Add playbook section on manually killing recalcitrant batch jobs (#160)

playbooks/hail_batch.md

@@ -17,6 +17,25 @@ Linked discussions:
 - [https://centrepopgen.slack.com/archives/C030X7WGFCL/p1712182771574679](https://centrepopgen.slack.com/archives/C030X7WGFCL/p1712182771574679)
 
 
+### Jobs that simply cannot be cancelled
+
+Hail has several interfaces for cancelling batches: there is a `Cancel` button in the batch web UI, and `hailctl batch cancel BATCHID` on the command line.
+But sometimes individual jobs cannot be cancelled.
+In particular, jobs marked `always_run` cannot be cancelled through the user interface.
+
+And sometimes a job gets into such a wedged state that Hail's usual cancellation mechanisms are ineffective, whether it is an `always_run` job or not.
+In those cases, operator assistance may be required to terminate the job's processes in a very manual Unix way:
+
+1. Check the job web UI page to see what instance (i.e., worker) the job is running on, which will be something like _batch-worker-default-abcde_.
+1. Go to the [Google console](https://console.cloud.google.com/), switch to the appropriate Hail project, and go to the VM Instance page (which can usually be found just by searching for _abcde_).
+1. From there, obtain the appropriate `gcloud compute ssh …` command to SSH into the worker.
+1. Once logged on to the worker, attach to Hail's Docker container by identifying it via `docker ps` and attaching with `docker exec -it CONTAINERHASH bash`. (This step may or may not be strictly necessary.)
+1. Now identify the Unix processes that correspond to the wedged job and are identifiably wedged themselves, and **carefully** terminate these processes in the usual `kill -9` way.
+
+Linked discussions:
+
+- [`always_run` job is stalling and cannot be cancelled](https://centrepopgen.slack.com/archives/C030X7WGFCL/p1718590807296239)
+
 
 ## Python jobs