From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from smtp.kernel.org (aws-us-west-2-korg-mail-1.web.codeaurora.org [10.30.226.201]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id 7A6EC2DAFC3; Mon, 27 Oct 2025 19:10:34 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=10.30.226.201 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1761592234; cv=none; b=cFkAy179hG+s+SXtrniGBGN4wba94LeiWYFSIPBYpX7IhNGOFwZXzrRPO9X21//GK548HuramyjeKKqkTjRAh40Cb7bmi9r3nU29MyeiSIExXsFfN+jq6nnXpM0VSZ+ykFnWsWqvXXSlRAirXXF1Fo9YPU7JQzQp+rePvKK0MGs= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1761592234; c=relaxed/simple; bh=e3UtPQ1jFCUJQWAmYORGEWT4RGNDM/uhZverNE4ztYc=; h=From:To:Cc:Subject:Date:Message-ID:In-Reply-To:References: MIME-Version; b=jY09vN4a4BhM1etsno76jkmcdgJ8iVhCPRVvNA4vRYQA52x8wlPCJieGUr+H9mMT/x1bMAwkxKYbNW+zvv5E1XkaciWYOpqp92NHJYSeDPE1VVU9nPXg1FzSx2eApOhVCz8XkKqLwESwF989rSiWu4rjfPV6mIoiygR2XozXCkA= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dkim=pass (1024-bit key) header.d=linuxfoundation.org header.i=@linuxfoundation.org header.b=iPOGE3ei; arc=none smtp.client-ip=10.30.226.201 Authentication-Results: smtp.subspace.kernel.org; dkim=pass (1024-bit key) header.d=linuxfoundation.org header.i=@linuxfoundation.org header.b="iPOGE3ei" Received: by smtp.kernel.org (Postfix) with ESMTPSA id 0B674C4CEFD; Mon, 27 Oct 2025 19:10:33 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=linuxfoundation.org; s=korg; t=1761592234; bh=e3UtPQ1jFCUJQWAmYORGEWT4RGNDM/uhZverNE4ztYc=; h=From:To:Cc:Subject:Date:In-Reply-To:References:From; b=iPOGE3ei/Yq93Ol8jfdVjFqMLBw0N0qOLFAdUtLIRUvojNPnCdpQKhYAl1dSbzkFZ GJwD2LXDEKELWUTa+TyqxHpXx8pgcSdMKRRQnzwE5FrcUus6OU0tC8TVHObyes1L99 8zKS8Epztp3yijooxn6YZktCXZVMwpNgZtc6AFco= From: Greg Kroah-Hartman To: stable@vger.kernel.org Cc: Greg Kroah-Hartman , patches@lists.linux.dev, Neal Cardwell , Eric Dumazet , Paolo Abeni , Sasha Levin Subject: [PATCH 6.1 043/157] tcp: fix tcp_tso_should_defer() vs large RTT Date: Mon, 27 Oct 2025 19:35:04 +0100 Message-ID: <20251027183502.452372246@linuxfoundation.org> X-Mailer: git-send-email 2.51.1 In-Reply-To: <20251027183501.227243846@linuxfoundation.org> References: <20251027183501.227243846@linuxfoundation.org> User-Agent: quilt/0.69 X-stable: review X-Patchwork-Hint: ignore Precedence: bulk X-Mailing-List: patches@lists.linux.dev List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Transfer-Encoding: 8bit 6.1-stable review patch. If anyone has any objections, please let me know. ------------------ From: Eric Dumazet [ Upstream commit 295ce1eb36ae47dc862d6c8a1012618a25516208 ] 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 Signed-off-by: Eric Dumazet Reviewed-by: Neal Cardwell Tested-by: Neal Cardwell Link: https://patch.msgid.link/20251011115742.1245771-1-edumazet@google.com [pabeni@redhat.com: fixed whitespace issue] Signed-off-by: Paolo Abeni Signed-off-by: Sasha Levin --- net/ipv4/tcp_output.c | 19 +++++++++++++++---- 1 file changed, 15 insertions(+), 4 deletions(-) diff --git a/net/ipv4/tcp_output.c b/net/ipv4/tcp_output.c index 40568365cdb3b..a8d8e2f294ff2 100644 --- a/net/ipv4/tcp_output.c +++ b/net/ipv4/tcp_output.c @@ -2184,7 +2184,8 @@ static bool tcp_tso_should_defer(struct sock *sk, struct sk_buff *skb, u32 max_segs) { const struct inet_connection_sock *icsk = inet_csk(sk); - u32 send_win, cong_win, limit, in_flight; + u32 send_win, cong_win, limit, in_flight, threshold; + u64 srtt_in_ns, expected_ack, how_far_is_the_ack; struct tcp_sock *tp = tcp_sk(sk); struct sk_buff *head; int win_divisor; @@ -2246,9 +2247,19 @@ static bool tcp_tso_should_defer(struct sock *sk, struct sk_buff *skb, head = tcp_rtx_queue_head(sk); if (!head) goto send_now; - 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) + + srtt_in_ns = (u64)(NSEC_PER_USEC >> 3) * tp->srtt_us; + /* When is the ACK expected ? */ + expected_ack = head->tstamp + srtt_in_ns; + /* How far from now is the ACK expected ? */ + how_far_is_the_ack = expected_ack - tp->tcp_clock_cache; + + /* If next ACK is likely to come too late, + * ie in more than min(1ms, half srtt), do not defer. + */ + threshold = min(srtt_in_ns >> 1, NSEC_PER_MSEC); + + if ((s64)(how_far_is_the_ack - threshold) > 0) goto send_now; /* Ok, it looks like it is advisable to defer. -- 2.51.0