Small tools used along the way. Most are one-offs, most are written in C

You need:

• Fairly new Linux kernel/distro. If a particular version is required, it is listed by the tool
• meson and ninja for building
• gcc (any version shipped with a fairly recent distro should work)

meson build
ninja -C build/

Binaries can be found in build/

Background: I needed to look at how PCIe latency could affect real-time tasks, and in particular how a PTP synchronized (syntonized) clock must be connected to the system time. This means moving data across the PCIe bus. AFAIK, there's no way to prioritize traffic in PCIe, so a NIC clock accurate to within "a few hundred ns" will not be as accurate for the system.

pcie_clock is fairly simple, it reads TSC before and after reading both CLOCK_REALTIME and ptp_fd. It attempts to reduce the interference from other tasks by running as sched_rr with priority 80. For further accuracy, the task should be shielded using cpuset (cset is useful!).

sudo cset shield -c 3 -k on
sudo cset shield -s -p $$
sudo ./build/pcie_clock -i eth2 -l 100000 -o ptp_clock.csv

This runs for 100k iterations (1ms period) using PTP clock on eth2 and stores the output to ptp_clock.csv on the format

$ head -n5 ptp_clock.csv
clock_realtime_s,tsc_real,tsc_ptp,real_ptp_ns,tsc_tot,real_tot_ns,clock_diff_s
1622195516.771866610,11918,29422,8683,41512,8745,36.999965
1622195516.772891545,434,31142,9161,31662,9183,36.999967
1622195516.773906788,480,26566,7832,27144,7875,36.999965
1622195516.774921432,484,35910,10572,36494,10615,36.999965

• clock_realtime: timestamp of system. Ideally synchronized using phc2sys from ptp timestamp.
• tsc_real: Number of CPU cycles from start to end of reading CLOCK_REALTIME
• tsc_ptp: Number of CPU cycles from start to end of reading the PTP clock via PCIe
• real_ptp_ns: elapsed system time measured using CLOCK_REALTIME to provide a stable mapping for TSC to actual time (think frequency scaling)
• tsc_tot: Number of CPU cycles spent reading '''both''' clock sources
• real_tot_ns: total time (CLOCK_REALTIME) for all accesses, kept for sanitycheck.
• clock_diff_s: Difference between system clock and PTP time (TAI)