Fix overflow for 32-bit jiffie counter on Android.

tools/perf/metrics/cpu.py

Index: tools/perf/metrics/cpu.py
diff --git a/tools/perf/metrics/cpu.py b/tools/perf/metrics/cpu.py
index 7666cb7fcf56e88ad704d92391d26501d5e7f483..07229d822234500387a5f63cca521f942ff7e9eb 100644
--- a/tools/perf/metrics/cpu.py
+++ b/tools/perf/metrics/cpu.py
@@ -65,7 +65,14 @@ def _SubtractCpuStats(cpu_stats, start_cpu_stats):
                         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