Re: [RFC mptcp-next] mptcp: TX timestamping at the msk level

[Geliang Tang <geliang@kernel.org>]

Hi David,

On Wed, 2026-09-23 at 04:27 +0100, David CARLIER wrote:
> Hi all,
> 
> After Paolo's comment on the MSG_ERRQUEUE series [1], I started
> looking
> at msk-level TX timestamping. As far as I can see nobody is on it, so
> I'd like to check the direction with you before writing code.
> 
> Today nothing arms tx_flags on the skbs built by
> mptcp_sendmsg_frag(),
> so MPTCP data never gets a TX report. Roughly what I have in mind:
> 
> - keep the tsflags and the key (data seq of the last byte) on the
>   dfrag, since the actual push happens after sendmsg returns;
> - arm the subflow skb covering that byte, keeping its tskey in
>   subflow seq space (tcp_fragment_tstamp() and tcp_ack_tstamp()
>   rely on that);
> - when forwarding SCHED/SND reports to the msk, rewrite ee_data to
>   the msk key. I'm not sure yet how to do the mapping. The easiest
>   option I see is a small per-subflow list of (subflow seq, data seq)
>   pairs for armed bytes;
> - emit ACK reports from __mptcp_clean_una() on DATA_ACK rather than
>   forwarding subflow ACKs;
> - seed the msk sk_tskey for OPT_ID the way TCP does.
> 
> The one I went back and forth on is reinjection: the same bytes can
> leave on several subflows. Plain TCP already reports again on
> retransmit, so reporting each transmission seems acceptable to me,
> but
> I'd like your opinion.
> 
> Geliang, your MSG_ZEROCOPY series [2] touches the same spots (dfrag
> layout, coalescing in mptcp_sendmsg_frag()). I'm happy to build on t
> of it. Do you know if it's still moving?

My MSG_ZEROCOPY series is still moving forward, but it will wait for a
while, since there are other series to handle first.

I found that I have a local patch "preserve the rx timestamp when
coalescing skbs", which I added while debugging some feature earlier.
It might be useful, and I'll include it in the next version of the
MSG_ZEROCOPY series too. Something like:

@@ -178,6 +178,13 @@ static bool __mptcp_try_coalesce(struct sock *sk,

            MPTCP_SKB_CB(from)->map_seq, MPTCP_SKB_CB(to)->map_seq,
            to->len, MPTCP_SKB_CB(from)->end_seq);
   MPTCP_SKB_CB(to)->end_seq = MPTCP_SKB_CB(from)->end_seq;
+
+  if (MPTCP_SKB_CB(from)->has_rxtstamp) {
+          MPTCP_SKB_CB(to)->has_rxtstamp = true;
+          to->tstamp = from->tstamp;
+          skb_hwtstamps(to)->hwtstamp = skb_hwtstamps(from)->hwtstamp;
+  }
+
   return true;
 }

Thanks,
-Geliang

> 
> Cheers
> 
> [1] https://lore.kernel.org/mptcp/d6cdbc83-86a6-4c2a-ad99-29cdb83566
> [2]
> https://lore.kernel.org/mptcp/32bac661bcae143f7e63905dfb781c98e95cdde5.1784260668.git.tanggeliang@kylinos.cn/