[Telemetry] Improve _GetProcJiffies operation with using grep.

tools/telemetry/telemetry/core/platform/linux_based_platform_backend.py

Index: tools/telemetry/telemetry/core/platform/linux_based_platform_backend.py
diff --git a/tools/telemetry/telemetry/core/platform/linux_based_platform_backend.py b/tools/telemetry/telemetry/core/platform/linux_based_platform_backend.py
index 6da162849aaa3303454f26d01023acba61c4f451..44bec8e2acb5855c8f333d69952e7e4200b0ac2c 100644
--- a/tools/telemetry/telemetry/core/platform/linux_based_platform_backend.py
+++ b/tools/telemetry/telemetry/core/platform/linux_based_platform_backend.py
@@ -53,8 +53,7 @@ class LinuxBasedPlatformBackend(platform_backend.PlatformBackend):
     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}
 
@@ -146,15 +145,15 @@ class LinuxBasedPlatformBackend(platform_backend.PlatformBackend):
       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)