The Cuckoo Escapement: field report, kernel patch, dashboard

What the Raspberry Pi time-server guides get wrong on a Pi 4, with the
measurements. The headline artifact is a four-line pps-gpio patch: PREEMPT_RT
force-threads IRQ handlers, and pps-gpio takes its timestamp inside its handler,
so the realtime kernel puts a scheduler between the electrical edge and the
clock. IRQF_NO_THREAD takes RMS offset from 2468 ns to 199 ns.

- kernel/     the patch
- dashboard/  live status page (position hidden by default)
- docs-site/  the write-up (Astro/Starlight, brass, no tutorial section)

dashboard/deploy/gpsntp-dashboard.service

@@ -0,0 +1,35 @@
+[Unit]
+Description=gps-ntp status dashboard
+Documentation=https://git.supported.systems/time-pi
+After=network-online.target gpsd.service chrony.service
+Wants=network-online.target
+
+[Service]
+Type=exec
+User=gpsntp
+Group=gpsntp
+ExecStart=/opt/gpsntp-dashboard/.venv/bin/gpsntp-dashboard
+Environment=GPSNTP_PORT=8080
+Environment=GPSNTP_HOST=0.0.0.0
+Restart=on-failure
+RestartSec=3
+
+# Keep the dashboard OFF cpu0. The PPS interrupt is handled on cpu0 and cannot
+# be moved (Pi 4 GPIO IRQs are demuxed through pinctrl-bcm2835 and refuse an
+# smp_affinity), so any load there directly adds jitter to the timestamp.
+# Measured: the dashboard cost ~36% more PPS jitter before this. It is a
+# monitoring tool; it must never perturb the clock it is watching.
+CPUAffinity=1
+Nice=10
+
+# Hardening. NoNewPrivileges is intentionally NOT set: the served-clients
+# panel shells `sudo -n chronyc -c clients`, allowed by a narrow sudoers rule.
+PrivateTmp=true
+ProtectHome=true
+ProtectControlGroups=true
+ProtectKernelTunables=true
+RestrictSUIDSGID=false
+LockPersonality=true
+
+[Install]
+WantedBy=multi-user.target