Re: [PATCH net-next] tcp: better handle TCP_TX_DELAY on established flows

[Paolo Abeni <pabeni@redhat.com>]

On 10/14/25 10:54 AM, Eric Dumazet wrote:
> On Tue, Oct 14, 2025 at 1:29 AM Eric Dumazet <edumazet@google.com> wrote:
>>
>> On Tue, Oct 14, 2025 at 1:22 AM Paolo Abeni <pabeni@redhat.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@google.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?!?
> 
> Not sure, look at the packetdrill test "`tc qdisc delete dev tun0 root
> 2>/dev/null ; tc qdisc add dev tun0 root pfifo limit 0`"
> 
> After "net: dev_queue_xmit() llist adoption" __dev_xmit_skb() might
> return NET_XMIT_SUCCESS instead of NET_XMIT_DROP
> 
> __tcp_transmit_skb() has some code to detect NET_XMIT_DROP
> immediately, instead of relying on a timer.
> 
> I can fix the 'single packet' case, but not the case of many packets
> being sent in //

What about using a nf rule to drop all the 'tun0' egress packet, instead
of a qdisc?

In any case I think the pending patches should be ok.

/P