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 8A9DAC433DB for ; Wed, 17 Mar 2021 01:08:01 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 5949764F96 for ; Wed, 17 Mar 2021 01:08:01 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S233409AbhCQBHT (ORCPT ); Tue, 16 Mar 2021 21:07:19 -0400 Received: from szxga02-in.huawei.com ([45.249.212.188]:3488 "EHLO szxga02-in.huawei.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S232818AbhCQBBJ (ORCPT ); Tue, 16 Mar 2021 21:01:09 -0400 Received: from DGGEMM401-HUB.china.huawei.com (unknown [172.30.72.57]) by szxga02-in.huawei.com (SkyGuard) with ESMTP id 4F0WyL2VQJzRNfr; Wed, 17 Mar 2021 08:59:22 +0800 (CST) Received: from dggpemm500005.china.huawei.com (7.185.36.74) by DGGEMM401-HUB.china.huawei.com (10.3.20.209) with Microsoft SMTP Server (TLS) id 14.3.498.0; Wed, 17 Mar 2021 09:01:06 +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 09:01:06 +0800 Subject: Re: [PATCH net-next] net: sched: remove unnecessay lock protection for skb_bad_txq/gso_skb To: Cong Wang CC: David Miller , Jakub Kicinski , Jamal Hadi Salim , Jiri Pirko , "Linux Kernel Network Developers" , LKML , References: <1615800610-34700-1-git-send-email-linyunsheng@huawei.com> From: Yunsheng Lin Message-ID: Date: Wed, 17 Mar 2021 09:01:05 +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: dggeme719-chm.china.huawei.com (10.1.199.115) To dggpemm500005.china.huawei.com (7.185.36.74) X-CFilter-Loop: Reflected Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 2021/3/17 2:41, Cong Wang wrote: > On Mon, Mar 15, 2021 at 2:29 AM Yunsheng Lin wrote: >> >> Currently qdisc_lock(q) is taken before enqueuing and dequeuing >> for lockless qdisc's skb_bad_txq/gso_skb queue, qdisc->seqlock is >> also taken, which can provide the same protection as qdisc_lock(q). >> >> This patch removes the unnecessay qdisc_lock(q) protection for >> lockless qdisc' skb_bad_txq/gso_skb queue. >> >> And dev_reset_queue() takes the qdisc->seqlock for lockless qdisc >> besides taking the qdisc_lock(q) when doing the qdisc reset, >> some_qdisc_is_busy() takes both qdisc->seqlock and qdisc_lock(q) >> when checking qdisc status. It is unnecessary to take both lock >> while the fast path only take one lock, so this patch also changes >> it to only take qdisc_lock(q) for locked qdisc, and only take >> qdisc->seqlock for lockless qdisc. >> >> Since qdisc->seqlock is taken for lockless qdisc when calling >> qdisc_is_running() in some_qdisc_is_busy(), use qdisc->running >> to decide if the lockless qdisc is running. > > What's the benefit here? Since qdisc->q.lock is also per-qdisc, > so there is no actual contention to take it when we already acquire > q->seqlock, right? Yes, there is no actual contention to take qdisc->q.lock while q->seqlock is acquired, but a cleanup or minor optimization. > > Also, is ->seqlock supposed to be used for protecting skb_bad_txq > etc.? From my understanding, it was introduced merely for replacing > __QDISC_STATE_RUNNING. If you want to extend it, you probably > have to rename it too. How about just using qdisc->q.lock for lockless qdisc too and remove dqisc->seqlock completely? > > Thanks. > > . >