home-network-k3s-impl/runbooks/node-cpu.md

# node-cpu — CPU pressure or run-queue depth high on a node

**Alerts:** `NodeCPUPressureHigh` (warning, 15m) and `NodeCPULoadHigh` (warning, 15m)
**File:** `platform/monitoring/alerts.yaml`, group `platform-health.nodes`
**Expressions:**
`rate(node_pressure_cpu_waiting_seconds_total[5m]) > 0.15`
`node_load5 / count without (cpu, mode) (node_cpu_seconds_total{mode="idle"}) > 1`

## What this means here

`NodeCPUPressureHigh` is PSI "some" for CPU: the fraction of wall-clock time during which at least
one task was stalled waiting for a CPU. Under 5% is healthy. Averaged CPU% — what `kubectl top`
shows — **hides this entirely**, which is why the alert uses PSI: it is the metric that surfaces the
"spikes cause blips of delay" symptom operators actually notice.

`NodeCPULoadHigh` is 5-minute load average divided by core count. Sustained above 1 means the run
queue is longer than the number of CPUs.

**Read them together.** CPU-PSI high *and* load high = genuine CPU contention. Load high *and* CPU-PSI
normal = the queue is full of tasks blocked on **I/O**, not CPU — go to `/runbooks/node-io`, which on
this cluster is the more dangerous of the two (etcd WAL and Longhorn replicas share one partition).

Measured history: steady CPU PSI ran **15–23%** with load at or above core count, which is what drove
the node resize from **6 to 8 vCPU**; afterwards PSI measured ~4.5–7.7%. So 0.15 is not an arbitrary
threshold, it is the level the cluster actually lived at before the resize.
## Running the commands in this runbook

`kubectl` does not work on the operator workstation. The toolchain lives only in the pinned dev
container, and the host's `k3s-oidc` context hangs on a browser prompt once its token expires —
that hang is not the cluster being down. Every command below runs as:

    infra/devcontainer/dev.sh run bash -c 'kubectl --context k3s-prod get nodes'

Always pass `--context k3s-prod` explicitly. `dev.sh` merges both clusters into one kubeconfig and
defaults its current-context to k3s-**dev**, so a command without `--context` silently answers
about the wrong cluster. (`CLUSTER=prod infra/devcontainer/dev.sh run kubectl ...` is equivalent.)
Do not run two `dev.sh` invocations at once — they share one container and interfere.

USE FULLY-QUALIFIED RESOURCE NAMES, ALWAYS. Three CRDs on this cluster register the kind `Backup`:
`longhorn.io/v1beta2`, `postgresql.cnpg.io/v1` and `velero.io/v1`. The bare short name resolves to
Longhorn's, which holds zero objects — so `kubectl get backup -A` prints **"No resources found"** on
a cluster holding 6 Velero backups and 27 CNPG backups. Verified on prod 2026-09-13. Reading that as
"the backups are gone" and acting on it turns a false negative into real data loss. Write
`backups.velero.io`, `backups.postgresql.cnpg.io`, `clusters.postgresql.cnpg.io`,
`settings.longhorn.io`, `volumes.longhorn.io`, `nodes.longhorn.io`.

Mutating kubectl (`apply`, `patch`, `annotate`, `rollout`) and `flux reconcile` are blocked for
agents here: every durable change goes through git (`git push origin main` **is** the deploy).
Where this runbook gives a mutating command it is for an operator, and it says so.

## First commands

Distinguish CPU contention from I/O contention — do this first:

```
ssh k3s-prod-1 'cat /proc/pressure/cpu; echo; cat /proc/pressure/io; echo; uptime; nproc'
```

Who is burning it:

```
infra/devcontainer/dev.sh run bash -c \
  'kubectl --context k3s-prod top nodes; echo; kubectl --context k3s-prod top pods -A --sort-by=cpu | head -20'
```

```
ssh k3s-prod-1 'top -b -n1 | head -20'
```

Are there VMs, and are any of them emulated (the worst case)?

```
infra/devcontainer/dev.sh run bash -c \
  'kubectl --context k3s-prod get vmi -A -o wide'
```

```
ssh k3s-prod-1 'grep -c -E "vmx|svm" /proc/cpuinfo; ls -l /dev/kvm'
```

## Likely causes, most likely first

### 1. It is actually I/O, not CPU
Most common misread. A node blocked on disk has a long run queue and normal CPU-PSI. Confirm with
`/proc/pressure/io` and go to `/runbooks/node-io`. On 2026-09-07 the node that died had I/O-PSI full
at 11.36 while CPU looked unremarkable.

### 2. A KubeVirt VM running under TCG emulation
`useEmulation: true` **silently forces TCG on KubeVirt v1.9** — it is set to `false` on purpose. A VM
that lost `/dev/kvm` or was authored with emulation burns a whole core per vCPU. Heavy sustained TCG
load has also degraded the Cilium datapath into intermittent LB-VIP unreachability (cleared by a
restart), so an "intermittent networking" report alongside this alert is probably one cause, not two.
Check `grep -c -E "vmx|svm" /proc/cpuinfo` (nested KVM is verified working; Proxmox nodes use
`cpu_type=host`).

### 3. A Longhorn rebuild's checksumming
Rebuilds are I/O-heavy but not free on CPU. Same remediation as `/runbooks/longhorn-degraded`: freeze
with `concurrent-replica-rebuild-per-node-limit` `0`, thaw to `1`.

### 4. Prometheus rule evaluation or a Loki query
`PersistentVolumeFillingUpPredicted` runs `predict_linear` over a 24h window across every PVC; a
Grafana panel doing the same interactively is heavier still. Bursty, self-limiting, usually harmless.

### 5. Genuine capacity
If nothing is anomalous the node is simply undersized again. The 6 → 8 vCPU resize is the precedent.
A bpg cores change **reboots the VM**, so roll **one node at a time, waiting for Longhorn
`robustness=healthy` between each**.

## When to escalate or stop

- **If I/O-PSI is also elevated, stop treating this as a CPU problem** and switch to
  `/runbooks/node-io`. Wrong runbook is the main risk here.
- Alone and steady, this alert is a capacity signal — schedule it, do not page on it. That is why it
  is a warning with a 15m `for`.
- **Do not drain or cordon** to shed CPU load; this cluster does not drain.
- Escalate to the operator for a vCPU change (it reboots the node) or to kill a VM you did not create.

---
Source of truth: `runbooks/node-cpu.md`. Index: `/runbooks/`
Records: `DECISIONS-LOG.md` (node vCPU 6→8 and the post-resize PSI measurements; KubeVirt
`useEmulation` finding).