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=-7.0 required=3.0 tests=HEADER_FROM_DIFFERENT_DOMAINS, INCLUDES_PATCH,MAILING_LIST_MULTI,SIGNED_OFF_BY,SPF_HELO_NONE,SPF_PASS, URIBL_BLOCKED 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 64A0FC28CC5 for ; Mon, 3 Jun 2019 02:11:40 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id 41A9C27AC9 for ; Mon, 3 Jun 2019 02:11:40 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1727274AbfFCCLj (ORCPT ); Sun, 2 Jun 2019 22:11:39 -0400 Received: from szxga07-in.huawei.com ([45.249.212.35]:38538 "EHLO huawei.com" rhost-flags-OK-OK-OK-FAIL) by vger.kernel.org with ESMTP id S1726941AbfFCCLi (ORCPT ); Sun, 2 Jun 2019 22:11:38 -0400 Received: from DGGEMS407-HUB.china.huawei.com (unknown [172.30.72.58]) by Forcepoint Email with ESMTP id D78CB515A48FE1F0898B; Mon, 3 Jun 2019 10:11:35 +0800 (CST) Received: from [127.0.0.1] (10.74.191.121) by DGGEMS407-HUB.china.huawei.com (10.3.19.207) with Microsoft SMTP Server id 14.3.439.0; Mon, 3 Jun 2019 10:11:29 +0800 Subject: Re: [PATCH v2 net-next] net: link_watch: prevent starvation when processing linkwatch wq To: Salil Mehta , "davem@davemloft.net" CC: "hkallweit1@gmail.com" , "f.fainelli@gmail.com" , "stephen@networkplumber.org" , "netdev@vger.kernel.org" , "linux-kernel@vger.kernel.org" , Linuxarm References: <1559293233-43017-1-git-send-email-linyunsheng@huawei.com> <8a93eecf7a7a4ffd81f1b7d08f1a7442@huawei.com> From: Yunsheng Lin Message-ID: <58960b44-63bb-21da-b995-2b9701a58126@huawei.com> Date: Mon, 3 Jun 2019 10:11:28 +0800 User-Agent: Mozilla/5.0 (Windows NT 6.1; WOW64; rv:52.0) Gecko/20100101 Thunderbird/52.2.0 MIME-Version: 1.0 In-Reply-To: <8a93eecf7a7a4ffd81f1b7d08f1a7442@huawei.com> Content-Type: text/plain; charset="utf-8" Content-Language: en-US Content-Transfer-Encoding: 7bit X-Originating-IP: [10.74.191.121] X-CFilter-Loop: Reflected Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 2019/5/31 19:17, Salil Mehta wrote: >> From: netdev-owner@vger.kernel.org [mailto:netdev- >> owner@vger.kernel.org] On Behalf Of Yunsheng Lin >> Sent: Friday, May 31, 2019 10:01 AM >> To: davem@davemloft.net >> Cc: hkallweit1@gmail.com; f.fainelli@gmail.com; >> stephen@networkplumber.org; netdev@vger.kernel.org; linux- >> kernel@vger.kernel.org; Linuxarm >> Subject: [PATCH v2 net-next] net: link_watch: prevent starvation when >> processing linkwatch wq >> >> When user has configured a large number of virtual netdev, such >> as 4K vlans, the carrier on/off operation of the real netdev >> will also cause it's virtual netdev's link state to be processed >> in linkwatch. Currently, the processing is done in a work queue, >> which may cause cpu and rtnl locking starvation problem. >> >> This patch releases the cpu and rtnl lock when link watch worker >> has processed a fixed number of netdev' link watch event. >> >> Currently __linkwatch_run_queue is called with rtnl lock, so >> enfore it with ASSERT_RTNL(); >> >> Signed-off-by: Yunsheng Lin >> --- >> V2: use cond_resched and rtnl_unlock after processing a fixed >> number of events >> --- >> net/core/link_watch.c | 17 +++++++++++++++++ >> 1 file changed, 17 insertions(+) >> >> diff --git a/net/core/link_watch.c b/net/core/link_watch.c >> index 7f51efb..07eebfb 100644 >> --- a/net/core/link_watch.c >> +++ b/net/core/link_watch.c >> @@ -168,9 +168,18 @@ static void linkwatch_do_dev(struct net_device >> *dev) >> >> static void __linkwatch_run_queue(int urgent_only) >> { >> +#define MAX_DO_DEV_PER_LOOP 100 >> + >> + int do_dev = MAX_DO_DEV_PER_LOOP; >> struct net_device *dev; >> LIST_HEAD(wrk); >> >> + ASSERT_RTNL(); >> + >> + /* Give urgent case more budget */ >> + if (urgent_only) >> + do_dev += MAX_DO_DEV_PER_LOOP; >> + >> /* >> * Limit the number of linkwatch events to one >> * per second so that a runaway driver does not >> @@ -200,6 +209,14 @@ static void __linkwatch_run_queue(int urgent_only) >> } >> spin_unlock_irq(&lweventlist_lock); >> linkwatch_do_dev(dev); >> + >> + if (--do_dev < 0) { >> + rtnl_unlock(); >> + cond_resched(); > > > > Sorry, missed in my earlier comment. I could see multiple problems here > and please correct me if I am wrong: > > 1. It looks like releasing the rtnl_lock here and then res-scheduling might > not be safe, especially when you have already held *lweventlist_lock* > (which is global and not per-netdev), and when you are trying to > reschedule. This can cause *deadlock* with itself. > > Reason: once you release the rtnl_lock() the similar leg of function > netdev_wait_allrefs() could be called for some other netdevice which > might end up in waiting for same global linkwatch event list lock > i.e. *lweventlist_lock*. lweventlist_lock has been released before releasing the rtnl_lock and rescheduling. > > 2. After releasing the rtnl_lock() we have not ensured that all the rcu > operations are complete. Perhaps we need to take rcu_barrier() before > retaking the rtnl_lock() Why do we need to ensure all the rcu operations are complete here? > > > > >> + do_dev = MAX_DO_DEV_PER_LOOP; > > > > Here, I think rcu_barrier() should exist. In netdev_wait_allrefs, rcu_barrier is indeed called between __rtnl_unlock and rtnl_lock and is added by below commit 0115e8e30d6f ("net: remove delay at device dismantle"), which seems to work with NETDEV_UNREGISTER_FINAL. And the NETDEV_UNREGISTER_FINAL is removed by commit 070f2d7e264a ("net: Drop NETDEV_UNREGISTER_FINAL"), which says something about whether the rcu_barrier is still needed. "dev_change_net_namespace() and netdev_wait_allrefs() have rcu_barrier() before NETDEV_UNREGISTER_FINAL call, and the source commits say they were introduced to delemit the call with NETDEV_UNREGISTER, but this patch leaves them on the places, since they require additional analysis, whether we need in them for something else." So the reason of calling rcu_barrier in netdev_wait_allrefs is unclear now. Also rcu_barrier in netdev_wait_allrefs is added to fix the device dismantle problem, so for linkwatch, maybe it is not needed. > > > >> + rtnl_lock(); >> + } >> + >> spin_lock_irq(&lweventlist_lock); >> } > > > . >