Unattended‑Upgrades Reboot Timing Uncertainty During Kernel Update

Diagnosing Reboot Timing in Unattended‑Upgrades

The goal is to determine why a kernel upgrade triggers a system reboot that may interrupt a deployment, yet the log at /var/log/unattended‑upgrades/unattended‑upgrades.log only records a generic “Reboot scheduled” message. The uncertainty lies in how the unattended‑upgrades service selects the exact reboot timestamp when a newer kernel is present, especially under load or when configuration options such as max‑reboot‑delay are set.

Constraints include the lack of detailed decision‑making logs, potential log rotation, and the fact that systemd‑reboot.service may honor or ignore user‑defined delays. Understanding the prioritization of critical services during the reboot sequence is also essential, as a premature reboot can leave services in an incomplete state.

Key questions for investigation:

• What criteria does unattended‑upgrades use to determine the exact reboot timestamp when a new kernel is installed?
• Does the max‑reboot‑delay setting influence the scheduled time when the system is under high load?
• How does systemd‑reboot.service prioritize services when the reboot is triggered by unattended‑upgrades?