From mboxrd@z Thu Jan 1 00:00:00 1970 From: David Miller Subject: Re: [PATCH net] tcp: warn on bogus MSS and try to amend it Date: Thu, 01 Dec 2016 15:29:49 -0500 (EST) Message-ID: <20161201.152949.1953888486413180001.davem@davemloft.net> References: <0d41deb00d57206f518e6bffae1b0be355bbc726.1480511277.git.marcelo.leitner@gmail.com> Mime-Version: 1.0 Content-Type: Text/Plain; charset=us-ascii Content-Transfer-Encoding: 7bit Cc: netdev@vger.kernel.org, jmaxwell37@gmail.com, alexandre.sidorenko@hpe.com, kuznet@ms2.inr.ac.ru, jmorris@namei.org, yoshfuji@linux-ipv6.org, kaber@trash.net, tlfalcon@linux.vnet.ibm.com, brking@linux.vnet.ibm.com, eric.dumazet@gmail.com To: marcelo.leitner@gmail.com Return-path: Received: from shards.monkeyblade.net ([184.105.139.130]:43814 "EHLO shards.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1757453AbcLAU3v (ORCPT ); Thu, 1 Dec 2016 15:29:51 -0500 In-Reply-To: <0d41deb00d57206f518e6bffae1b0be355bbc726.1480511277.git.marcelo.leitner@gmail.com> Sender: netdev-owner@vger.kernel.org List-ID: From: Marcelo Ricardo Leitner Date: Wed, 30 Nov 2016 11:14:32 -0200 > There have been some reports lately about TCP connection stalls caused > by NIC drivers that aren't setting gso_size on aggregated packets on rx > path. This causes TCP to assume that the MSS is actually the size of the > aggregated packet, which is invalid. > > Although the proper fix is to be done at each driver, it's often hard > and cumbersome for one to debug, come to such root cause and report/fix > it. > > This patch amends this situation in two ways. First, it adds a warning > on when this situation occurs, so it gives a hint to those trying to > debug this. It also limit the maximum probed MSS to the adverised MSS, > as it should never be any higher than that. > > The result is that the connection may not have the best performance ever > but it shouldn't stall, and the admin will have a hint on what to look > for. > > Tested with virtio by forcing gso_size to 0. > > Cc: Jonathan Maxwell > Signed-off-by: Marcelo Ricardo Leitner I totally agree with this change, however I think the warning message can be improved in two ways: > len = skb_shinfo(skb)->gso_size ? : skb->len; > if (len >= icsk->icsk_ack.rcv_mss) { > - icsk->icsk_ack.rcv_mss = len; > + icsk->icsk_ack.rcv_mss = min_t(unsigned int, len, > + tcp_sk(sk)->advmss); > + if (icsk->icsk_ack.rcv_mss != len) > + pr_warn_once("Seems your NIC driver is doing bad RX acceleration. TCP performance may be compromised.\n"); We know it's a bad GRO implementation that causes this so let's be specific in the message, perhaps something like: Driver has suspect GRO implementation, TCP performance may be compromised. Also, we have skb->dev available here most likely, so prefixing the message with skb->dev->name would make analyzing this situation even easier for someone hitting this. I'm not certain if an skb->dev==NULL check is necessary here or not, but it is definitely something you need to consider. Thanks!