pn01, pn02: enable memtest86 and update stability docs

Enable memtest86 in systemd-boot menu on both PN51 units to allow
extended memory testing. Update stability document with March crash
data from pstore/Loki — crashes now traced to sched_ext scheduler
kernel oops, suggesting possible memory corruption.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

docs/plans/pn51-stability.md

@@ -134,16 +134,78 @@ These appear on both units and can be ignored:
 - **Positive**: The system auto-rebooted this time (likely hardware watchdog), unlike previous freezes that required manual power cycle
 - `processor.max_cstate=1` may have extended uptime (2d21h vs previous 1h and 5.5h) but did not prevent the freeze
 
+### 2026-02-27 to 2026-03-03: Relative Stability
+
+- pn02 ran without crashes for approximately one week after the third freeze
+- pn01 continued to be completely stable throughout this period
+- Auto-upgrade reboots continued daily (~4am) on both units — these are planned and healthy
+
+### 2026-03-04: pn02 Fourth Crash — sched_ext Kernel Oops (pstore captured)
+
+- pn02 crashed after ~5.8 days uptime (504566s)
+- **First crash captured by pstore** — kernel oops and panic stack traces preserved across reboot
+- Journal corruption confirmed: `system.journal corrupted or uncleanly shut down`
+- **Crash location**: `RIP: 0010:set_next_task_scx+0x6e/0x210` — crash in the **sched_ext (SCX) scheduler** subsystem
+- **Call trace**: `sysvec_apic_timer_interrupt` → `cpuidle_enter_state` — crashed during CPU idle, triggered by APIC timer interrupt
+- **CR2**: `ffffffffffffff89` — dereferencing an obviously invalid kernel pointer
+- **Kernel**: 6.12.74 (NixOS 25.11)
+- **Significance**: This is the first crash with actual diagnostic output. Previous crashes were silent sub-NMI freezes. The sched_ext scheduler path is a new finding — earlier crashes were assumed to be hardware-level.
+
+### 2026-03-06: pn02 Fifth Crash
+
+- pn02 crashed again — journal corruption on next boot
+- No pstore data captured for this crash
+
+### 2026-03-07: pn02 Sixth and Seventh Crashes — Two in One Day
+
+**First crash (~11:06 UTC):**
+- ~26.6 hours uptime (95994s)
+- **pstore captured both Oops and Panic**
+- **Crash location**: Scheduler code path — `pick_next_task_fair` → `__pick_next_task`
+- **CR2**: `000000c000726000` — invalid pointer dereference
+- **Notable**: `dbus-daemon` segfaulted ~50 minutes before the kernel crash (`segfault at 0` in `libdbus-1.so.3.32.4` on CPU 0) — may indicate memory corruption preceding the kernel crash
+
+**Second crash (~21:15 UTC):**
+- Journal corruption confirmed on next boot
+- No pstore data captured
+
+### 2026-03-07: pn01 Status
+
+- pn01 has had **zero crashes** since initial setup on Feb 21
+- Zero journal corruptions, zero pstore dumps in 30 days
+- Same BOOT_ID maintained between daily auto-upgrade reboots — consistently clean shutdown/reboot cycles
+- All 8 reboots in 30 days are planned auto-upgrade reboots
+- **pn01 is fully stable**
+
+## Crash Summary
+
+| Date | Uptime Before Crash | Crash Type | Diagnostic Data |
+|------|---------------------|------------|-----------------|
+| Feb 21 | ~1h | Silent freeze | None — sub-NMI |
+| Feb 22 | ~5.5h | Silent freeze | None — sub-NMI |
+| Feb 27 | ~2d 21h | Silent freeze | None — sub-NMI, rasdaemon empty |
+| Mar 4 | ~5.8d | **Kernel oops** | pstore: `set_next_task_scx` (sched_ext) |
+| Mar 6 | Unknown | Crash | Journal corruption only |
+| Mar 7 | ~26.6h | **Kernel oops + panic** | pstore: `pick_next_task_fair` (scheduler) + dbus segfault |
+| Mar 7 | Unknown | Crash | Journal corruption only |
+
 ## Conclusion
 
-**pn02 is unreliable.** After exhausting mitigations (fTPM disabled, BIOS updated, WiFi/BT disabled, ErP disabled, amdgpu blacklisted, processor.max_cstate=1, NMI watchdog, rasdaemon), the unit still hard-freezes every few days. The freeze occurs below NMI level with zero diagnostic output, pointing to a board-level hardware defect. The unit is not suitable for hypervisor use or any workload requiring reliability.
+**pn02 is unreliable.** After exhausting mitigations (fTPM disabled, BIOS updated, WiFi/BT disabled, ErP disabled, amdgpu blacklisted, processor.max_cstate=1, NMI watchdog, rasdaemon), the unit still crashes every few days. 26 reboots in 30 days (7 unclean crashes + daily auto-upgrade reboots).
 
