Fix overflow for 32-bit jiffie counter on Android.

tools/perf/metrics/cpu.py

 # Copyright 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.
 
 from metrics import Metric
 from telemetry.value import scalar
 
 
 class CpuMetric(Metric):
   """Calulates CPU load over a span of time."""
@@ skipping 43 matching lines @@
   Returns:
     A dict of process type names (Browser, Renderer, etc.) to ratios of cpu
     time used to total time elapsed.
   """
   cpu_usage = {}
   for process_type in cpu_stats:
     assert process_type in start_cpu_stats, 'Mismatching process types'
     # Skip any process_types that are empty.
     if (not cpu_stats[process_type]) or (not start_cpu_stats[process_type]):
       continue
     cpu_process_time = (cpu_stats[process_type]['CpuProcessTime'] -

[Sami, 2014/06/30 15:52:26]

Does this field have the same problem? If so, maybe add a utility function that
does the subtraction with wraparound handling.

[Victor Starodub, 2014/06/30 16:15:02]

No, this field is counted from the process start rather than system boot.

                         start_cpu_stats[process_type]['CpuProcessTime'])
     total_time = (cpu_stats[process_type]['TotalTime'] -
                   start_cpu_stats[process_type]['TotalTime'])
-    assert total_time > 0, 'Expected total_time > 0, was: %d' % total_time
+    # Fix overflow for 32-bit jiffie counter, 64-bit counter will not overflow.
+    # Android devices start with a value close to an overflow, so correction is

[Sami, 2014/06/30 15:52:26]

Looks like this isn't really Android-specific but something that Linux does on
purpose, e.g.:

http://books.google.co.uk/books?id=h0lltXyJ8aIC&pg=PT250&lpg=PT250&dq=linux+j...

[Victor Starodub, 2014/06/30 16:15:02]

Thanks. Fixed the comment

+    # necessary.
+    if total_time < 0:
+      total_time += 2**32
+    # Assert that the arguments were given in the correct order.
+    assert total_time > 0 and total_time < 2**31, (
+        'Expected total_time > 0, was: %d' % total_time)
     cpu_usage[process_type] = float(cpu_process_time) / total_time
   return cpu_usage