host-ctr: remove KillMode=mixed and don't create the container task under the unit's cgroup

[etungsten]

Issue number:
Fixes #1237

Description of changes:

Author: Erikson Tung <etung@amazon.com>
Date:   Fri Dec 4 14:31:26 2020 -0800

    host-containers@: remove KillMode=mixed
    
    We don't need systemd to go and actively try kill all processes
    of the unit's cgroup.

Author: Erikson Tung <etung@amazon.com>
Date:   Mon Dec 14 13:47:04 2020 -0800

    host-ctr: don't create the container task under the service's cgroup
    
    We no longer need to launch the host container task under the systemd
    service unit's cgroup. We're managing the life-cycle of the host
    container directly without the help of systemd

Testing done:
Built aws-k8s-1.17 x86 ami and launched instance.
Restarting host-containerd doesn't kill any running host-containers as expected.

Terms of contribution:

By submitting this pull request, I agree that this contribution is dual-licensed under the terms of both the Apache License, version 2.0, and the MIT license.

[bcressey]

Which cgroup are the tasks created under now? Still host-containerd's, or something else?

[etungsten]

Which cgroup are the tasks created under now? Still host-containerd's, or something else?

They're all launched under host-containerd's cgroup:

● host-containerd.service - containerd runtime for host containers
     Loaded: loaded (/x86_64-bottlerocket-linux-gnu/sys-root/usr/lib/systemd/system/host-containerd.service; enabled; vendor preset: enabled)
     Active: active (running) since Tue 2020-12-15 06:13:18 UTC; 1min 8s ago
       Docs: https://containerd.io
   Main PID: 2928 (containerd)
      Tasks: 34
     Memory: 685.3M
     CGroup: /system.slice/host-containerd.service
             ├─2928 /usr/bin/containerd --config /etc/host-containerd/config.toml
             ├─3660 /x86_64-bottlerocket-linux-gnu/sys-root/usr/bin/containerd-shim-runc-v2 -namespace default -id control -address /run/host-containerd/containerd.sock
             ├─3736 /usr/bin/amazon-ssm-agent
             ├─3836 /x86_64-bottlerocket-linux-gnu/sys-root/usr/bin/containerd-shim-runc-v2 -namespace default -id admin -address /run/host-containerd/containerd.sock
             ├─3858 /usr/sbin/sshd -e -D
             ├─3961 sshd: ec2-user [priv]
             ├─3972 sshd: ec2-user@pts/0
             ├─3974 -bash
             ├─4017 sudo sheltie
             ├─4018 nsenter -t 1 -a /proc/4017/root/opt/bin/bash
             ├─4019 /proc/4017/root/opt/bin/bash
             ├─4401 systemctl status host-containerd
             └─4402 more

[zmrow]

🏐