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Thu, 12 Mar 2020 00:42:47 +0000 References: <20200311173356.38181-1-petrm@mellanox.com> <20200311173356.38181-4-petrm@mellanox.com> <2629782f-24e7-bf34-6251-ab0afe22ff03@gmail.com> User-agent: mu4e 1.3.3; emacs 26.3 From: Petr Machata To: Eric Dumazet Cc: netdev@vger.kernel.org, Jakub Kicinski , Roman Mashak , jhs@mojatatu.com, xiyou.wangcong@gmail.com, davem@davemloft.net, jiri@mellanox.com, mlxsw@mellanox.com Subject: Re: [PATCH net-next v2 3/6] net: sched: RED: Introduce an ECN tail-dropping mode In-reply-to: <2629782f-24e7-bf34-6251-ab0afe22ff03@gmail.com> Date: Thu, 12 Mar 2020 01:42:44 +0100 Message-ID: <87imjaxv23.fsf@mellanox.com> Content-Type: text/plain X-ClientProxiedBy: FR2P281CA0023.DEUP281.PROD.OUTLOOK.COM (2603:10a6:d10:14::10) To HE1PR05MB4746.eurprd05.prod.outlook.com (2603:10a6:7:a3::22) MIME-Version: 1.0 X-MS-Exchange-MessageSentRepresentingType: 1 Received: from yaviefel (89.176.246.183) by FR2P281CA0023.DEUP281.PROD.OUTLOOK.COM (2603:10a6:d10:14::10) with Microsoft SMTP Server (version=TLS1_2, cipher=TLS_ECDHE_RSA_WITH_AES_256_GCM_SHA384) id 15.20.2793.15 via Frontend Transport; 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X-Microsoft-Antispam-Message-Info: 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 X-MS-Exchange-AntiSpam-MessageData: iwcosb/GriEYPRZUWwjF1rbbXoikEvmd0NT5RFML/Tk6h5UjFfxJyASDHtFvdT0Ficndrlme/duOX4V3gSjDigXtVw7j2neLT4Gc6hRoHZLv9izPNRDDodl/zK3LhpT+0Zm7lqf/6hgwadgBK2+eTQ== X-OriginatorOrg: Mellanox.com X-MS-Exchange-CrossTenant-Network-Message-Id: 7289d7c2-b955-4e03-69bb-08d7c61e48a4 X-MS-Exchange-CrossTenant-OriginalArrivalTime: 12 Mar 2020 00:42:46.8936 (UTC) X-MS-Exchange-CrossTenant-FromEntityHeader: Hosted X-MS-Exchange-CrossTenant-Id: a652971c-7d2e-4d9b-a6a4-d149256f461b X-MS-Exchange-CrossTenant-MailboxType: HOSTED X-MS-Exchange-CrossTenant-UserPrincipalName: 9GFa3ZeDKkKiCI42OF9RY1aFFmVIlP0SSnbeGkVsQpPIHfRPQgl7pja7GSQEm4GtkDF6t8LDzCkfaDgiWVK7jw== X-MS-Exchange-Transport-CrossTenantHeadersStamped: HE1PR05MB4700 Sender: netdev-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: netdev@vger.kernel.org Eric Dumazet writes: > On 3/11/20 10:33 AM, Petr Machata wrote: >> When the RED Qdisc is currently configured to enable ECN, the RED algorithm >> is used to decide whether a certain SKB should be marked. If that SKB is >> not ECN-capable, it is early-dropped. >> >> It is also possible to keep all traffic in the queue, and just mark the >> ECN-capable subset of it, as appropriate under the RED algorithm. Some >> switches support this mode, and some installations make use of it. >> >> To that end, add a new RED flag, TC_RED_TAILDROP. When the Qdisc is >> configured with this flag, non-ECT traffic is enqueued (and tail-dropped >> when the queue size is exhausted) instead of being early-dropped. >> > > I find the naming of the feature very confusing. > > When enabling this new feature, we no longer drop packets > that could not be CE marked. > Tail drop is already in the packet scheduler, you want to disable it. > > > How this feature has been named elsewhere ??? > (you mentioned in your cover letter : > "Some switches support this mode, and some installations make use of it.") The two interfaces that I know about are Juniper and Cumulus. I don't know either from direct experience, but from the manual, Cumulus seems to allow enablement of either ECN on its own[0], or ECN with RED[1]. (Or RED on its own I presume, but I couldn't actually find that.) In Juniper likewise, "on ECN-enabled queues, the switch [...] uses the tail-drop algorithm to drop non-ECN-capable packets during periods of congestion"[2]. You need to direct non-ECT traffic to a different queue and configure RED on that to get the RED+ECN behavior ala Linux. So this is unlike the RED qdisc, where RED is implied, and needs to be turned off again by an anti-RED flag. The logic behind the chosen flag name is that the opposite of early dropping is tail dropping. Note that Juniper actually calls it that as well. That said, I agree that from the perspective of the qdisc itself the name does not make sense. We can make it "nodrop", or "nored", or maybe "keep_non_ect"... I guess "nored" is closest to the desired effect. [0] https://docs.cumulusnetworks.com/cumulus-linux-40/Layer-1-and-Switch-Ports/Buffer-and-Queue-Management/ [1] https://docs.cumulusnetworks.com/version/cumulus-linux-37/Network-Solutions/RDMA-over-Converged-Ethernet-RoCE/ [2] https://www.juniper.net/documentation/en_US/junos/topics/concept/cos-qfx-series-tail-drop-understanding.html