Re: [PATCH net] tcp: fix tcp_tso_should_defer() vs large RTT

[Neal Cardwell <ncardwell@google.com>]

On Sat, Oct 11, 2025 at 7:57 AM Eric Dumazet <edumazet@google.com> wrote:
>
> Neal reported that using neper tcp_stream with TCP_TX_DELAY
> set to 50ms would often lead to flows stuck in a small cwnd mode,
> regardless of the congestion control.
>
> While tcp_stream sets TCP_TX_DELAY too late after the connect(),
> it highlighted two kernel bugs.
>
> The following heuristic in tcp_tso_should_defer() seems wrong
> for large RTT:
>
> delta = tp->tcp_clock_cache - head->tstamp;
> /* If next ACK is likely to come too late (half srtt), do not defer */
> if ((s64)(delta - (u64)NSEC_PER_USEC * (tp->srtt_us >> 4)) < 0)
>       goto send_now;
>
> If next ACK is expected to come in more than 1 ms, we should
> not defer because we prefer a smooth ACK clocking.
>
> While blamed commit was a step in the good direction, it was not
> generic enough.
>
> Another patch fixing TCP_TX_DELAY for established flows
> will be proposed when net-next reopens.
>
> Fixes: 50c8339e9299 ("tcp: tso: restore IW10 after TSO autosizing")
> Reported-by: Neal Cardwell <ncardwell@google.com>
> Signed-off-by: Eric Dumazet <edumazet@google.com>
> ---

Thanks, Eric! Great catch! The patch looks great to me, and I tested
that it fixes the issue I was seeing with neper tcp_stream with
TCP_TX_DELAY.

Reviewed-by: Neal Cardwell <ncardwell@google.com>
Tested-by: Neal Cardwell <ncardwell@google.com>

neal