Adding instrumentation to locate the source of jankiness.

chrome/browser/task_manager/task_manager.cc

Index: chrome/browser/task_manager/task_manager.cc
diff --git a/chrome/browser/task_manager/task_manager.cc b/chrome/browser/task_manager/task_manager.cc
index 3d7b02c3f20f40d3ad1ba17de88aa5da1a7b9900..068221c770f76ec8501f0f3a2823ea4a663f5f65 100644
--- a/chrome/browser/task_manager/task_manager.cc
+++ b/chrome/browser/task_manager/task_manager.cc
@@ -10,6 +10,7 @@
 #include "base/prefs/pref_registry_simple.h"
 #include "base/prefs/pref_service.h"
 #include "base/process/process_metrics.h"
+#include "base/profiler/scoped_tracker.h"
 #include "base/stl_util.h"
 #include "base/strings/string16.h"
 #include "base/strings/string_number_conversions.h"
@@ -1229,6 +1230,11 @@ void TaskManagerModel::NotifyV8HeapStats(base::ProcessId renderer_id,
 
 void TaskManagerModel::NotifyBytesRead(const net::URLRequest& request,
                                        int byte_count) {
+  // TODO(vadimt): Remove ScopedTracker below once crbug.com/423948 is fixed.
+  tracked_objects::ScopedTracker tracking_profile(
+      FROM_HERE_WITH_EXPLICIT_FUNCTION(
+          "423948 TaskManagerModel::NotifyBytesRead"));

[mmenke, 2014/11/17 15:38:16]

I suggest adding another one just above the bytes_read_buffer_.push_back call. 
That seems the most likely culprit for OnRawBytesRead.

[vadimt, 2014/11/18 01:14:27]

Done.

+
   DCHECK(BrowserThread::CurrentlyOn(BrowserThread::IO));
   if (!is_updating_byte_count_)
     return;