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,URIBL_BLOCKED,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 92A1AC433DB for ; Mon, 22 Mar 2021 00:56:08 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 579CD61930 for ; Mon, 22 Mar 2021 00:56:08 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S230015AbhCVAzi (ORCPT ); Sun, 21 Mar 2021 20:55:38 -0400 Received: from szxga01-in.huawei.com ([45.249.212.187]:5099 "EHLO szxga01-in.huawei.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S229979AbhCVAzM (ORCPT ); Sun, 21 Mar 2021 20:55:12 -0400 Received: from dggeml406-hub.china.huawei.com (unknown [172.30.72.55]) by szxga01-in.huawei.com (SkyGuard) with ESMTP id 4F3bb62HRPzYMlt; Mon, 22 Mar 2021 08:53:22 +0800 (CST) Received: from dggpemm500005.china.huawei.com (7.185.36.74) by dggeml406-hub.china.huawei.com (10.3.17.50) with Microsoft SMTP Server (TLS) id 14.3.498.0; Mon, 22 Mar 2021 08:55:09 +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; Mon, 22 Mar 2021 08:55:09 +0800 Subject: Re: [Linuxarm] Re: [RFC v2] net: sched: implement TCQ_F_CAN_BYPASS for lockless qdisc To: Cong Wang CC: "Jason A. Donenfeld" , =?UTF-8?Q?Toke_H=c3=b8iland-J=c3=b8rgensen?= , "Jakub Kicinski" , 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> <87eegddhsj.fsf@toke.dk> <3bae7b26-9d7f-15b8-d466-ff5c26d08b35@huawei.com> From: Yunsheng Lin Message-ID: <9d045462-051e-0cde-24d0-349dd397e2b7@huawei.com> Date: Mon, 22 Mar 2021 08:55:09 +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: 8bit X-Originating-IP: [10.69.30.204] X-ClientProxiedBy: dggeme701-chm.china.huawei.com (10.1.199.97) To dggpemm500005.china.huawei.com (7.185.36.74) X-CFilter-Loop: Reflected Precedence: bulk List-ID: X-Mailing-List: netdev@vger.kernel.org On 2021/3/20 2:15, Cong Wang wrote: > On Thu, Mar 18, 2021 at 12:33 AM Yunsheng Lin wrote: >> >> On 2021/3/17 21:45, Jason A. Donenfeld wrote: >>> On 3/17/21, Toke Høiland-Jørgensen wrote: >>>> Cong Wang writes: >>>> >>>>> 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... >>>> >>>> Just a thought, have you guys considered adopting the lockless MSPC ring >>>> buffer recently introduced into Wireguard in commit: >>>> >>>> 8b5553ace83c ("wireguard: queueing: get rid of per-peer ring buffers") >>>> >>>> Jason indicated he was willing to work on generalising it into a >>>> reusable library if there was a use case for it. I haven't quite though >>>> through the details of whether this would be such a use case, but >>>> figured I'd at least mention it :) >>> >>> That offer definitely still stands. Generalization sounds like a lot of fun. >>> >>> Keep in mind though that it's an eventually consistent queue, not an >>> immediately consistent one, so that might not match all use cases. It >>> works with wg because we always trigger the reader thread anew when it >>> finishes, but that doesn't apply to everyone's queueing setup. >> >> Thanks for mentioning this. >> >> "multi-producer, single-consumer" seems to match the lockless qdisc's >> paradigm too, for now concurrent enqueuing/dequeuing to the pfifo_fast's >> queues() is not allowed, it is protected by producer_lock or consumer_lock. >> >> So it would be good to has lockless concurrent enqueuing, while dequeuing >> can be protected by qdisc_lock() or q->seqlock, which meets the "multi-producer, >> single-consumer" paradigm. > > I don't think so. Usually we have one queue for each CPU so we can expect > each CPU has a lockless qdisc assigned, but we can not assume this in > the code, so we still have to deal with multiple CPU's sharing a lockless qdisc, > and we usually enqueue and dequeue in process context, so it means we could > have multiple producers and multiple consumers. For lockless qdisc, dequeuing is always within the qdisc_run_begin() and qdisc_run_end(), so multiple consumers is protected with each other by q->seqlock . For enqueuing, multiple consumers is protected by producer_lock, see pfifo_fast_enqueue() -> skb_array_produce() -> ptr_ring_produce(). I am not sure if lockless MSPC can work with the process context, but even if not, the enqueuing is also protected by rcu_read_lock_bh(), which provides some kind of atomicity, so that producer_lock can be reomved when lockless MSPC is used. > > On the other hand, I don't think the problems we have been fixing are the ring > buffer implementation itself, they are about the high-level qdisc > state transitions. > > Thanks. > > . >