Lots of paladin have been failing over the last 2 runs:

https://luci-milo.appspot.com/buildbot/chromeos/elm-paladin/3684
https://luci-milo.appspot.com/buildbot/chromeos/hana-paladin/524
https://luci-milo.appspot.com/buildbot/chromeos/veyron_mighty-paladin/6229
https://luci-milo.appspot.com/buildbot/chromeos/veyron_speedy-paladin/6224

failing to start HWTest suites, one example below:

08/11 04:12:06.277 INFO |          autoserv:0671| Results placed in /usr/local/autotest/results/134391881-chromeos-test/hostless
08/11 04:12:06.278 DEBUG|          autoserv:0679| autoserv is running in drone cros-autotest-shard5.hot.corp.google.com.
08/11 04:12:06.278 DEBUG|          autoserv:0680| autoserv command was: /usr/local/autotest/server/autoserv -p -r /usr/local/autotest/results/134391881-chromeos-test/hostless -u chromeos-test -l hana-paladin/R62-9831.0.0-rc1-test_suites/control.suite_attr_wrapper -s --lab True -P 134391881-chromeos-test/hostless -n /usr/local/autotest/results/drone_tmp/attach.51836
08/11 04:12:06.278 INFO |           pidfile:0016| Logged pid 30607 to /usr/local/autotest/results/134391881-chromeos-test/hostless/.autoserv_execute
08/11 04:12:06.279 DEBUG|          autoserv:0379| faulthandler registered on SIGTERM.
08/11 04:12:06.279 DEBUG|          base_job:0357| Persistent state global_properties.test_retry now set to 0
08/11 04:12:06.279 DEBUG|          base_job:0357| Persistent state global_properties.tag now set to '134391881-chromeos-test/hostless'
08/11 04:12:06.286 DEBUG|          base_job:0357| Persistent state global_properties.max_result_size_KB now set to 20000
08/11 04:12:06.339 INFO |    connectionpool:0207| Starting new HTTP connection (1): metadata.google.internal
08/11 04:12:06.414 NOTIC|      cros_logging:0037| ts_mon was set up.
08/11 04:12:06.503 INFO |        server_job:0771| I am PID 30607
08/11 04:12:06.509 WARNI|        subcommand:0085| parallel_simple was called with an empty arglist, did you forget to pass in a list of machines?
08/11 04:12:06.510 WARNI|        server_job:0831| Not checking if job_repo_url contains autotest packages on []
08/11 04:12:06.510 INFO |        server_job:0854| Processing control file
08/11 04:12:06.554 ERROR|           control:0064| Unable to find the attribute boolean expression used to trigger tests in suite_args {'job_retry': True, 'test_args': None, 'suite_args': "{'attr_filter': u'(suite:bvt-arc) and (subsystem:default)'}", 'num': 6, 'timeout_mins': 90, 'max_runtime_mins': 1440, 'devserver_url': 'http://100.115.185.227:8082', 'wait_for_results': True, 'priority': 70, 'board': 'hana', 'build': 'hana-paladin/R62-9831.0.0-rc1', 'timeout': 24, 'check_hosts': True, 'offload_failures_only': True, 'max_retries': 5, 'pool': 'cq', 'builds': {'cros-version': 'hana-paladin/R62-9831.0.0-rc1'}, 'test_source_build': 'hana-paladin/R62-9831.0.0-rc1', 'file_bugs': False, 'run_prod_code': False, 'delay_minutes': 0, 'job_keyvals': {'cidb_build_stage_id': 52597882, 'cidb_build_id': 1745870, 'datastore_parent_key': ['Build', 1745870, 'BuildStage', 52597882]}}, please use attr_filter keyword to specify it. suite aborting
08/11 04:12:06.555 ERROR|        server_job:0864| Exception escaped control file, job aborting:
Traceback (most recent call last):
  File "/usr/local/autotest/server/server_job.py", line 856, in run
    self._execute_code(server_control_file, namespace)
  File "/usr/local/autotest/server/server_job.py", line 1357, in _execute_code
    execfile(code_file, namespace, namespace)
  File "/usr/local/autotest/results/134391881-chromeos-test/hostless/control.srv", line 59, in <module>
    attr = args_dict['attr_filter']
KeyError: 'attr_filter'
08/11 04:12:06.558 INFO |        server_job:0214| INFO	----	----	timestamp=1502449926	job_abort_reason='attr_filter'	localtime=Aug 11 04:12:06