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=-10.9 required=3.0 tests=BAYES_00, HEADER_FROM_DIFFERENT_DOMAINS,INCLUDES_PATCH,MAILING_LIST_MULTI,NICE_REPLY_A, SPF_HELO_NONE,SPF_PASS,USER_AGENT_SANE_1 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 35155C47092 for ; Wed, 2 Jun 2021 01:21:07 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 19C07613D6 for ; Wed, 2 Jun 2021 01:21:07 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S229966AbhFBBWs (ORCPT ); Tue, 1 Jun 2021 21:22:48 -0400 Received: from szxga02-in.huawei.com ([45.249.212.188]:2939 "EHLO szxga02-in.huawei.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S229851AbhFBBWq (ORCPT ); Tue, 1 Jun 2021 21:22:46 -0400 Received: from dggemv711-chm.china.huawei.com (unknown [172.30.72.55]) by szxga02-in.huawei.com (SkyGuard) with ESMTP id 4FvrkN66GSz68yG; Wed, 2 Jun 2021 09:18:04 +0800 (CST) Received: from dggpemm500005.china.huawei.com (7.185.36.74) by dggemv711-chm.china.huawei.com (10.1.198.66) with Microsoft SMTP Server (version=TLS1_2, cipher=TLS_ECDHE_RSA_WITH_AES_128_GCM_SHA256) id 15.1.2176.2; Wed, 2 Jun 2021 09:21:02 +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; Wed, 2 Jun 2021 09:21:02 +0800 Subject: Re: [Linuxarm] Re: [PATCH net-next 2/3] net: sched: implement TCQ_F_CAN_BYPASS for lockless qdisc To: Jakub Kicinski CC: Yunsheng Lin , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , 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> <3a307707-9fb5-d73a-01f9-93aaf5c7a437@huawei.com> <428f92d8-f4a2-13cf-8dcc-b38d48a42965@huawei.com> <20210531215146.5ca802a5@kicinski-fedora-PC1C0HJN.hsd1.ca.comcast.net> <20210601134856.12573333@kicinski-fedora-PC1C0HJN.hsd1.ca.comcast.net> From: Yunsheng Lin Message-ID: <20e9bf35-444c-8c35-97ec-de434fc80d73@huawei.com> Date: Wed, 2 Jun 2021 09:21:01 +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: <20210601134856.12573333@kicinski-fedora-PC1C0HJN.hsd1.ca.comcast.net> Content-Type: text/plain; charset="utf-8" Content-Language: en-US Content-Transfer-Encoding: 7bit X-Originating-IP: [10.69.30.204] X-ClientProxiedBy: dggeme710-chm.china.huawei.com (10.1.199.106) 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 2021/6/2 4:48, Jakub Kicinski wrote: > On Tue, 1 Jun 2021 16:18:54 +0800 Yunsheng Lin wrote: >>> I see, thanks! That explains the need. Perhaps we can rephrase the >>> comment? Maybe: >>> >>> + /* Retest nolock_qdisc_is_empty() within the protection >>> + * of q->seqlock to protect from racing with requeuing. >>> + */ >> >> Yes if we still decide to preserve the nolock_qdisc_is_empty() rechecking >> under q->seqlock. > > Sounds good. > >>>> --- a/net/sched/sch_generic.c >>>> +++ b/net/sched/sch_generic.c >>>> @@ -38,6 +38,15 @@ EXPORT_SYMBOL(default_qdisc_ops); >>>> static void qdisc_maybe_clear_missed(struct Qdisc *q, >>>> const struct netdev_queue *txq) >>>> { >>>> + set_bit(__QDISC_STATE_DRAINING, &q->state); >>>> + >>>> + /* Make sure DRAINING is set before clearing MISSED >>>> + * to make sure nolock_qdisc_is_empty() always return >>>> + * false for aoviding transmitting a packet directly >>>> + * bypassing the requeued packet. >>>> + */ >>>> + smp_mb__after_atomic(); >>>> + >>>> clear_bit(__QDISC_STATE_MISSED, &q->state); >>>> >>>> /* Make sure the below netif_xmit_frozen_or_stopped() >>>> @@ -52,8 +61,6 @@ static void qdisc_maybe_clear_missed(struct Qdisc *q, >>>> */ >>>> if (!netif_xmit_frozen_or_stopped(txq)) >>>> set_bit(__QDISC_STATE_MISSED, &q->state); >>>> - else >>>> - set_bit(__QDISC_STATE_DRAINING, &q->state); >>>> } >>> >>> But this would not be enough because we may also clear MISSING >>> in pfifo_fast_dequeue()? >> >> For the MISSING clearing in pfifo_fast_dequeue(), it seems it >> looks like the data race described in RFC v3 too? >> >> CPU1 CPU2 CPU3 >> qdisc_run_begin(q) . . >> . MISSED is set . >> MISSED is cleared . . >> q->dequeue() . . >> . enqueue skb1 check MISSED # true >> qdisc_run_end(q) . . >> . . qdisc_run_begin(q) # true >> . MISSED is set send skb2 directly > > Not sure what you mean. CPU1 CPU2 CPU3 qdisc_run_begin(q) . . . MISSED is set . MISSED is cleared . . another dequeuing . . . . . . enqueue skb1 nolock_qdisc_is_empty() # true qdisc_run_end(q) . . . . qdisc_run_begin(q) # true . . send skb2 directly . MISSED is set . As qdisc is indeed empty at the point when MISSED is clear and another dequeue is retried by CPU1, MISSED setting is not under q->seqlock, so it seems retesting MISSED under q->seqlock does not seem to make any difference? and it seems like the case that does not need handling as we agreed previously? > > . >