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=-5.2 required=3.0 tests=HEADER_FROM_DIFFERENT_DOMAINS, MAILING_LIST_MULTI,SIGNED_OFF_BY,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 6381FC432C0 for ; Mon, 2 Dec 2019 10:47:30 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id 44EA6214AF for ; Mon, 2 Dec 2019 10:47:30 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1727332AbfLBKr0 (ORCPT ); Mon, 2 Dec 2019 05:47:26 -0500 Received: from szxga07-in.huawei.com ([45.249.212.35]:54290 "EHLO huawei.com" rhost-flags-OK-OK-OK-FAIL) by vger.kernel.org with ESMTP id S1726276AbfLBKr0 (ORCPT ); Mon, 2 Dec 2019 05:47:26 -0500 Received: from DGGEMS408-HUB.china.huawei.com (unknown [172.30.72.59]) by Forcepoint Email with ESMTP id 1FDA555B4F6FC4535465; Mon, 2 Dec 2019 18:47:23 +0800 (CST) Received: from [127.0.0.1] (10.177.246.209) by DGGEMS408-HUB.china.huawei.com (10.3.19.208) with Microsoft SMTP Server id 14.3.439.0; Mon, 2 Dec 2019 18:47:12 +0800 Subject: Re: vfio_pin_map_dma cause synchronize_sched wait too long To: Paolo Bonzini , Alex Williamson CC: , , , "Longpeng(Mike)" , Gonglei , Huangzhichao References: <2e53a9f0-3225-d416-98ff-55bd337330bc@huawei.com> <34c53520-4144-fe71-528a-8df53e7f4dd1@redhat.com> <561fb205-16be-ae87-9cfe-61e6a3b04dc5@huawei.com> <42c907fd-6c09-fbb6-d166-60e6827edff5@redhat.com> From: "Longpeng (Mike)" Message-ID: <8a14a25d-61ce-5fb3-edc2-2c69b18f8e36@huawei.com> Date: Mon, 2 Dec 2019 18:47:11 +0800 User-Agent: Mozilla/5.0 (Windows NT 6.1; WOW64; rv:60.0) Gecko/20100101 Thunderbird/60.7.2 MIME-Version: 1.0 In-Reply-To: <42c907fd-6c09-fbb6-d166-60e6827edff5@redhat.com> Content-Type: text/plain; charset="gbk" Content-Transfer-Encoding: 8bit X-Originating-IP: [10.177.246.209] X-CFilter-Loop: Reflected Sender: kvm-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: kvm@vger.kernel.org ÔÚ 2019/12/2 18:06, Paolo Bonzini дµÀ: > On 02/12/19 10:42, Longpeng (Mike) wrote: >>> cond_resched in vfio_iommu_map. Perhaps you could add one to >>> vfio_pin_pages_remote and/or use vfio_pgsize_bitmap to cap the >>> number of pages that it returns. >> Um ... There's only one running task (qemu-kvm of the VM1) on that >> CPU, so maybe the cond_resched() is ineffective ? > > Note that synchronize_sched() these days is just a synonym of > synchronize_rcu, so this makes me wonder if you're running on an older > kernel and whether you are missing this commit: > Yep. I'm running on an older kernel and I've missed this patchset. Thanks a lot :) > > commit 92aa39e9dc77481b90cbef25e547d66cab901496 > Author: Paul E. McKenney > Date: Mon Jul 9 13:47:30 2018 -0700 > > rcu: Make need_resched() respond to urgent RCU-QS needs > > The per-CPU rcu_dynticks.rcu_urgent_qs variable communicates an urgent > need for an RCU quiescent state from the force-quiescent-state > processing > within the grace-period kthread to context switches and to > cond_resched(). > Unfortunately, such urgent needs are not communicated to need_resched(), > which is sometimes used to decide when to invoke cond_resched(), for > but one example, within the KVM vcpu_run() function. As of v4.15, this > can result in synchronize_sched() being delayed by up to ten seconds, > which can be problematic, to say nothing of annoying. > > This commit therefore checks rcu_dynticks.rcu_urgent_qs from within > rcu_check_callbacks(), which is invoked from the scheduling-clock > interrupt handler. If the current task is not an idle task and is > not executing in usermode, a context switch is forced, and either way, > the rcu_dynticks.rcu_urgent_qs variable is set to false. If the current > task is an idle task, then RCU's dyntick-idle code will detect the > quiescent state, so no further action is required. Similarly, if the > task is executing in usermode, other code in rcu_check_callbacks() and > its called functions will report the corresponding quiescent state. > > Reported-by: Marius Hillenbrand > Reported-by: David Woodhouse > Suggested-by: Peter Zijlstra > Signed-off-by: Paul E. McKenney > > > Thanks, > > Paolo > > > . > -- Regards, Longpeng(Mike)