Re: [PATCH net v4] net/udp_gso: Allow TX timestamp with UDP GSO

[Willem de Bruijn]

On Mon, Jun 17, 2019 at 3:05 PM Fred Klassen <fklassen@xxxxxxxxxxx> wrote:
>
> Fixes an issue where TX Timestamps are not arriving on the error queue
> when UDP_SEGMENT CMSG type is combined with CMSG type SO_TIMESTAMPING.
> This can be illustrated with an updated updgso_bench_tx program which
> includes the '-T' option to test for this condition. It also introduces
> the '-P' option which will call poll() before reading the error queue.
>
>     ./udpgso_bench_tx -4ucTPv -S 1472 -l2 -D 172.16.120.18
>     poll timeout
>     udp tx:      0 MB/s        1 calls/s      1 msg/s
>
> The "poll timeout" message above indicates that TX timestamp never
> arrived.
>
> This patch preserves tx_flags for the first UDP GSO segment. Only the
> first segment is timestamped, even though in some cases there may be
> benefital in timestamping both the first and last segment.
>
> Factors in deciding on first segment timestamp only:
>
> - Timestamping both first and last segmented is not feasible. Hardware
> can only have one outstanding TS request at a time.
>
> - Timestamping last segment may under report network latency of the
> previous segments. Even though the doorbell is suppressed, the ring
> producer counter has been incremented.
>
> - Timestamping the first segment has the upside in that it reports
> timestamps from the application's view, e.g. RTT.
>
> - Timestamping the first segment has the downside that it may
> underreport tx host network latency. It appears that we have to pick
> one or the other. And possibly follow-up with a config flag to choose
> behavior.
>
> v2: Remove tests as noted by Willem de Bruijn <willemb@xxxxxxxxxx>
>     Moving tests from net to net-next
>
> v3: Update only relevant tx_flag bits as per
>     Willem de Bruijn <willemb@xxxxxxxxxx>
>
> v4: Update comments and commit message as per
>     Willem de Bruijn <willemb@xxxxxxxxxx>
>
> Fixes: ee80d1ebe5ba ("udp: add udp gso")
> Signed-off-by: Fred Klassen <fklassen@xxxxxxxxxxx>

Acked-by: Willem de Bruijn <willemb@xxxxxxxxxx>