+// optimization.
+enum CPUUsageWatcherError : uint8_t {
+  ClockGetTimeError = 1,
+  GetNumberOfProcessorsError,
+  GetProcessTimesError,
+  GetSystemTimesError,
+  HostStatisticsError,
+  ProcStatError,
+};
+
+namespace detail {
+
+template <>
+struct UnusedZero<CPUUsageWatcherError> : UnusedZeroEnum<CPUUsageWatcherError> {
+};
+
+}  // namespace detail
+
+class CPUUsageHangAnnotator : public BackgroundHangAnnotator {
+ public:
+};
+
+class CPUUsageWatcher : public BackgroundHangAnnotator {
+ public:
+#ifdef CPU_USAGE_WATCHER_ACTIVE
+  CPUUsageWatcher()
+      : mInitialized(false),
+        mExternalUsageThreshold(0),
+        mExternalUsageRatio(0),
+        mProcessUsageTime(0),
+        mProcessUpdateTime(0),
+        mGlobalUsageTime(0),
+        mGlobalUpdateTime(0),
+        mNumCPUs(0) {}
+#endif
+
+  Result<Ok, CPUUsageWatcherError> Init();
+
+  void Uninit();
+
+  // Updates necessary values to allow AnnotateHang to function. This must be
+  // called on some semi-regular basis, as it will calculate the mean CPU
+  // usage values between now and the last time it was called.
+  Result<Ok, CPUUsageWatcherError> CollectCPUUsage();
+
+  void AnnotateHang(BackgroundHangAnnotations& aAnnotations) final;
+
+ private:
+#ifdef CPU_USAGE_WATCHER_ACTIVE
+  bool mInitialized;
+  // The threshold above which we will mark a hang as occurring under high
+  // external CPU usage conditions
+  float mExternalUsageThreshold;
+  // The CPU usage (0-1) external to our process, averaged between the two
+  // most recent monitor thread runs
+  float mExternalUsageRatio;
+  // The total cumulative CPU usage time by our process as of the last
+  // CollectCPUUsage or Startup
+  uint64_t mProcessUsageTime;
+  // A time value in the same units as mProcessUsageTime used to
+  // determine the ratio of CPU usage time to idle time
+  uint64_t mProcessUpdateTime;
+  // The total cumulative CPU usage time by all processes as of the last
+  // CollectCPUUsage or Startup
+  uint64_t mGlobalUsageTime;
+  // A time value in the same units as mGlobalUsageTime used to
+  // determine the ratio of CPU usage time to idle time
+  uint64_t mGlobalUpdateTime;
+  // The number of virtual cores on our machine
+  uint64_t mNumCPUs;
+#endif
+};
+
+}  // namespace mozilla
+
+#endif  // mozilla_CPUUsageWatcher_h