-**pn01 remains stable** so far but has historically crashed as well, just less frequently. Continuing to monitor.
+The pstore crash dumps from March reveal a new dimension: at least some crashes are **kernel scheduler bugs in sched_ext**, not just silent hardware-level freezes. The `set_next_task_scx` and `pick_next_task_fair` crash sites, combined with the dbus-daemon segfault before one crash, suggest possible memory corruption that manifests in the scheduler. It's unclear whether this is:
+1. A sched_ext kernel bug exposed by the PN51's hardware quirks (unstable TSC, C-state behavior)
+2. Hardware-induced memory corruption that happens to hit scheduler data structures
+3. A pure software bug in the 6.12.74 kernel's sched_ext implementation
+
+**pn01 is stable** — zero crashes in 30 days of continuous operation. Both units have identical kernel and NixOS configuration (minus pn02's diagnostic mitigations), so the difference points toward a hardware defect specific to the pn02 board.
 
 ## Next Steps
 
+- **pn02 memtest**: Run memtest86 for 24h+ (available in systemd-boot menu). The crash signatures (userspace segfaults before kernel panics, corrupted pointers in scheduler structures) are consistent with intermittent RAM errors that a quick pass wouldn't catch. If memtest finds errors, swap the DIMM.
 - **pn02**: Consider scrapping or repurposing for non-critical workloads that tolerate random reboots (auto-recovery via hardware watchdog is now working)
-- **pn01**: Continue monitoring. If it remains stable long-term, may still be viable for light workloads
+- **pn02 investigation**: Could try disabling sched_ext (`boot.kernelParams = [ "sched_ext.enabled=0" ]` or equivalent) to test whether the crashes stop — would help distinguish kernel bug from hardware defect
+- **pn01**: Continue monitoring. If it remains stable long-term, it is viable for light workloads
 - If pn01 eventually crashes, apply the same mitigations (amdgpu blacklist, max_cstate=1) to see if they help
 - For the Incus hypervisor plan: likely need different hardware. Evaluating GMKtec G3 (Intel) as an alternative. Note: mixed Intel/AMD cluster complicates live migration
 
@@ -160,11 +222,10 @@ hardware.rasdaemon.enable = true;
 hardware.rasdaemon.record = true;
 ```
 
-**On next freeze, one of two things happens:**
-1. **NMI watchdog catches it** -> kernel panic with stack trace in logs -> auto-reboot after 10s -> we get diagnostic info
-2. **Hard lockup below NMI level** -> SP5100 TCO hardware watchdog (10min timeout) reboots it -> confirms board-level defect
+**Crash recovery is working**: pstore now captures kernel oops/panic data, and the system auto-reboots via `panic=10` or SP5100 TCO hardware watchdog.
 
 **After reboot, check:**
 - `ras-mc-ctl --summary` — overview of hardware errors
 - `ras-mc-ctl --errors` — detailed error list
 - `journalctl -b -1 -p err` — kernel logs from crashed boot (if panic was logged)
+- pstore data is automatically archived by `systemd-pstore.service` and forwarded to Loki via promtail

hosts/pn01/configuration.nix

@@ -12,6 +12,7 @@
   ];
 
   boot.loader.systemd-boot.enable = true;
+  boot.loader.systemd-boot.memtest86.enable = true;
   boot.loader.efi.canTouchEfiVariables = true;
 
   networking.hostName = "pn01";

hosts/pn02/configuration.nix

@@ -12,6 +12,7 @@
   ];
 
   boot.loader.systemd-boot.enable = true;
+  boot.loader.systemd-boot.memtest86.enable = true;
   boot.loader.efi.canTouchEfiVariables = true;
   boot.blacklistedKernelModules = [ "amdgpu" ];
   boot.kernelParams = [ "panic=10" "nmi_watchdog=1" "processor.max_cstate=1" ];