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=-5.9 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 6DC3AC47094 for ; Mon, 31 May 2021 01:10:29 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 541B761278 for ; Mon, 31 May 2021 01:10:29 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S230036AbhEaBMG (ORCPT ); Sun, 30 May 2021 21:12:06 -0400 Received: from szxga03-in.huawei.com ([45.249.212.189]:2413 "EHLO szxga03-in.huawei.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S230031AbhEaBMC (ORCPT ); Sun, 30 May 2021 21:12:02 -0400 Received: from dggemv703-chm.china.huawei.com (unknown [172.30.72.54]) by szxga03-in.huawei.com (SkyGuard) with ESMTP id 4FtcZ73RDtz66wN; Mon, 31 May 2021 09:06:39 +0800 (CST) Received: from dggpemm500005.china.huawei.com (7.185.36.74) by dggemv703-chm.china.huawei.com (10.3.19.46) with Microsoft SMTP Server (version=TLS1_2, cipher=TLS_ECDHE_RSA_WITH_AES_128_GCM_SHA256) id 15.1.2176.2; Mon, 31 May 2021 09:10:20 +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 15.1.2176.2; Mon, 31 May 2021 09:10:20 +0800 Subject: Re: [Linuxarm] Re: [PATCH net-next 2/3] net: sched: implement TCQ_F_CAN_BYPASS for lockless qdisc From: Yunsheng Lin To: Jakub Kicinski , Yunsheng Lin CC: , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , References: <1622170197-27370-1-git-send-email-linyunsheng@huawei.com> <1622170197-27370-3-git-send-email-linyunsheng@huawei.com> <20210528180012.676797d6@kicinski-fedora-PC1C0HJN.hsd1.ca.comcast.net> <20210528213218.2b90864c@kicinski-fedora-PC1C0HJN.hsd1.ca.comcast.net> <20210529114919.4f8b1980@kicinski-fedora-PC1C0HJN.hsd1.ca.comcast.net> <9cc9f513-7655-07df-3c74-5abe07ae8321@gmail.com> <20210530132111.3a974275@kicinski-fedora-PC1C0HJN.hsd1.ca.comcast.net> <3c2fbc70-841f-d90b-ca13-1f058169be50@huawei.com> Message-ID: <3a307707-9fb5-d73a-01f9-93aaf5c7a437@huawei.com> Date: Mon, 31 May 2021 09:10:19 +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: <3c2fbc70-841f-d90b-ca13-1f058169be50@huawei.com> Content-Type: text/plain; charset="utf-8" Content-Language: en-US Content-Transfer-Encoding: 7bit X-Originating-IP: [10.69.30.204] X-ClientProxiedBy: dggeme707-chm.china.huawei.com (10.1.199.103) 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 2021/5/31 8:40, Yunsheng Lin wrote: > On 2021/5/31 4:21, Jakub Kicinski wrote: >> On Sun, 30 May 2021 09:37:09 +0800 Yunsheng Lin wrote: >>> On 2021/5/30 2:49, Jakub Kicinski wrote: >>>> The fact that MISSED is only cleared under q->seqlock does not matter, >>>> because setting it and ->enqueue() are not under any lock. If the thread >>>> gets interrupted between: >>>> >>>> if (q->flags & TCQ_F_CAN_BYPASS && nolock_qdisc_is_empty(q) && >>>> qdisc_run_begin(q)) { >>>> >>>> and ->enqueue() we can't guarantee that something else won't come in, >>>> take q->seqlock and clear MISSED. >>>> >>>> thread1 thread2 thread3 >>>> # holds seqlock >>>> qdisc_run_begin(q) >>>> set(MISSED) >>>> pfifo_fast_dequeue >>>> clear(MISSED) >>>> # recheck the queue >>>> qdisc_run_end() >>>> ->enqueue() >>>> q->flags & TCQ_F_CAN_BYPASS.. >>>> qdisc_run_begin() # true >>>> sch_direct_xmit() >>>> qdisc_run_begin() >>>> set(MISSED) >>>> >>>> Or am I missing something? >>>> >>>> Re-checking nolock_qdisc_is_empty() may or may not help. >>>> But it doesn't really matter because there is no ordering >>>> requirement between thread2 and thread3 here. >>> >>> I were more focued on explaining that using MISSED is reliable >>> as sch_may_need_requeuing() checking in RFCv3 [1] to indicate a >>> empty qdisc, and forgot to mention the data race described in >>> RFCv3, which is kind of like the one described above: >>> >>> "There is a data race as below: >>> >>> CPU1 CPU2 >>> qdisc_run_begin(q) . >>> . q->enqueue() >>> sch_may_need_requeuing() . >>> return true . >>> . . >>> . . >>> q->enqueue() . >>> >>> When above happen, the skb enqueued by CPU1 is dequeued after the >>> skb enqueued by CPU2 because sch_may_need_requeuing() return true. >>> If there is not qdisc bypass, the CPU1 has better chance to queue >>> the skb quicker than CPU2. >>> >>> This patch does not take care of the above data race, because I >>> view this as similar as below: >>> >>> Even at the same time CPU1 and CPU2 write the skb to two socket >>> which both heading to the same qdisc, there is no guarantee that >>> which skb will hit the qdisc first, becuase there is a lot of >>> factor like interrupt/softirq/cache miss/scheduling afffecting >>> that." >>> >>> Does above make sense? Or any idea to avoid it? >>> >>> 1. https://patchwork.kernel.org/project/netdevbpf/patch/1616404156-11772-1-git-send-email-linyunsheng@huawei.com/ >> >> We agree on this one. >> >> Could you draw a sequence diagram of different CPUs (like the one >> above) for the case where removing re-checking nolock_qdisc_is_empty() >> under q->seqlock leads to incorrect behavior? > > When nolock_qdisc_is_empty() is not re-checking under q->seqlock, we > may have: > > > CPU1 CPU2 > qdisc_run_begin(q) . > . enqueue skb1 > deuqueue skb1 and clear MISSED . > . nolock_qdisc_is_empty() return true > requeue skb . > q->enqueue() . > set MISSED . > . . > qdisc_run_end(q) . > . qdisc_run_begin(q) > . transmit skb2 directly > . transmit the requeued skb1 > > The problem here is that skb1 and skb2 are from the same CPU, which > means they are likely from the same flow, so we need to avoid this, > right? CPU1 CPU2 qdisc_run_begin(q) . . enqueue skb1 dequeue skb1 . . . netdevice stopped and MISSED is clear . . nolock_qdisc_is_empty() return true requeue skb . . . . . . . qdisc_run_end(q) . . qdisc_run_begin(q) . transmit skb2 directly . transmit the requeued skb1 The above sequence diagram seems more correct, it is basically about how to avoid transmitting a packet directly bypassing the requeued packet. > >> >> If there is no such case would you be willing to repeat the benchmark >> with and without this test? >> >> Sorry for dragging the review out.. >> >> . >> > _______________________________________________ > Linuxarm mailing list -- linuxarm@openeuler.org > To unsubscribe send an email to linuxarm-leave@openeuler.org >