service/ci.md

@@ -184,6 +184,27 @@ for images for which there aren't TPA-maintained alternatives).
    hub.docker.com and build our own ecosystem of images, built `FROM
    scratch` or from `debootstrap`
 
+## Finding a runner
+
+Runners are registered with the GitLab rails app under a given
+code name. Say you're running a job on "#356 (bkQZPa1B) TPA-managed
+runner groups, includes ci-runner-x86-02 and ci-runner-x86-03, maybe
+more". That code name (`bkQZPa1B`) should be present in the runner, in
+`/etc/gitlab-runner/config.toml`:
+
+```
+root@ci-runner-x86-02:~# grep bkQZPa1B /etc/gitlab-runner/config.toml
+token = "glrt-t1_bkQZPa1Bf5GxtcyTQrbL"
+```
+
+Inversely, if you're on a VM and are wondering which runner is
+associated with that configuration, you need to look at a *substring*
+of the `token` variable, specifically the first 8 characters following
+the underscore.
+
+Also note that multiple runners, on different machines, can be
+registered with the same token.
+
 ## Pager playbook
 
 <!-- information about common errors from the monitoring system and -->