From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org X-Spam-Level: X-Spam-Status: No, score=-10.0 required=3.0 tests=BAYES_00,DKIMWL_WL_HIGH, DKIM_SIGNED,DKIM_VALID,HEADER_FROM_DIFFERENT_DOMAINS,MAILING_LIST_MULTI, SIGNED_OFF_BY,SPF_HELO_NONE,SPF_PASS,URIBL_BLOCKED,USER_AGENT_GIT autolearn=unavailable autolearn_force=no version=3.4.0 Received: from mail.kernel.org (mail.kernel.org [198.145.29.99]) by smtp.lore.kernel.org (Postfix) with ESMTP id 61AA7C433E0 for ; Mon, 10 Aug 2020 15:28:51 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 34D8922CF7 for ; Mon, 10 Aug 2020 15:28:51 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=default; t=1597073331; bh=/WDC1K6IwjpeqrSiEcyEoHJxhvNKb6efaSsBjR82Dj8=; h=From:To:Cc:Subject:Date:In-Reply-To:References:List-ID:From; b=wCEChY6BomQSSVnOL31/dp7rF9zTlNuVuFsfaQDFBTbOl5Ggb+O2zzhTUKB1hqO/F Qwq5wqjmaDBaNuCFtaGkl7hWroJC7D11LR8Fwr3xn3O97vIQdEPdLVKsajo3hNQcXA X9ureuUotmF+YPO6hbRLXr69hyrosoZQitMBCDro= Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1728473AbgHJP2u (ORCPT ); Mon, 10 Aug 2020 11:28:50 -0400 Received: from mail.kernel.org ([198.145.29.99]:34880 "EHLO mail.kernel.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1728899AbgHJP2p (ORCPT ); Mon, 10 Aug 2020 11:28:45 -0400 Received: from localhost (83-86-89-107.cable.dynamic.v4.ziggo.nl [83.86.89.107]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by mail.kernel.org (Postfix) with ESMTPSA id 1FE9F22B47; Mon, 10 Aug 2020 15:28:43 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=default; t=1597073324; bh=/WDC1K6IwjpeqrSiEcyEoHJxhvNKb6efaSsBjR82Dj8=; h=From:To:Cc:Subject:Date:In-Reply-To:References:From; b=a94mm2W6eFdEaUMQCfjyZKAm14l9SfZQqdr1sCxlbZoAEHVUka4ZGZBUBoi+vOKwP AzsPc45xjHE5t4lHxapg5cJvDdRKnCNbG3CYnhMa5UE5fEMjwy4HRRhsmsJs2IHb04 BjzxnaXmE+EoJ+KfpbBexzxhXDEJNEjJtF9dthMc= From: Greg Kroah-Hartman To: linux-kernel@vger.kernel.org Cc: Greg Kroah-Hartman , stable@vger.kernel.org, Jianfeng Wang , Neal Cardwell , Eric Dumazet , Kevin Yang , Yuchung Cheng , "David S. Miller" Subject: [PATCH 5.4 65/67] tcp: apply a floor of 1 for RTT samples from TCP timestamps Date: Mon, 10 Aug 2020 17:21:52 +0200 Message-Id: <20200810151812.702529304@linuxfoundation.org> X-Mailer: git-send-email 2.28.0 In-Reply-To: <20200810151809.438685785@linuxfoundation.org> References: <20200810151809.438685785@linuxfoundation.org> User-Agent: quilt/0.66 MIME-Version: 1.0 Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 8bit Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org From: Jianfeng Wang [ Upstream commit 730e700e2c19d87e578ff0e7d8cb1d4a02b036d2 ] For retransmitted packets, TCP needs to resort to using TCP timestamps for computing RTT samples. In the common case where the data and ACK fall in the same 1-millisecond interval, TCP senders with millisecond- granularity TCP timestamps compute a ca_rtt_us of 0. This ca_rtt_us of 0 propagates to rs->rtt_us. This value of 0 can cause performance problems for congestion control modules. For example, in BBR, the zero min_rtt sample can bring the min_rtt and BDP estimate down to 0, reduce snd_cwnd and result in a low throughput. It would be hard to mitigate this with filtering in the congestion control module, because the proper floor to apply would depend on the method of RTT sampling (using timestamp options or internally-saved transmission timestamps). This fix applies a floor of 1 for the RTT sample delta from TCP timestamps, so that seq_rtt_us, ca_rtt_us, and rs->rtt_us will be at least 1 * (USEC_PER_SEC / TCP_TS_HZ). Note that the receiver RTT computation in tcp_rcv_rtt_measure() and min_rtt computation in tcp_update_rtt_min() both already apply a floor of 1 timestamp tick, so this commit makes the code more consistent in avoiding this edge case of a value of 0. Signed-off-by: Jianfeng Wang Signed-off-by: Neal Cardwell Signed-off-by: Eric Dumazet Acked-by: Kevin Yang Acked-by: Yuchung Cheng Signed-off-by: David S. Miller Signed-off-by: Greg Kroah-Hartman --- net/ipv4/tcp_input.c | 2 ++ 1 file changed, 2 insertions(+) --- a/net/ipv4/tcp_input.c +++ b/net/ipv4/tcp_input.c @@ -2944,6 +2944,8 @@ static bool tcp_ack_update_rtt(struct so u32 delta = tcp_time_stamp(tp) - tp->rx_opt.rcv_tsecr; if (likely(delta < INT_MAX / (USEC_PER_SEC / TCP_TS_HZ))) { + if (!delta) + delta = 1; seq_rtt_us = delta * (USEC_PER_SEC / TCP_TS_HZ); ca_rtt_us = seq_rtt_us; }