Telemetry: Don't fail on setting performance mode without root.

build/android/pylib/perf/perf_control.py

Index: build/android/pylib/perf/perf_control.py
diff --git a/build/android/pylib/perf/perf_control.py b/build/android/pylib/perf/perf_control.py
index 6f74575b9e6021fa60fd3d9401d36f05a4fb91c8..70389071a6b46037f23e7c891af7045486e0ef57 100644
--- a/build/android/pylib/perf/perf_control.py
+++ b/build/android/pylib/perf/perf_control.py
@@ -2,12 +2,12 @@
 # Use of this source code is governed by a BSD-style license that can be
 # found in the LICENSE file.
 
+import atexit
 import logging
 
 from pylib import android_commands
 from pylib.device import device_utils
 
-
 class PerfControl(object):
   """Provides methods for setting the performance mode of a device."""
   _SCALING_GOVERNOR_FMT = (
@@ -28,15 +28,23 @@ class PerfControl(object):
     self._have_mpdecision = self._device.FileExists('/system/bin/mpdecision')
 
   def SetHighPerfMode(self):
+    """Sets the highest possible performance mode for the device."""
+    if not self._device.old_interface.IsRootEnabled():
+      message = 'Need root for performance mode. Results may be NOISY!!'
+      logging.warning(message)
+      # Add an additional warning at exit, such that it's clear that any results
+      # may be different/noisy (due to the lack of intended performance mode).
+      atexit.register(logging.warning, message)
+      return
     # TODO(epenner): Enable on all devices (http://crbug.com/383566)
     if 'Nexus 4' == self._device.old_interface.GetProductModel():
       self._ForceAllCpusOnline(True)
       if not self._AllCpusAreOnline():
-        logging.warning('Failed to force CPUs online. Results may be noisy!')
+        logging.warning('Failed to force CPUs online. Results may be NOISY!')
     self._SetScalingGovernorInternal('performance')
 
   def SetPerfProfilingMode(self):
-    """Sets the highest possible performance mode for the device."""
+    """Enables all cores for reliable perf profiling."""
     self._ForceAllCpusOnline(True)
     self._SetScalingGovernorInternal('performance')
     if not self._AllCpusAreOnline():
@@ -46,6 +54,8 @@ class PerfControl(object):
 
   def SetDefaultPerfMode(self):
     """Sets the performance mode for the device to its default mode."""
+    if not self._device.old_interface.IsRootEnabled():
+      return
     product_model = self._device.GetProp('ro.product.model')
     governor_mode = {
         'GT-I9300': 'pegasusq',