netdev - Re: [PATCH net] tcp: fix tcp_tso_should

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite: Windows password security audit tool. GUI, reports in PDF.

[<prev] [next>] [<thread-prev] [day] [month] [year] [list]

Message-ID: <CADVnQyk7M2Z0fz3ub_qpVyW16mpEbRWjRudpRzzTvOMA3j9fLw@mail.gmail.com>
Date: Sat, 11 Oct 2025 13:58:20 -0400
From: Neal Cardwell <ncardwell@...gle.com>
To: Eric Dumazet <edumazet@...gle.com>
Cc: "David S . Miller" <davem@...emloft.net>, Jakub Kicinski <kuba@...nel.org>, 
	Paolo Abeni <pabeni@...hat.com>, Simon Horman <horms@...nel.org>, 
	Willem de Bruijn <willemb@...gle.com>, Kuniyuki Iwashima <kuniyu@...gle.com>, netdev@...r.kernel.org, 
	eric.dumazet@...il.com
Subject: Re: [PATCH net] tcp: fix tcp_tso_should_defer() vs large RTT

On Sat, Oct 11, 2025 at 7:57 AM Eric Dumazet <edumazet@...gle.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@...gle.com>
> Signed-off-by: Eric Dumazet <edumazet@...gle.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@...gle.com>
Tested-by: Neal Cardwell <ncardwell@...gle.com>

neal