# 90-mirror-wall-selfheal.conf — make a hung wall recover on its own.
#
# Installed by install-kiosk.sh to /etc/sysctl.d/. Applied at boot by
# systemd-sysctl, and immediately by `sysctl --system` in the installer.
#
# The trade this makes: a wall whose kernel has oopsed, deadlocked or soft-locked
# is a panel showing a frozen frame forever, in a room nobody is in. Rebooting
# into itself is always the better outcome, so every one of these escalates to a
# reboot. Reasoning and the incident behind it: ../kiosk-silverblue.md
# ("When the wall hangs").
#
# To revert a single line: drop a file with a higher-numbered name in the same
# directory, e.g. /etc/sysctl.d/91-mirror-wall-quiet.conf containing
# "kernel.panic_on_oops = 0", then `sudo sysctl --system`.

# The lockup detectors themselves (default on, pinned here so a future base
# image change cannot silently turn them off).
kernel.watchdog = 1

# A soft lockup (a CPU spinning in kernel mode with preemption off) or a hard
# lockup (interrupts disabled; caught by the NMI watchdog) panics instead of
# only printing "watchdog: BUG: soft lockup" into a journal nobody reads.
kernel.softlockup_panic = 1
kernel.hardlockup_panic = 1

# An oops used to leave the box nominally up with its display driver dead — the
# worst case, because PID 1 stays healthy and so nothing else would reset it.
kernel.panic_on_oops = 1

# Panic, then reboot 30s later. Long enough for the console to be read and for
# the filesystems to be sync'ed by the panic path; short enough that nobody
# waits for it. 0 would mean "sit there forever", which is the bug.
kernel.panic = 30

# Not set here on purpose: kernel.hung_task_panic does not exist in the Fedora
# 44 kernel build (CONFIG_DETECT_HUNG_TASK is off). A task blocked on I/O is
# caught by the watchdog in 90-mirror-wall-watchdog.conf instead.