netdev - Re: [PATCH net-next] tcp: better handle TCP_TX

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Message-ID: <CANn89iKu7jjnjc1QdUrvbetti2AGhKe0VR+srecrpJ2s-hfkKA@mail.gmail.com>
Date: Tue, 14 Oct 2025 01:29:49 -0700
From: Eric Dumazet <edumazet@...gle.com>
To: Paolo Abeni <pabeni@...hat.com>
Cc: "David S . Miller" <davem@...emloft.net>, Jakub Kicinski <kuba@...nel.org>, 
	Simon Horman <horms@...nel.org>, Neal Cardwell <ncardwell@...gle.com>, 
	Willem de Bruijn <willemb@...gle.com>, Kuniyuki Iwashima <kuniyu@...gle.com>, netdev@...r.kernel.org, 
	eric.dumazet@...il.com
Subject: Re: [PATCH net-next] tcp: better handle TCP_TX_DELAY on established flows

On Tue, Oct 14, 2025 at 1:22 AM Paolo Abeni <pabeni@...hat.com> wrote:
>
> On 10/13/25 4:59 PM, Eric Dumazet wrote:
> > Some applications uses TCP_TX_DELAY socket option after TCP flow
> > is established.
> >
> > Some metrics need to be updated, otherwise TCP might take time to
> > adapt to the new (emulated) RTT.
> >
> > This patch adjusts tp->srtt_us, tp->rtt_min, icsk_rto
> > and sk->sk_pacing_rate.
> >
> > This is best effort, and for instance icsk_rto is reset
> > without taking backoff into account.
> >
> > Signed-off-by: Eric Dumazet <edumazet@...gle.com>
>
> The CI is consistently reporting pktdrill failures on top of this patch:
>
> # selftests: net/packetdrill: tcp_user_timeout_user-timeout-probe.pkt
> # TAP version 13
> # 1..2
> # tcp_user_timeout_user-timeout-probe.pkt:35: error in Python code
> # Traceback (most recent call last):
> #   File "/tmp/code_T7S7S4", line 202, in <module>
> #     assert tcpi_probes == 6, tcpi_probes; \
> # AssertionError: 0
> # tcp_user_timeout_user-timeout-probe.pkt: error executing code:
> 'python3' returned non-zero status 1
>
> To be accurate, the patches batch under tests also includes:
>
> https://patchwork.kernel.org/project/netdevbpf/list/?series=1010780
>
> but the latter looks even more unlikely to cause the reported issues?!?
>
> Tentatively setting this patch to changes request, to for CI's sake.

I will take a look, thanks.

I ran our ~2000 packetdrill tests for the tcp_tso_should_defer() fix,
but had no coverage yet for TCP_TX_DELAY, and started adding
packetdrill tests for that.