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=-5.3 required=3.0 tests=BAYES_00, HEADER_FROM_DIFFERENT_DOMAINS,MAILING_LIST_MULTI,NICE_REPLY_A,SPF_HELO_NONE, SPF_PASS,USER_AGENT_SANE_1 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 1B931C433E0 for ; Wed, 17 Mar 2021 01:15:31 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id CB3E964F8C for ; Wed, 17 Mar 2021 01:15:30 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S229621AbhCQBO6 (ORCPT ); Tue, 16 Mar 2021 21:14:58 -0400 Received: from szxga01-in.huawei.com ([45.249.212.187]:5093 "EHLO szxga01-in.huawei.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S229508AbhCQBOm (ORCPT ); Tue, 16 Mar 2021 21:14:42 -0400 Received: from DGGEMM404-HUB.china.huawei.com (unknown [172.30.72.57]) by szxga01-in.huawei.com (SkyGuard) with ESMTP id 4F0XFv57DFzYKq0; Wed, 17 Mar 2021 09:12:51 +0800 (CST) Received: from dggpemm500005.china.huawei.com (7.185.36.74) by DGGEMM404-HUB.china.huawei.com (10.3.20.212) with Microsoft SMTP Server (TLS) id 14.3.498.0; Wed, 17 Mar 2021 09:14:35 +0800 Received: from [127.0.0.1] (10.69.30.204) by dggpemm500005.china.huawei.com (7.185.36.74) with Microsoft SMTP Server (version=TLS1_2, cipher=TLS_ECDHE_RSA_WITH_AES_128_CBC_SHA256) id 15.1.2106.2; Wed, 17 Mar 2021 09:14:35 +0800 Subject: Re: [RFC v2] net: sched: implement TCQ_F_CAN_BYPASS for lockless qdisc To: Cong Wang , Jakub Kicinski CC: David Miller , Vladimir Oltean , Alexei Starovoitov , Daniel Borkmann , Andrii Nakryiko , Eric Dumazet , "Wei Wang" , "Cong Wang ." , Taehee Yoo , Linux Kernel Network Developers , LKML , , Marc Kleine-Budde , References: <1615603667-22568-1-git-send-email-linyunsheng@huawei.com> <1615777818-13969-1-git-send-email-linyunsheng@huawei.com> <20210315115332.1647e92b@kicinski-fedora-pc1c0hjn.dhcp.thefacebook.com> From: Yunsheng Lin Message-ID: Date: Wed, 17 Mar 2021 09:14:34 +0800 User-Agent: Mozilla/5.0 (Windows NT 10.0; WOW64; rv:52.0) Gecko/20100101 Thunderbird/52.2.0 MIME-Version: 1.0 In-Reply-To: Content-Type: text/plain; charset="utf-8" Content-Language: en-US Content-Transfer-Encoding: 7bit X-Originating-IP: [10.69.30.204] X-ClientProxiedBy: dggeme702-chm.china.huawei.com (10.1.199.98) To dggpemm500005.china.huawei.com (7.185.36.74) X-CFilter-Loop: Reflected Precedence: bulk List-ID: X-Mailing-List: linux-can@vger.kernel.org On 2021/3/17 6:48, Cong Wang wrote: > On Mon, Mar 15, 2021 at 2:07 PM Jakub Kicinski wrote: >> >> I thought pfifo was supposed to be "lockless" and this change >> re-introduces a lock between producer and consumer, no? > > It has never been truly lockless, it uses two spinlocks in the ring buffer > implementation, and it introduced a q->seqlock recently, with this patch > now we have priv->lock, 4 locks in total. So our "lockless" qdisc ends > up having more locks than others. ;) I don't think we are going to a > right direction... Yes, we have 4 locks in total, but lockless qdisc only use two locks in this patch, which are priv->lock and q->seqlock. The qdisc at least uses two locks, which is qdisc_lock(q) and q->busylock, which seems to have bigger contention when concurrent accessing to the same qdisc. If we want to reduce the total number of lock, we can use qdisc_lock(q) for lockless qdisc and remove q->seqlock:) > > Thanks. > > . >