[xdp-hints] Re: [PATCH bpf-next V1 5/5] selftests/bpf: xdp_hw_metadata track more timestamps

[Jesper Dangaard Brouer <jbrouer@redhat.com>]

On 17/04/2023 16.57, Jesper Dangaard Brouer wrote:
> 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
> 

For QA verification testing, I want to mention that this fix[0] were
applied, in-order to get "rx_timestamp" working on igc:

  [0] 
https://lore.kernel.org/all/20230414154902.2950535-1-yoong.siang.song@intel.com/


> 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
>