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=-3.8 required=3.0 tests=BAYES_00, HEADER_FROM_DIFFERENT_DOMAINS,MAILING_LIST_MULTI,SPF_HELO_NONE,SPF_PASS autolearn=no 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 DE8B5C4167B for ; Fri, 11 Dec 2020 10:06:23 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id A0C7223F34 for ; Fri, 11 Dec 2020 10:06:23 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S2405796AbgLKKGQ convert rfc822-to-8bit (ORCPT ); Fri, 11 Dec 2020 05:06:16 -0500 Received: from eu-smtp-delivery-151.mimecast.com ([185.58.86.151]:20158 "EHLO eu-smtp-delivery-151.mimecast.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S2405764AbgLKKF2 (ORCPT ); Fri, 11 Dec 2020 05:05:28 -0500 Received: from AcuMS.aculab.com (156.67.243.126 [156.67.243.126]) (Using TLS) by relay.mimecast.com with ESMTP id uk-mta-249-bCdh2ug7OF-rDdSrCZLxFQ-1; Fri, 11 Dec 2020 10:03:48 +0000 X-MC-Unique: bCdh2ug7OF-rDdSrCZLxFQ-1 Received: from AcuMS.Aculab.com (fd9f:af1c:a25b:0:43c:695e:880f:8750) by AcuMS.aculab.com (fd9f:af1c:a25b:0:43c:695e:880f:8750) with Microsoft SMTP Server (TLS) id 15.0.1347.2; Fri, 11 Dec 2020 10:03:47 +0000 Received: from AcuMS.Aculab.com ([fe80::43c:695e:880f:8750]) by AcuMS.aculab.com ([fe80::43c:695e:880f:8750%12]) with mapi id 15.00.1347.000; Fri, 11 Dec 2020 10:03:47 +0000 From: David Laight To: 'Ioana Ciornei' CC: 'Daniel Thompson' , "linux-netdev@vger.kernel.org" , "linux-kernel@vger.kernel.org" Subject: RE: [PATCH RESEND net-next 1/2] dpaa2-eth: send a scatter-gather FD instead of realloc-ing Thread-Topic: [PATCH RESEND net-next 1/2] dpaa2-eth: send a scatter-gather FD instead of realloc-ing Thread-Index: AQHWzxqS1niBehB/rU2uwbP/Q9O4LKnxoRyAgAADk4CAAATMgA== Date: Fri, 11 Dec 2020 10:03:47 +0000 Message-ID: References: <20200629184712.12449-2-ioana.ciornei () nxp ! com> <20201210173156.mbizovo6rxvkda73@holly.lan> <20201211093856.gj3do6ntnmxsqtzp@skbuf> In-Reply-To: <20201211093856.gj3do6ntnmxsqtzp@skbuf> Accept-Language: en-GB, en-US X-MS-Has-Attach: X-MS-TNEF-Correlator: x-ms-exchange-transport-fromentityheader: Hosted x-originating-ip: [10.202.205.107] MIME-Version: 1.0 Authentication-Results: relay.mimecast.com; auth=pass smtp.auth=C51A453 smtp.mailfrom=david.laight@aculab.com X-Mimecast-Spam-Score: 0 X-Mimecast-Originator: aculab.com Content-Language: en-US Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 8BIT Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org From: Ioana Ciornei > Sent: 11 December 2020 09:39 > > On Fri, Dec 11, 2020 at 09:30:43AM +0000, David Laight wrote: > > From: Daniel Thompson > > > Sent: 10 December 2020 17:32 > > > > > > On Mon, Jun 29, 2020 at 06:47:11PM +0000, Ioana Ciornei wrote: > > > > Instead of realloc-ing the skb on the Tx path when the provided headroom > > > > is smaller than the HW requirements, create a Scatter/Gather frame > > > > descriptor with only one entry. > > > > Is it worth simplifying the code by permanently allocating (and dma-mapping) > > the extra structure for every ring entry. > > It is (probably) only one page and 1 iommu entry. > > > That is exactly what I was thinking. At the moment the SGT structure is > pre-allocated but not pre-mapped. > > I'll let you know how it goes. How much does the dma-map actually cost? For short fragments it is probably worth copying into a pre-allocated pre-mapped transmit buffer area. You'd want to do aligned full-word copies and use separate cache lines for each frame. It does make tx setup more error prone - since you need the space in the tx buffer area as well as in the tx ring. For one OS (not sun's) on a sparc mbus+sbus system one of my colleagues measured a cutoff point of about 1k. The copy to tx buffer path also helps with the pathological skb that are 1500 bytes in 1 byte fragments. (Maybe skb can't get that bad, but I've seen that on other OS.) David - Registered Address Lakeside, Bramley Road, Mount Farm, Milton Keynes, MK1 1PT, UK Registration No: 1397386 (Wales)