forgejo-runner-optimiser/docs/background/identify-process-cgroup-by-pid.md

# Identifying a Process's Cgroup by PID

Read `/proc/<PID>/cgroup` to find which cgroup (and therefore which container) a process belongs to.

## /proc/PID/cgroup

```bash
cat /proc/<PID>/cgroup
```

Shows all cgroup controllers the process belongs to:
```
12:blkio:/user.slice
11:memory:/user.slice/user-1000.slice
...
0::/user.slice/user-1000.slice/session-1.scope
```

On cgroup v2, the path after `::` is the cgroup path under `/sys/fs/cgroup/`.

## Other Methods

```bash
# ps format options
ps -o pid,cgroup -p <PID>

# systemd systems
systemd-cgls --unit <unit-name>
systemd-cgls  # whole tree
```

## Quick One-Liners

```bash
cat /proc/self/cgroup  # current shell
cat /proc/$$/cgroup    # also current shell
cat /proc/1234/cgroup  # specific PID
```