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=-2.3 required=3.0 tests=HEADER_FROM_DIFFERENT_DOMAINS, MAILING_LIST_MULTI,SPF_PASS,URIBL_BLOCKED,USER_AGENT_MUTT 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 595DDC43A1D for ; Thu, 12 Jul 2018 02:13:48 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id 12E9D2146F for ; Thu, 12 Jul 2018 02:13:48 +0000 (UTC) DMARC-Filter: OpenDMARC Filter v1.3.2 mail.kernel.org 12E9D2146F Authentication-Results: mail.kernel.org; dmarc=fail (p=none dis=none) header.from=linux.vnet.ibm.com Authentication-Results: mail.kernel.org; spf=none smtp.mailfrom=linux-kernel-owner@vger.kernel.org Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S2403898AbeGLCU5 (ORCPT ); Wed, 11 Jul 2018 22:20:57 -0400 Received: from mx0a-001b2d01.pphosted.com ([148.163.156.1]:46192 "EHLO mx0a-001b2d01.pphosted.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S2403883AbeGLCU5 (ORCPT ); Wed, 11 Jul 2018 22:20:57 -0400 Received: from pps.filterd (m0098399.ppops.net [127.0.0.1]) by mx0a-001b2d01.pphosted.com (8.16.0.22/8.16.0.22) with SMTP id w6C244Xh125313 for ; Wed, 11 Jul 2018 22:13:45 -0400 Received: from e17.ny.us.ibm.com (e17.ny.us.ibm.com [129.33.205.207]) by mx0a-001b2d01.pphosted.com with ESMTP id 2k5teexgj4-1 (version=TLSv1.2 cipher=AES256-GCM-SHA384 bits=256 verify=NOT) for ; Wed, 11 Jul 2018 22:13:44 -0400 Received: from localhost by e17.ny.us.ibm.com with IBM ESMTP SMTP Gateway: Authorized Use Only! Violators will be prosecuted for from ; Wed, 11 Jul 2018 22:13:43 -0400 Received: from b01cxnp23032.gho.pok.ibm.com (9.57.198.27) by e17.ny.us.ibm.com (146.89.104.204) with IBM ESMTP SMTP Gateway: Authorized Use Only! Violators will be prosecuted; (version=TLSv1/SSLv3 cipher=AES256-GCM-SHA384 bits=256/256) Wed, 11 Jul 2018 22:13:41 -0400 Received: from b01ledav003.gho.pok.ibm.com (b01ledav003.gho.pok.ibm.com [9.57.199.108]) by b01cxnp23032.gho.pok.ibm.com (8.14.9/8.14.9/NCO v10.0) with ESMTP id w6C2De7g18153840 (version=TLSv1/SSLv3 cipher=DHE-RSA-AES256-GCM-SHA384 bits=256 verify=FAIL); Thu, 12 Jul 2018 02:13:40 GMT Received: from b01ledav003.gho.pok.ibm.com (unknown [127.0.0.1]) by IMSVA (Postfix) with ESMTP id 1F36BB2064; Wed, 11 Jul 2018 22:13:39 -0400 (EDT) Received: from b01ledav003.gho.pok.ibm.com (unknown [127.0.0.1]) by IMSVA (Postfix) with ESMTP id DDBAAB205F; Wed, 11 Jul 2018 22:13:38 -0400 (EDT) Received: from paulmck-ThinkPad-W541 (unknown [9.85.161.121]) by b01ledav003.gho.pok.ibm.com (Postfix) with ESMTP; Wed, 11 Jul 2018 22:13:38 -0400 (EDT) Received: by paulmck-ThinkPad-W541 (Postfix, from userid 1000) id 06C8916C41DD; Wed, 11 Jul 2018 19:16:00 -0700 (PDT) Date: Wed, 11 Jul 2018 19:15:59 -0700 From: "Paul E. McKenney" To: Christian Borntraeger Cc: David Woodhouse , peterz@infradead.org, mhillenb@amazon.de, linux-kernel@vger.kernel.org, kvm@vger.kernel.org Subject: Re: [PATCH v2] kvm/x86: Inform RCU of quiescent state when entering guest mode Reply-To: paulmck@linux.vnet.ibm.com References: <20180711174843.GX3593@linux.vnet.ibm.com> <20180711180101.3711464-1-dwmw2@infradead.org> <20180711182053.GA3593@linux.vnet.ibm.com> <20180711183645.GA23820@linux.vnet.ibm.com> <62e945ee-a58e-f9b3-279c-74cd0f5809da@de.ibm.com> <20180711202752.GC3593@linux.vnet.ibm.com> <2f7ba67e-0442-13cc-628c-2dca56520c21@de.ibm.com> <20180711213259.GF3593@linux.vnet.ibm.com> <20180711233727.GA9888@linux.vnet.ibm.