See for example this log: https://logs.chromium.org/v/?s=chrome%2Fbb%2Fchromium.perf%2FAndroid_Nexus6_WebView_Perf%2F2484%2F%2B%2Frecipes%2Fsteps%2Ftest_pre_run%2F0%2Fsteps%2Fs__trigger__performance_webview_test_suite_on_Android_device_Nexus_6%2F0%2Fstdout

Healthy bots: ['build203-b7--device1', 'build203-b7--device3', 'build203-b7--device2', 'build203-b7--device5', 'build203-b7--device4', 'build203-b7--device7', 'build203-b7--device6', 'build202-b7--device4', 'build202-b7--device5', 'build202-b7--device6', 'build202-b7--device7', 'build202-b7--device1', 'build202-b7--device2', 'build202-b7--device3']
Dead Bots: ['build204-b7--device1']
Traceback (most recent call last):
  File "/b/c/b/Android_Nexus6_WebView_Perf/src/testing/trigger_scripts/perf_device_trigger.py", line 301, in <module>
    sys.exit(main())
  File "/b/c/b/Android_Nexus6_WebView_Perf/src/testing/trigger_scripts/perf_device_trigger.py", line 298, in main
    return triggerer.trigger_tasks(args, remaining)
  File "/b/c/b/Android_Nexus6_WebView_Perf/src/testing/trigger_scripts/base_test_triggerer.py", line 246, in trigger_tasks
    selected_configs = self.select_config_indices(args, verbose)
  File "/b/c/b/Android_Nexus6_WebView_Perf/src/testing/trigger_scripts/perf_device_trigger.py", line 122, in select_config_indices
    return self._select_config_indices_with_soft_affinity(args, verbose)
  File "/b/c/b/Android_Nexus6_WebView_Perf/src/testing/trigger_scripts/perf_device_trigger.py", line 131, in _select_config_indices_with_soft_affinity
    raise ValueError('Not enough available machines exist in in swarming'
ValueError: Not enough available machines exist in in swarmingpool. Contact labs to rack in more hardware
step returned non-zero exit code: 1

When I read that log it doesn't make any sense because there is a long list of Healthy bots, but actually for whatever reason we cannot run against them. These logs need to explain a little more. Like if we can't run because of device affinity or a maintenance mutex, then we should print that out so that sheriffs can figure this out.

Relevant code: https://cs.chromium.org/chromium/src/testing/trigger_scripts/perf_device_trigger.py?q=trigger_scripts/perf_device_trigger.py&sq=package:chromium&dr

See potentially related issue 878332.