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

[David Miller <davem@davemloft.net>]

From: Fred Klassen <fklassen@appneta.com>
Date: Mon, 17 Jun 2019 12:05:07 -0700

> 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.
 ...
> 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.
 ...
> Fixes: ee80d1ebe5ba ("udp: add udp gso")
> Signed-off-by: Fred Klassen <fklassen@appneta.com>

Applied and queued up for -stable, thanks.