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20180711233727.GA9888@linux.vnet.ibm.com> User-Agent: Mutt/1.5.21 (2010-09-15) X-TM-AS-GCONF: 00 x-cbid: 18071202-0040-0000-0000-0000044D8CAA X-IBM-SpamModules-Scores: X-IBM-SpamModules-Versions: BY=3.00009354; HX=3.00000241; KW=3.00000007; PH=3.00000004; SC=3.00000266; SDB=6.01060050; UDB=6.00544097; IPR=6.00837970; MB=3.00022109; MTD=3.00000008; XFM=3.00000015; UTC=2018-07-12 02:13:42 X-IBM-AV-DETECTION: SAVI=unused REMOTE=unused XFE=unused x-cbparentid: 18071202-0041-0000-0000-00000853B277 Message-Id: <20180712021559.GA413@linux.vnet.ibm.com> X-Proofpoint-Virus-Version: vendor=fsecure engine=2.50.10434:,, definitions=2018-07-11_07:,, signatures=0 X-Proofpoint-Spam-Details: rule=outbound_notspam policy=outbound score=0 priorityscore=1501 malwarescore=0 suspectscore=0 phishscore=0 bulkscore=0 spamscore=0 clxscore=1015 lowpriorityscore=0 mlxscore=0 impostorscore=0 mlxlogscore=999 adultscore=0 classifier=spam adjust=0 reason=mlx scancount=1 engine=8.0.1-1806210000 definitions=main-1807120022 Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Wed, Jul 11, 2018 at 04:37:27PM -0700, Paul E. McKenney wrote: > On Wed, Jul 11, 2018 at 02:32:59PM -0700, Paul E. McKenney wrote: > > On Wed, Jul 11, 2018 at 11:11:19PM +0200, Christian Borntraeger wrote: > > > > > > > > > On 07/11/2018 10:27 PM, Paul E. McKenney wrote: > > > > On Wed, Jul 11, 2018 at 08:39:36PM +0200, Christian Borntraeger wrote: > > > >> > > > >> > > > >> On 07/11/2018 08:36 PM, Paul E. McKenney wrote: > > > >>> On Wed, Jul 11, 2018 at 11:20:53AM -0700, Paul E. McKenney wrote: > > > >>>> On Wed, Jul 11, 2018 at 07:01:01PM +0100, David Woodhouse wrote: > > > >>>>> From: David Woodhouse > > > >>>>> > > > >>>>> RCU can spend long periods of time waiting for a CPU which is actually in > > > >>>>> KVM guest mode, entirely pointlessly. Treat it like the idle and userspace > > > >>>>> modes, and don't wait for it. > > > >>>>> > > > >>>>> Signed-off-by: David Woodhouse > > > >>>> > > > >>>> And idiot here forgot about some of the debugging code in RCU's dyntick-idle > > > >>>> code. I will reply with a fixed patch. > > > >>>> > > > >>>> The code below works just fine as long as you don't enable CONFIG_RCU_EQS_DEBUG, > > > >>>> so should be OK for testing, just not for mainline. > > > >>> > > > >>> And here is the updated code that allegedly avoids splatting when run with > > > >>> CONFIG_RCU_EQS_DEBUG. > > > >>> > > > >>> Thoughts? > > > >>> > > > >>> Thanx, Paul > > > >>> > > > >>> ------------------------------------------------------------------------ > > > >>> > > > >>> commit 12cd59e49cf734f907f44b696e2c6e4b46a291c3 > > > >>> Author: David Woodhouse > > > >>> Date: Wed Jul 11 19:01:01 2018 +0100 > > > >>> > > > >>> kvm/x86: Inform RCU of quiescent state when entering guest mode > > > >>> > > > >>> RCU can spend long periods of time waiting for a CPU which is actually in > > > >>> KVM guest mode, entirely pointlessly. Treat it like the idle and userspace > > > >>> modes, and don't wait for it. > > > >>> > > > >>> Signed-off-by: David Woodhouse > > > >>> Signed-off-by: Paul E. McKenney > > > >>> [ paulmck: Adjust to avoid bad advice I gave to dwmw, avoid WARN_ON()s. ] > > > >>> > > > >>> diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c > > > >>> index 0046aa70205a..b0c82f70afa7 100644 > > > >>> --- a/arch/x86/kvm/x86.c > > > >>> +++ b/arch/x86/kvm/x86.c > > > >>> @@ -7458,7 +7458,9 @@ static int vcpu_enter_guest(struct kvm_vcpu *vcpu) > > > >>> vcpu->arch.switch_db_regs &= ~KVM_DEBUGREG_RELOAD; > > > >>> } > > > >>> > > > >>> + rcu_kvm_enter(); > > > >>> kvm_x86_ops->run(vcpu); > > > >>> + rcu_kvm_exit(); > > > >> > > > >> As indicated in my other mail. This is supposed to be handled in the guest_enter|exit_ calls around > > > >> the run function. This would also handle other architectures. So if the guest_enter_irqoff code is > > > >> not good enough, we should rather fix that instead of adding another rcu hint. > > > > > > > > Something like this, on top of the earlier patch? I am not at all > > > > confident of this patch because there might be other entry/exit > > > > paths I am missing. Plus there might be RCU uses on the arch-specific > > > > patch to and from the guest OS. > > > > > > > > Thoughts? > > > > > > > > > > If you instrment guest_enter/exit, you should cover all cases and all architectures as far > > > as I can tell. FWIW, we did this rcu_note thing back then actually handling this particular > > > case of long running guests blocking rcu for many seconds. And I am pretty sure that > > > this did help back then. > > > > And my second patch on the email you replied to replaced the only call > > to rcu_virt_note_context_switch(). So maybe it covers what it needs to, > > but yes, there might well be things I missed. Let's see what David > > comes up with. > > > > What changed was RCU's reactions to longish grace periods. It used to > > be very aggressive about forcing the scheduler to do otherwise-unneeded > > context switches, which became a problem somewhere between v4.9 and v4.15. > > I therefore reduced the number of such context switches, which in turn > > caused KVM to tell RCU about quiescent states way too infrequently. > > > > The advantage of the rcu_kvm_enter()/rcu_kvm_exit() approach is that > > it tells RCU of an extended duration in the guest, which means that > > RCU can ignore the corresponding CPU, which in turn allows the guest > > to proceed without any RCU-induced interruptions. > > > > Does that make sense, or am I missing something? I freely admit to > > much ignorance of both kvm and s390! ;-) > > But I am getting some rcutorture near misses on the commit that > introduces rcu_kvm_enter() and rcu_kvm_exit() to the x86 arch-specific > vcpu_enter_guest() function. These near misses occur when running > rcutorture scenarios TREE01 and TREE03, and in my -rcu tree rather > than the v4.15 version of this patch. > > Given that I am making pervasive changes to the way that RCU works, > it might well be that this commit is an innocent bystander. I will > run tests overnight and let you know what comes up. And thus far, the verdict is "intermittent". I am starting a new series with CONFIG_RCU_EQS_DEBUG=y, which should detect any mismatched rcu_kvm_enter()/rcu_kvm_exit() pairs. Thanx, Paul