bb32347876
To correlate the hardware RX timestamp with something, add tracking of two software timestamps both clock source CLOCK_TAI (see description in man clock_gettime(2)). XDP metadata is extended with xdp_timestamp for capturing when XDP received the packet. Populated with BPF helper bpf_ktime_get_tai_ns(). I could not find a BPF helper for getting CLOCK_REALTIME, which would have been preferred. In userspace when AF_XDP sees the packet another software timestamp is recorded via clock_gettime() also clock source CLOCK_TAI. Example output shortly after loading igc driver: poll: 1 (0) skip=1 fail=0 redir=2 xsk_ring_cons__peek: 1 0x12557a8: rx_desc[1]->addr=100000000009000 addr=9100 comp_addr=9000 rx_hash: 0x82A96531 with RSS type:0x1 rx_timestamp: 1681740540304898909 (sec:1681740540.3049) XDP RX-time: 1681740577304958316 (sec:1681740577.3050) delta sec:37.0001 (37000059.407 usec) AF_XDP time: 1681740577305051315 (sec:1681740577.3051) delta sec:0.0001 (92.999 usec) 0x12557a8: complete idx=9 addr=9000 The first observation is that the 37 sec difference between RX HW vs XDP timestamps, which indicate hardware is likely clock source CLOCK_REALTIME, because (as of this writing) CLOCK_TAI is initialised with a 37 sec offset. The 93 usec (microsec) difference between XDP vs AF_XDP userspace is the userspace wakeup time. On this hardware it was caused by CPU idle sleep states, which can be reduced by tuning /dev/cpu_dma_latency. View current requested/allowed latency bound via: hexdump --format '"%d\n"' /dev/cpu_dma_latency More explanation of the output and how this can be used to identify clock drift for the HW clock can be seen here[1]: [1] https://github.com/xdp-project/xdp-project/blob/master/areas/hints/xdp_hints_kfuncs02_driver_igc.org Signed-off-by: Jesper Dangaard Brouer <brouer@redhat.com> Signed-off-by: Daniel Borkmann <daniel@iogearbox.net> Acked-by: Stanislav Fomichev <sdf@google.com> Acked-by: Song Yoong Siang <yoong.siang.song@intel.com> Link: https://lore.kernel.org/bpf/168182466298.616355.2544377890818617459.stgit@firesoul
Linux kernel
============
There are several guides for kernel developers and users. These guides can
be rendered in a number of formats, like HTML and PDF. Please read
Documentation/admin-guide/README.rst first.
In order to build the documentation, use ``make htmldocs`` or
``make pdfdocs``. The formatted documentation can also be read online at:
https://www.kernel.org/doc/html/latest/
There are various text files in the Documentation/ subdirectory,
several of them using the Restructured Text markup notation.
Please read the Documentation/process/changes.rst file, as it contains the
requirements for building and running the kernel, and information about
the problems which may result by upgrading your kernel.