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

tools/perf/measurements/rasterize_and_record.py

Index: tools/perf/measurements/rasterize_and_record.py
diff --git a/tools/perf/measurements/rasterize_and_record.py b/tools/perf/measurements/rasterize_and_record.py
index 4ed5a0942e0808f28f9c0fc9cac36980f07bc3a7..92b2f65c1d305bfdb755bc497a81a63a6c5740d5 100644
--- a/tools/perf/measurements/rasterize_and_record.py
+++ b/tools/perf/measurements/rasterize_and_record.py
@@ -148,7 +148,11 @@ class RasterizeAndRecord(page_measurement.PageMeasurement):
         });
     """)
 
-    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("""
@@ -166,10 +170,17 @@ class RasterizeAndRecord(page_measurement.PageMeasurement):
     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()