# freeze-watcher

A systemd daemon that continuously samples Linux system state and dumps a detailed forensic snapshot whenever freeze-like conditions are detected (D-state spikes, PSI pressure, blocked processes, load avg). Includes a Qt GUI for live metrics and snapshot browsing.

Built to diagnose intermittent system freezes that don't show up in normal resource monitors — specifically the kind where some apps stall but the mouse keeps moving.

## What it captures

Each snapshot is a single text file in `/var/log/freeze-watcher/snap-YYYYMMDD-HHMMSS.log` containing:

- `loadavg`, `procs_running`, `procs_blocked`, `intr`, `ctxt`
- PSI cpu / io / memory pressure (`some` and `full` averages)
- D-state processes with kernel `wchan` (where exactly they're stuck)
- Top 15 by CPU and RSS
- `meminfo` summary
- `iostat -xt` snapshot
- Top interrupt sources by total + per-CPU breakdown
- Full `softirq` table
- Last 50 dmesg lines
- Last 60s of journal warnings/errors

The wchan column is the key one — that's how you tell `btrfs_lock_root_node` lock contention from `usb_sg_wait` USB stalls from `folio_wait_bit_common` page-fault stalls.

## Components

| Path | What |
|---|---|
| `src/freeze-watcher.sh` | The daemon — samples every 1s, captures on threshold |
| `src/freeze-monitor-gui` | PySide6 GUI: live metrics + snapshot browser + tray icon |
| `systemd/freeze-watcher.service` | systemd unit |
| `desktop/freeze-monitor.desktop` | App launcher entry |

## Install

### Via Makefile (system-wide install to `/usr/local`)

```
sudo make install
sudo make enable
```

### As an Arch package

```
./packaging/build-package.sh
sudo pacman -U freeze-watcher-*.pkg.tar.zst
sudo systemctl enable --now freeze-watcher.service
```

## Update workflow

When you change something:

```
# 1. Edit files in src/ or systemd/
# 2. Reinstall
sudo make install

# 3. If the daemon script changed, restart it
sudo systemctl restart freeze-watcher.service

# 4. If the GUI changed, just relaunch it (it's a regular Qt app)
```

If you're using the Arch package: `./packaging/build-package.sh && sudo pacman -U *.pkg.tar.zst` will rebuild and upgrade in place.

## Tuning thresholds

Open `src/freeze-watcher.sh` and edit the constants at the top:

```
PSI_IO_THRESHOLD=40       # io pressure avg10 % to trigger
PSI_CPU_THRESHOLD=50      # cpu pressure avg10 % to trigger
DSTATE_THRESHOLD=5        # processes in D state
BLOCKED_THRESHOLD=5       # procs_blocked from /proc/stat
LOAD_THRESHOLD=20         # 1-min load average
COOLDOWN=30               # seconds between captures
SAMPLE_INTERVAL=1         # seconds between samples
```

## GUI

Launch from app menu (search "Freeze Watcher") or `freeze-monitor-gui`.

- Live PSI / D-state / load metrics with green/orange/red color coding (1s refresh)
- Service status with start/stop buttons (uses `pkexec`)
- Captured event list — click any row for full snapshot view
- Closes to system tray; quit from the tray menu
- Notification when a new snapshot is captured

## Reading a snapshot

The most informative section is **D-state processes** — the `wchan` tells you what kernel function each process is blocked in. Common ones:

| wchan | Meaning |
|---|---|
| `btrfs_lock_root_node` | btrfs metadata lock contention (often qgroups + snapshots) |
| `btrfs_search_slot` | btrfs tree walk waiting for lock |
| `btrfs_sync_log` | fsync waiting for transaction commit |
| `folio_wait_bit_common` | Waiting for memory page (often swap-in) |
| `usb_sg_wait` | USB device stall |
| `io_schedule` | Generic I/O wait |

Cross-reference `wchan` with PSI io and PSI cpu to distinguish lock contention (low PSI) from genuine resource pressure (high PSI).

## Future ideas

- Per-cgroup PSI capture
- eBPF tracing for long-running syscalls
- Web UI option (no Qt requirement)
- Auto-prune old snapshots
- Configurable `/etc/freeze-watcher.conf` instead of inline thresholds
- Integration with desktop notifications via `notify-send`
- Optional perf-profile capture during event

## License

MIT