dumpcap: timestamp is years ahead when in pcapng format

[Isaac Boukris]

On Tue, Sep 19, 2023 at 9:00 PM Stephen Hemminger
<stephen at networkplumber.org> wrote:
>
> On Tue, 19 Sep 2023 19:35:55 +0300
> Isaac Boukris <iboukris at gmail.com> wrote:
>
> > Looking with git log, i found the original line was:
> > return pcapng_time.ns + (delta * NSEC_PER_SEC) / rte_get_tsc_hz();
> >
> > Testing that does show a wrapping issue, e.g. (it stays around 08:05).
> >
> > 2023-09-19 08:05:24.372037 IP _gateway.domain > Rocky8.38358: 31975
> > NXDomain 0/0/0 (46)                                              10
> > 2023-09-19 08:05:21.577497 ARP, Request who-has _gateway tell Rocky8,
> > length 46
> > 2023-09-19 08:05:21.577599 ARP, Reply _gateway is-at 00:50:56:f8:92:76
> > (oui Unknown), length 46                                     13
> > 2023-09-19 08:05:22.833897 IP 192.168.202.1.50886 >
> > 239.255.255.250.ssdp: UDP, length 174
> >
> > However with my change it looks fine and always increments. I dropped
> > all the parenthesis:
> > return pcapng_time.ns + delta / pcapng_time.tsc_hz * NSEC_PER_SEC;
>
> The issue is that timestamping is in the fast path and that 64 bit divide is slow.
> Looking at other alternatives.

Then perhaps we can keep the division optimization and just get rid of
the overflow check, relying on the change to multiply by NSEC_PER_SEC
after the division.

With the below change only the first packet is from 2257 while all
subsequent packets are fine. But if I keep the overflow check and only
change to multiply after the division, then all packets are shown from
2257.

[admin at Rocky8 dpdk]$ git diff lib/pcapng/rte_pcapng.c
diff --git a/lib/pcapng/rte_pcapng.c b/lib/pcapng/rte_pcapng.c
index 80d08e1..fa545cd 100644
--- a/lib/pcapng/rte_pcapng.c
+++ b/lib/pcapng/rte_pcapng.c
@@ -79,7 +79,7 @@ static uint64_t pcapng_tsc_to_ns(uint64_t cycles)
         * Currently all TSCs operate below 5GHz.
         */
        delta = cycles - pcapng_time.cycles;
-       if (unlikely(delta >= pcapng_time.tsc_hz)) {
+       if (0 && unlikely(delta >= pcapng_time.tsc_hz)) {
                if (likely(delta < pcapng_time.tsc_hz * 2)) {
                        delta -= pcapng_time.tsc_hz;
                        pcapng_time.cycles += pcapng_time.tsc_hz;
@@ -92,8 +92,9 @@ static uint64_t pcapng_tsc_to_ns(uint64_t cycles)
                }
        }

-       return pcapng_time.ns + rte_reciprocal_divide_u64(delta * NSEC_PER_SEC,
-
&pcapng_time.tsc_hz_inverse);
+       return pcapng_time.ns + rte_reciprocal_divide_u64(delta,
+
&pcapng_time.tsc_hz_inverse) * NSEC_PER_SEC;
 }