Telemetry: Don't silently compare histograms from different processes.

We used to do so, and the resulting data doesn't make sense.

BUG=224262
NOTRY=true


Committed: https://src.chromium.org/viewvc/chrome?view=rev&revision=192526

Review URL: https://chromiumcodereview.appspot.com/13119014

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@192533 0039d316-1c4b-4281-b951-d872f2087c98

base/metrics/histogram_base.cc

@@ -13,6 +13,7 @@
 #include "base/metrics/histogram_samples.h"
 #include "base/metrics/sparse_histogram.h"
 #include "base/pickle.h"
+#include "base/process_util.h"
 #include "base/values.h"
 
 namespace base {
@@ -119,6 +120,7 @@ void HistogramBase::WriteJSON(std::string* output) const {
   root.SetInteger("flags", flags());
   root.Set("params", parameters.release());
   root.Set("buckets", buckets.release());
+  root.SetInteger("pid", GetCurrentProcId());
   serializer.Serialize(root);
 }
 

tools/perf/perf_tools/histogram.py

@@ -13,12 +13,18 @@ def SubtractHistogram(histogram_json, start_histogram_json):
   if 'buckets' not in start_histogram:
     return histogram_json
 
+  histogram = json.loads(histogram_json)
+  if ('pid' in start_histogram and 'pid' in histogram
+      and start_histogram['pid'] != histogram['pid']):
+    raise Exception(
+        'Trying to compare histograms from different processes (%d and %d)'
+        % (start_histogram['pid'], histogram['pid']))
+
   start_histogram_buckets = dict()
   for b in start_histogram['buckets']:
     start_histogram_buckets[b['low']] = b['count']
 
   new_buckets = []
-  histogram = json.loads(histogram_json)
   for b in histogram['buckets']:
     new_bucket = b
     low = b['low']