Ryan Malloy
6881489bf6
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)
36 lines
1.1 KiB
Desktop File
36 lines
1.1 KiB
Desktop File
[Unit]
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Description=gps-ntp status dashboard
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Documentation=https://git.supported.systems/time-pi
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After=network-online.target gpsd.service chrony.service
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Wants=network-online.target
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[Service]
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Type=exec
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User=gpsntp
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Group=gpsntp
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ExecStart=/opt/gpsntp-dashboard/.venv/bin/gpsntp-dashboard
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Environment=GPSNTP_PORT=8080
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Environment=GPSNTP_HOST=0.0.0.0
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Restart=on-failure
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RestartSec=3
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# Keep the dashboard OFF cpu0. The PPS interrupt is handled on cpu0 and cannot
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# be moved (Pi 4 GPIO IRQs are demuxed through pinctrl-bcm2835 and refuse an
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# smp_affinity), so any load there directly adds jitter to the timestamp.
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# Measured: the dashboard cost ~36% more PPS jitter before this. It is a
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# monitoring tool; it must never perturb the clock it is watching.
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CPUAffinity=1
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Nice=10
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# Hardening. NoNewPrivileges is intentionally NOT set: the served-clients
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# panel shells `sudo -n chronyc -c clients`, allowed by a narrow sudoers rule.
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PrivateTmp=true
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ProtectHome=true
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ProtectControlGroups=true
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ProtectKernelTunables=true
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RestrictSUIDSGID=false
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LockPersonality=true
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[Install]
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WantedBy=multi-user.target
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