[telemetry] Drop data for sysfs power monitor if distribution changes but total does not.

The log showed that the sum across all C-states didn't change, but C0 went down and WFI went up. AFAIK it shouldn't be possible for any of them to go down, so we'll just drop the data in this case.

BUG=426430
TEST=None.
TBR=tonyg

Review URL: https://codereview.chromium.org/679813002

Cr-Commit-Position: refs/heads/master@{#302492}

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

@@ -170,6 +170,11 @@ class SysfsPowerMonitor(power_monitor.PowerMonitor):
       for state in initial[cpu]:
         current_cpu[state] = final[cpu][state] - initial[cpu][state]
         total += current_cpu[state]
+      if total == 0:
+        # Somehow it's possible for initial and final to have the same sum,
+        # but a different distribution, making total == 0. crbug.com/426430
+        cpu_stats[cpu] = collections.defaultdict(int)
+        continue
       for state in current_cpu:
         current_cpu[state] /= (float(total) / 100.0)
         # Calculate the average c-state residency across all CPUs.

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

@@ -195,6 +195,26 @@ class SysfsPowerMonitorMonitorTest(unittest.TestCase):
         self.assertAlmostEqual(results[cpu][freq],
                                self.expected_freq_percents[cpu][freq])
 
+  def testComputeCpuStatsWithMissingData(self):
+    results = sysfs_power_monitor.SysfsPowerMonitor.ComputeCpuStats(
+        {'cpu1': {}}, {'cpu1': {}})
+    self.assertEqual(results['cpu1'][12345], 0)
+
+    results = sysfs_power_monitor.SysfsPowerMonitor.ComputeCpuStats(
+        {'cpu1': {123: 0}}, {'cpu1': {123: 0}})
+    self.assertEqual(results['cpu1'][123], 0)
+
+    results = sysfs_power_monitor.SysfsPowerMonitor.ComputeCpuStats(
+        {'cpu1': {123: 456}}, {'cpu1': {123: 456}})
+    self.assertEqual(results['cpu1'][123], 0)
+
+  def testComputeCpuStatsWithNumberChange(self):
+    results = sysfs_power_monitor.SysfsPowerMonitor.ComputeCpuStats(
+        {'cpu1': {'C0': 10, 'WFI': 20}},
+        {'cpu1': {'C0': 20, 'WFI': 10}})
+    self.assertEqual(results['cpu1']['C0'], 0)
+    self.assertEqual(results['cpu1']['WFI'], 0)
+
   def testGetCpuStateForAndroidDevices(self):
     class PlatformStub(object):
       def __init__(self, run_command_return_value):