[telemetry] Detect and correct overflow in Intel energy MSRs.

Failure seen here:
http://build.chromium.org/p/chromium.perf/builders/Win%207%20Perf%20%281%29/builds/52
AssertionError: Negative energy consumption. (Starting energy was 65522.6123962.)

BUG=336556, 336558
TEST=None.
R=tonyg

Review URL: https://codereview.chromium.org/607733002

Cr-Commit-Position: refs/heads/master@{#296880}

tools/telemetry/telemetry/core/platform/linux_platform_backend.py

@@ -103,7 +103,7 @@ class LinuxPlatformBackend(
   def StopMonitoringPower(self):
     return self._power_monitor.StopMonitoringPower()
 
-  def ReadMsr(self, msr_number):
+  def ReadMsr(self, msr_number, start=0, length=64):
     cmd = ['/usr/sbin/rdmsr', '-d', str(msr_number)]
     (out, err) = subprocess.Popen(cmd,
                                   stdout=subprocess.PIPE,
@@ -114,7 +114,7 @@ class LinuxPlatformBackend(
       result = int(out)
     except ValueError:
       raise OSError('Cannot interpret rdmsr output: %s' % out)
-    return result
+    return result >> start & ((1 << length) - 1)
 
   def _IsIpfwKernelModuleInstalled(self):
     return 'ipfw_mod' in subprocess.Popen(

tools/telemetry/telemetry/core/platform/platform_backend.py

@@ -226,10 +226,16 @@ class PlatformBackend(object):
   def StopMonitoringPower(self):
     raise NotImplementedError()
 
-  def ReadMsr(self, msr_number):
+  def ReadMsr(self, msr_number, start=0, length=64):
     """Read a CPU model-specific register (MSR).
 
     Which MSRs are available depends on the CPU model.
     On systems with multiple CPUs, this function may run on any CPU.
+
+    Args:
+      msr_number: The number of the register to read.
+      start: The least significant bit to read, zero-indexed.
+          (Said another way, the number of bits to right-shift the MSR value.)
+      length: The number of bits to read. MSRs are 64 bits, even on 32-bit CPUs.
     """
     raise NotImplementedError()

tools/telemetry/telemetry/core/platform/power_monitor/msr_power_monitor.py

@@ -78,8 +78,9 @@ class MsrPowerMonitor(power_monitor.PowerMonitor):
 
     energy_consumption_j = self._PackageEnergyJoules() - self._start_energy_j
     average_temp_c = (self._TemperatureCelsius() + self._start_temp_c) / 2.
-    assert energy_consumption_j >= 0, ('Negative energy consumption. (Starting '
-                                       'energy was %s.)' % self._start_energy_j)
+    if energy_consumption_j < 0:  # Correct overflow.
+      # The energy portion of the MSR is 4 bytes.
+      energy_consumption_j += 2 ** 32 * self._EnergyMultiplier()
 
     self._start_energy_j = None
     self._start_temp_c = None
@@ -96,19 +97,18 @@ class MsrPowerMonitor(power_monitor.PowerMonitor):
 
   @decorators.Cache
   def _EnergyMultiplier(self):
-    return 0.5 ** ((self._backend.ReadMsr(MSR_RAPL_POWER_UNIT) >> 8) & 0x1f)
+    return 0.5 ** self._backend.ReadMsr(MSR_RAPL_POWER_UNIT, 8, 5)
 
   def _PackageEnergyJoules(self):
-    return (self._backend.ReadMsr(MSR_PKG_ENERGY_STATUS) *
+    return (self._backend.ReadMsr(MSR_PKG_ENERGY_STATUS, 0, 32) *
             self._EnergyMultiplier())
 
   def _TemperatureCelsius(self):
-    tcc_activation_temp = (
-        self._backend.ReadMsr(IA32_TEMPERATURE_TARGET) >> 16 & 0x7f)
+    tcc_activation_temp = self._backend.ReadMsr(IA32_TEMPERATURE_TARGET, 16, 7)
     if tcc_activation_temp <= 0:
       tcc_activation_temp = 105
-    package_temp_headroom = (
-        self._backend.ReadMsr(IA32_PACKAGE_THERM_STATUS) >> 16 & 0x7f)
+    package_temp_headroom = self._backend.ReadMsr(
+        IA32_PACKAGE_THERM_STATUS, 16, 7)
     return tcc_activation_temp - package_temp_headroom
 
   def _CheckMSRs(self):

tools/telemetry/telemetry/core/platform/win_platform_backend.py

@@ -346,7 +346,7 @@ class WinPlatformBackend(desktop_platform_backend.DesktopPlatformBackend):
       self.CloseMsrServer()
     atexit.register(TerminateProcess, self._msr_server_handle)
 
-  def ReadMsr(self, msr_number):
+  def ReadMsr(self, msr_number, start=0, length=64):
     self._StartMsrServerIfNeeded()
     if not self._msr_server_handle:
       raise OSError('Unable to start MSR server.')
@@ -357,4 +357,4 @@ class WinPlatformBackend(desktop_platform_backend.DesktopPlatformBackend):
       response = sock.recv(8)
     finally:
       sock.close()
-    return struct.unpack('Q', response)[0]
+    return struct.unpack('Q', response)[0] >> start & ((1 << length) - 1)