[Telemetry] Improve _GetProcJiffies operation with using grep.

tools/telemetry/telemetry/core/platform/linux_based_platform_backend.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.
 
 try:
   import resource  # pylint: disable=F0401
 except ImportError:
   resource = None  # Not available on all platforms
 
 from telemetry.core import exceptions
@@ skipping 35 matching lines @@
       return results
     stats = stats.split()
     utime = float(stats[13])
     stime = float(stats[14])
     cpu_process_jiffies = utime + stime
     clock_ticks = self.GetClockTicks()
     results.update({'CpuProcessTime': cpu_process_jiffies / clock_ticks})
     return results
 
   def GetCpuTimestamp(self):
-    timer_list = self.GetFileContents('/proc/timer_list')
-    total_jiffies = float(self._GetProcJiffies(timer_list))
+    total_jiffies = self._GetProcJiffies()
     clock_ticks = self.GetClockTicks()
     return {'TotalTime': total_jiffies / clock_ticks}
 
   def GetMemoryStats(self, pid):
     status_contents = self._GetProcFileForPid(pid, 'status')
     stats = self._GetProcFileForPid(pid, 'stat').split()
     status = self._GetProcFileDict(status_contents)
     if not status or not stats or 'Z' in status['State']:
       return {}
     vm = int(stats[22])
@@ skipping 71 matching lines @@
   def _ConvertKbToByte(self, value):
     return int(value.replace('kB', '')) * 1024
 
   def _GetProcFileDict(self, contents):
     retval = {}
     for line in contents.splitlines():
       key, value = line.split(':')
       retval[key.strip()] = value.strip()
     return retval
 
-  def _GetProcJiffies(self, timer_list):
+  def _GetProcJiffies(self):
     """Parse '/proc/timer_list' output and returns the first jiffies attribute.
 
     Multi-CPU machines will have multiple 'jiffies:' lines, all of which will be
     essentially the same.  Return the first one."""
-    if isinstance(timer_list, str):
-      timer_list = timer_list.splitlines()
-    for line in timer_list:
-      if line.startswith('jiffies:'):
-        _, value = line.split(':')
-        return value
-    raise Exception('Unable to find jiffies from /proc/timer_list')
+    first_jiffies_timer_line = self.RunCommand(
+        ['grep', '-m', '1', 'jiffies:','/proc/timer_list'])

[aiolos (Not reviewing), 2015/04/24 23:55:15]

I'm a little wary of this change because it makes the code more fragile due to
dependence on the formatting of the files. If the format changed so that
'last-jiffies' or 'next-jiffies' had the colon appear right after it instead of
with while space between them, this would break, and we probably would never
know since we would still be getting a reasonable value. 1st Q: How much would
we throw off our tests if that happened? 2nd Q:  Are we willing to take that
risk?

[jbudorick, 2015/04/24 23:56:56]

Agreed, this seems like too much of an optimization. What about just grepping
for "jiffies" in /proc/timer_list and then doing some amount of processing with
the output?

[aiolos (Not reviewing), 2015/04/25 00:53:55]

I'm much happier with that idea.

[nednguyen, 2015/04/25 02:02:07]

Good call, done.  I use regex so the parsing is more resilient than the original
version/

+    if not first_jiffies_timer_line:
+      raise Exception('Unable to find jiffies from /proc/timer_list')
+    # The line should look something like 'jiffies: 4315883489'.
+    _, value = first_jiffies_timer_line.split(':')
+    return float(value)