Teach `benchmark.mojo` to save the collected trace file to disk.

mojo/devtools/common/mojo_benchmark

 #!/usr/bin/env python
 # Copyright 2015 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.
 
 """Runner for Mojo application benchmarks."""
 
 import argparse
 import logging
 import sys
+import time
 
 from devtoolslib import shell_arguments
 from devtoolslib import shell_config
 
 
 _DESCRIPTION = """Runner for Mojo application benchmarks.
 
 |benchmark_list_file| has to be a valid Python program that sets a |benchmarks|
 global variable, containing entries of the following form:
 
@@ skipping 24 matching lines @@
 _BENCHMARK_APP = 'https://core.mojoapps.io/benchmark.mojo'
 _CACHE_SERVICE_URL = 'mojo:url_response_disk_cache'
 
 # Additional time in seconds allocated per shell run to accommodate start-up.
 # The shell should terminate before hitting this time out, it is an error if it
 # doesn't.
 _EXTRA_TIMEOUT = 20
 
 
 def _run_benchmark(shell, shell_args, name, app, duration_seconds, measurements,
-                   cold_start, verbose):
+                   cold_start, verbose, save_traces):
   """Runs `benchmark.mojo` in shell with correct arguments, parses and
   presents the benchmark results.
   """
   timeout = duration_seconds + _EXTRA_TIMEOUT
   benchmark_args = []
   benchmark_args.append('--app=' + app)
   benchmark_args.append('--duration=' + str(duration_seconds))
+  if save_traces:
+    trace_output_file = 'benchmark-%s-%s-%s.trace' % (
+        name.replace(' ', '_'),
+        'cold_start' if cold_start else 'warm_start',
+        time.strftime('%Y%m%d%H%M%S'))
+    benchmark_args.append('--trace-output=' + trace_output_file)
+
   for measurement in measurements:
     benchmark_args.append(measurement)
 
   shell_args = list(shell_args)
   shell_args.append(_BENCHMARK_APP)
   shell_args.append('--args-for=%s %s' % (_BENCHMARK_APP,
                                           ' '.join(benchmark_args)))
 
   if cold_start:
     shell_args.append('--args-for=%s %s' % (_CACHE_SERVICE_URL, '--clear'))
@@ skipping 20 matching lines @@
       print line
   return True
 
 
 def main():
   parser = argparse.ArgumentParser(
       formatter_class=argparse.RawDescriptionHelpFormatter,
       description=_DESCRIPTION)
   parser.add_argument('benchmark_list_file', type=file,
                       help='a file listing benchmarks to run')
+  parser.add_argument('--save-traces', action='store_true',
+                      help='save the traces produced by benchmarks to disk')
 
   # Common shell configuration arguments.
   shell_config.add_shell_arguments(parser)
   script_args = parser.parse_args()
   config = shell_config.get_shell_config(script_args)
 
   try:
     shell, common_shell_args = shell_arguments.get_shell(config, [])
   except shell_arguments.ShellConfigurationException as e:
     print e
     return 1
 
   target_os = 'android' if script_args.android else 'linux'
   benchmark_list_params = {"target_os": target_os}
   exec script_args.benchmark_list_file in benchmark_list_params
   benchmark_list = benchmark_list_params['benchmarks']
 
   succeeded = True
   for benchmark_spec in benchmark_list:
     name = benchmark_spec['name']
     app = benchmark_spec['app']
     duration = benchmark_spec['duration']
     shell_args = benchmark_spec.get('shell-args', []) + common_shell_args
     measurements = benchmark_spec['measurements']
     _run_benchmark(shell, shell_args, name, app, duration, measurements,
-                   cold_start=True, verbose=script_args.verbose)
+                   cold_start=True, verbose=script_args.verbose,
+                   save_traces=script_args.save_traces)
     _run_benchmark(shell, shell_args, name, app, duration, measurements,
-                   cold_start=False, verbose=script_args.verbose)
+                   cold_start=False, verbose=script_args.verbose,
+                   save_traces=script_args.save_traces)
 
   return 0 if succeeded else 1
 
 if __name__ == '__main__':
   sys.exit(main())