From patchwork Sun Mar 21 13:48:48 2021 Content-Type: text/plain; charset="utf-8" MIME-Version: 1.0 Content-Transfer-Encoding: 7bit X-Patchwork-Submitter: Vincent MAILHOL X-Patchwork-Id: 406573 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=-11.8 required=3.0 tests=BAYES_00, HEADER_FROM_DIFFERENT_DOMAINS, INCLUDES_PATCH, MAILING_LIST_MULTI, SPF_HELO_NONE, SPF_PASS, 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 B9814C433E4 for ; Sun, 21 Mar 2021 13:49:57 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 8C66D61943 for ; Sun, 21 Mar 2021 13:49:57 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S229995AbhCUNtV (ORCPT ); Sun, 21 Mar 2021 09:49:21 -0400 Received: from smtp07.smtpout.orange.fr ([80.12.242.129]:45758 "EHLO smtp.smtpout.orange.fr" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S229815AbhCUNtQ (ORCPT ); Sun, 21 Mar 2021 09:49:16 -0400 Received: from tomoyo.flets-east.jp ([153.202.107.157]) by mwinf5d13 with ME id j1oz240023PnFJp031p6zA; Sun, 21 Mar 2021 14:49:14 +0100 X-ME-Helo: tomoyo.flets-east.jp X-ME-Auth: bWFpbGhvbC52aW5jZW50QHdhbmFkb28uZnI= X-ME-Date: Sun, 21 Mar 2021 14:49:14 +0100 X-ME-IP: 153.202.107.157 From: Vincent Mailhol To: Marc Kleine-Budde , linux-can@vger.kernel.org, linux-kernel@vger.kernel.org, netdev@vger.kernel.org, Eric Dumazet , Dave Taht Cc: Peter Zijlstra , Randy Dunlap , "David S . Miller" , Jakub Kicinski , Vincent Mailhol Subject: [PATCH v3 0/1] Allow drivers to modify dql.min_limit value Date: Sun, 21 Mar 2021 22:48:48 +0900 Message-Id: <20210321134849.463560-1-mailhol.vincent@wanadoo.fr> X-Mailer: git-send-email 2.26.2 MIME-Version: 1.0 Precedence: bulk List-ID: X-Mailing-List: netdev@vger.kernel.org Abstract: would like to directly set dql.min_limit value inside a driver to improve BQL performances of a CAN USB driver. CAN packets have a small PDU: for classical CAN maximum size is roughly 16 bytes (8 for payload and 8 for arbitration, CRC and others). I am writing an CAN driver for an USB interface. To compensate the extra latency introduced by the USB, I want to group several CAN frames and do one USB bulk send. To this purpose, I implemented BQL in my driver. However, the BQL algorithms can take time to adjust, especially if there are small bursts. The best way I found is to directly modify the dql.min_limit and set it to some empirical values. This way, even during small burst events I can have a good throughput. Slightly increasing the dql.min_limit has no measurable impact on the latency as long as frames fit in the same USB packet (i.e. BQL overheard is negligible compared to USB overhead). The BQL was not designed for USB nor was it designed for CAN's small PDUs which probably explains why I am the first one to ever have thought of using dql.min_limit within the driver. The code I wrote looks like: > #ifdef CONFIG_BQL > netdev_get_tx_queue(netdev, 0)->dql.min_limit = ; > #endif Using #ifdef to set up some variables is not a best practice. I am sending this RFC to see if we can add a function to set this dql.min_limit in a more pretty way. For your reference, this RFQ is a follow-up of a discussion on the linux-can mailing list: https://lore.kernel.org/linux-can/20210309125708.ei75tr5vp2sanfh6@pengutronix.de/ Thank you for your comments. Yours sincerely, Vincent ** Changelog ** RFC v2 -> v3 - More verbose commit description. - Fix kernel documentation. RFC v1 -> RFC v2 - Fix incorect #ifdef use. Reference: https://lore.kernel.org/linux-can/20210309153547.q7zspf46k6terxqv@pengutronix.de/ Link to RFC v1: https://lore.kernel.org/linux-can/20210309152354.95309-1-mailhol.vincent@wanadoo.fr/T/#t Vincent Mailhol (1): netdev: add netdev_queue_set_dql_min_limit() include/linux/netdevice.h | 18 ++++++++++++++++++ 1 file changed, 18 insertions(+)