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=-10.8 required=3.0 tests=BAYES_00, HEADER_FROM_DIFFERENT_DOMAINS,INCLUDES_PATCH,MAILING_LIST_MULTI,NICE_REPLY_A, SPF_HELO_NONE,SPF_PASS,URIBL_BLOCKED,USER_AGENT_SANE_1 autolearn=ham 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 E916FC47080 for ; Mon, 31 May 2021 12:40:19 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id BECF46135C for ; Mon, 31 May 2021 12:40:19 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S231521AbhEaMl6 (ORCPT ); Mon, 31 May 2021 08:41:58 -0400 Received: from szxga03-in.huawei.com ([45.249.212.189]:2418 "EHLO szxga03-in.huawei.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S231327AbhEaMl6 (ORCPT ); Mon, 31 May 2021 08:41:58 -0400 Received: from dggemv711-chm.china.huawei.com (unknown [172.30.72.53]) by szxga03-in.huawei.com (SkyGuard) with ESMTP id 4Ftvt91Mq0z67Q8; Mon, 31 May 2021 20:36:33 +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; Mon, 31 May 2021 20:40:03 +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 20:40:02 +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> <3a307707-9fb5-d73a-01f9-93aaf5c7a437@huawei.com> Message-ID: <428f92d8-f4a2-13cf-8dcc-b38d48a42965@huawei.com> Date: Mon, 31 May 2021 20:40:01 +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: <3a307707-9fb5-d73a-01f9-93aaf5c7a437@huawei.com> Content-Type: text/plain; charset="utf-8" Content-Language: en-US Content-Transfer-Encoding: 8bit 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: linux-can@vger.kernel.org On 2021/5/31 9:10, Yunsheng Lin wrote: > 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? I had did some interesting testing to show how adjust a small number of code has some notiable performance degrade. 1. I used below patch to remove the nolock_qdisc_is_empty() testing under q->seqlock. @@ -3763,17 +3763,6 @@ static inline int __dev_xmit_skb(struct sk_buff *skb, struct Qdisc *q, if (q->flags & TCQ_F_NOLOCK) { if (q->flags & TCQ_F_CAN_BYPASS && nolock_qdisc_is_empty(q) && qdisc_run_begin(q)) { - /* Retest nolock_qdisc_is_empty() within the protection - * of q->seqlock to ensure qdisc is indeed empty. - */ - if (unlikely(!nolock_qdisc_is_empty(q))) { - rc = q->enqueue(skb, q, &to_free) & NET_XMIT_MASK; - __qdisc_run(q); - qdisc_run_end(q); - - goto no_lock_out; - } - qdisc_bstats_cpu_update(q, skb); if (sch_direct_xmit(skb, q, dev, txq, NULL, true) && !nolock_qdisc_is_empty(q)) @@ -3786,7 +3775,6 @@ static inline int __dev_xmit_skb(struct sk_buff *skb, struct Qdisc *q, rc = q->enqueue(skb, q, &to_free) & NET_XMIT_MASK; qdisc_run(q); -no_lock_out: if (unlikely(to_free)) kfree_skb_list(to_free); return rc; which has the below performance improvement: threads v1 v1 + above patch delta 1 3.21Mpps 3.20Mpps -0.3% 2 5.56Mpps 5.94Mpps +4.9% 4 5.58Mpps 5.60Mpps +0.3% 8 2.76Mpps 2.77Mpps +0.3% 16 2.23Mpps 2.23Mpps +0.0% v1 = this patchset. 2. After the above testing, it seems worthwhile to remove the nolock_qdisc_is_empty() testing under q->seqlock, so I used below patch to make sure nolock_qdisc_is_empty() always return false for netdev queue stopped case。 --- 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); } which has the below performance data: threads v1 v1 + above two patch delta 1 3.21Mpps 3.20Mpps -0.3% 2 5.56Mpps 5.94Mpps +4.9% 4 5.58Mpps 5.02Mpps -10% 8 2.76Mpps 2.77Mpps +0.3% 16 2.23Mpps 2.23Mpps +0.0% So the adjustment in qdisc_maybe_clear_missed() seems to have caused about 10% performance degradation for 4 threads case. And the cpu topdown perf data suggested that icache missed and bad Speculation play the main factor to those performance difference. I tried to control the above factor by removing the inline function and add likely and unlikely tag for netif_xmit_frozen_or_stopped() in sch_generic.c. And after removing the inline mark for function in sch_generic.c and add likely/unlikely tag for netif_xmit_frozen_or_stopped() checking in in sch_generic.c, we got notiable performance improvement for 1/2 threads case(some performance improvement for ip forwarding test too), but not for 4 threads case. So it seems we need to ignore the performance degradation for 4 threads case? or any idea? >>> >>> Sorry for dragging the review out.. >>> >>> . >>> >> _______________________________________________ >> Linuxarm mailing list -- linuxarm@openeuler.org >> To unsubscribe send an email to linuxarm-leave@openeuler.org >> > _______________________________________________ > Linuxarm mailing list -- linuxarm@openeuler.org > To unsubscribe send an email to linuxarm-leave@openeuler.org >