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kured: fix sentinel-gate OOM — 256Mi limit + self-restart leak guard

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The k8s-master gate pod OOM-killed child kubectls 149x/7d (accelerating:
0/day → 15 → 134) while master sat in pending-reboot. Root cause: only the
pending-reboot node's gate pod runs the kubectl-heavy hot path each cycle,
and the immortal bash loop slowly leaks (kubectl forks + Check-4 process
substitution) past the 64Mi cgroup limit. PID 1 bash survives each kill, so
the pod never restarts — just silent oom_events.

Fix: raise limit 64Mi→256Mi (headroom for ~30-50Mi kubectl forks) + add a
MAX_ITER=72 self-exit (~6h) so kubelet restarts the pod fresh and the leak
can never accumulate, regardless of how long a node stays pending-reboot.

Docs: post-mortem + automated-upgrades.md gate note.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>
This commit is contained in:
Viktor Barzin 2026-05-31 14:49:04 +00:00
parent 0c64fc2948
commit 51313ee088
3 changed files with 120 additions and 10 deletions
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docs/architecture/automated-upgrades.md
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@ -219,7 +219,7 @@ Independent of the service-upgrade pipeline above. Drives apt package updates +
### Stack
- **In-guest**: `unattended-upgrades` runs apt upgrades within Allowed-Origins (`-security`, `-updates`, ESM). Package-Blacklist excludes runtime components (`containerd`, `containerd.io`, `runc`, `cri-tools`, `kubernetes-cni`, `calico-*`, `cni-plugins-*`, `docker-ce`). `apt-mark hold` on `kubelet`, `kubeadm`, `kubectl` (and runtime pkgs as belt-and-braces). `Automatic-Reboot=false` — kured handles reboots.
- **Reboot driver**: `kured` (chart `kured-5.11.0`, app `1.21.0`). Window 02:00-06:00 Europe/London every day of the week (Mon-Fri-only restriction dropped 2026-05-16 — see PM), period=1h, concurrency=1, reboot-delay=30s, drainTimeout=30m.
- **Reboot gate (sentinel)**: `kured-sentinel-gate` DaemonSet creates `/var/run/gated-reboot-required` only when (a) host needs reboot, (b) all nodes Ready, (c) all calico-node pods Running, (d) **no node has transitioned Ready in the last 24h** (24h soak window).
- **Reboot gate (sentinel)**: `kured-sentinel-gate` DaemonSet creates `/var/run/gated-reboot-required` only when (a) host needs reboot, (b) all nodes Ready, (c) all calico-node pods Running, (d) **no node has transitioned Ready in the last 24h** (24h soak window). The gate runs as an immortal `bash` loop that forks `kubectl` each cycle; the pod whose host has a pending reboot runs the full kubectl-heavy path indefinitely and slowly leaks. Mitigated 2026-05-31 (limit 64Mi→256Mi + `MAX_ITER=72` self-exit ≈6h so kubelet restarts it fresh) — see PM `2026-05-31-kured-sentinel-gate-oom.md`.
- **Reboot gate (Prometheus)**: kured `--prometheus-url` polls `prometheus-server.monitoring.svc:80` before each drain. ANY firing alert blocks unless it matches the ignore-regex `^(Watchdog|RebootRequired|KuredNodeWasNotDrained|InfoInhibitor)$`.
- **Health alert library**: 10 alerts in the `Upgrade Gates` group (`prometheus_chart_values.tpl`): `KubeAPIServerDown`, `KubeStateMetricsDown`, `PrometheusRuleEvaluationFailing`, `PVCStuckPending`, `RecentNodeReboot` (the explicit 24h soak signal), `MysqlStandaloneDown`, `ClusterPodReadyRatioDropped`, `NodeMemoryPressure`, `NodeDiskPressure`, `KubeQuotaAlmostFull`. Plus the existing 200+ alerts in the cluster-wide library (anything firing blocks kured).
- **Notifications**: kured `notifyUrl` posts drain-start/drain-finish to Slack via Vault `secret/kured.slack_kured_webhook`. Alertmanager separately routes critical alerts to `#alerts`.