telemetry: Make trace profiler work with trace-based benchmarks.

tools/perf/measurements/rasterize_and_record.py

 # Copyright (c) 2012 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.
 
 import time
 
 from metrics import smoothness
 from telemetry.page import page_measurement
 
 class StatsCollector(object):
@@ skipping 130 matching lines @@
     # first frame requested has more variance in the number of pixels
     # rasterized.
     tab.ExecuteJavaScript("""
         window.__rafFired = false;
         window.webkitRequestAnimationFrame(function() {
           chrome.gpuBenchmarking.setNeedsDisplayOnAllLayers();
           window.__rafFired  = true;
         });
     """)
 
-    tab.browser.StartTracing('webkit,webkit.console,benchmark', 60)
+    # We don't need to start tracing, if a trace profiler is already running.
+    # Note that all trace event categories are enabled in this case, which
+    # increases the risk of a trace buffer overflow.
+    if self.options.profiler != 'trace':

[tonyg, 2013/09/11 21:31:44]

Making this Measurement aware of profilers which are running seems fragile to
me. It also means that any Measurements which use Start/StopTracing() will have
to be careful to do this dance.

I'd much prefer a design where the Start/StopTracing() methods themselves were
robust against multiple users.

What if StartTracing() incremented a counter and StopTracing() would decrement
but only actually stop if the count hits zero. Then GetTraceResultAndReset()
could check whether tracing is still running, and if so, just queue up a Reset
operation for after the tracing is actually stopped.

Anyway, kind of thinking aloud here, please push back if I'm crazy.

+      tab.browser.StartTracing('webkit.console,benchmark', 60)
     self._metrics.Start()
 
     tab.ExecuteJavaScript("""
         console.time("measureNextFrame");
         window.__rafFired = false;
         window.webkitRequestAnimationFrame(function() {
           chrome.gpuBenchmarking.setNeedsDisplayOnAllLayers();
           window.__rafFired  = true;
         });
     """)
     # Wait until the frame was drawn.
     # Needs to be adjusted for every device and for different
     # raster_record_repeat counts.
     # TODO(ernstm): replace by call-back.
     time.sleep(float(self.options.stop_wait_time))
     tab.ExecuteJavaScript('console.timeEnd("measureNextFrame")')
 
+    # Always stop tracing at this point. If a trace profiler is running, it will
+    # be stopped here.
     tab.browser.StopTracing()
     self._metrics.Stop()
 
-    timeline = tab.browser.GetTraceResultAndReset().AsTimelineModel()
+    # Fetch the results without resetting, if a trace profiler is running. The
+    # profiler needs to read the results later on.
+    if self.options.profiler == 'trace':
+      timeline = tab.browser.GetTraceResult().AsTimelineModel()
+    else:
+      timeline = tab.browser.GetTraceResultAndReset().AsTimelineModel()
     collector = StatsCollector(timeline)
     collector.GatherRenderingStats()
 
     rendering_stats = self._metrics.end_values
 
     results.Add('best_rasterize_time', 'seconds',
                 collector.total_best_rasterize_time / 1.e3,
                 data_type='unimportant')
     results.Add('best_record_time', 'seconds',
                 collector.total_best_record_time / 1.e3,
                 data_type='unimportant')
     results.Add('total_pixels_rasterized', 'pixels',
                 collector.total_pixels_rasterized,
                 data_type='unimportant')
     results.Add('total_pixels_recorded', 'pixels',
                 collector.total_pixels_recorded,
                 data_type='unimportant')
 
     if self.options.report_all_results:
       for k, v in rendering_stats.iteritems():
         results.Add(k, '', v)