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 76042C43A1D for ; Wed, 11 Jul 2018 23:45:09 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id 29E3020C10 for ; Wed, 11 Jul 2018 23:45:09 +0000 (UTC) DMARC-Filter: OpenDMARC Filter v1.3.2 mail.kernel.org 29E3020C10 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 S2390852AbeGKXvt (ORCPT ); Wed, 11 Jul 2018 19:51:49 -0400 Received: from mx0a-001b2d01.pphosted.com ([148.163.156.1]:58444 "EHLO mx0a-001b2d01.pphosted.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S2388150AbeGKXvt (ORCPT ); Wed, 11 Jul 2018 19:51:49 -0400 Received: from pps.filterd (m0098410.ppops.net [127.0.0.1]) by mx0a-001b2d01.pphosted.com (8.16.0.22/8.16.0.22) with SMTP id w6BNhs57108668 for ; Wed, 11 Jul 2018 19:45:05 -0400 Received: from e15.ny.us.ibm.com (e15.ny.us.ibm.com [129.33.205.205]) by mx0a-001b2d01.pphosted.com with ESMTP id 2k5umc8awp-1 (version=TLSv1.2 cipher=AES256-GCM-SHA384 bits=256 verify=NOT) for ; Wed, 11 Jul 2018 19:45:05 -0400 Received: from localhost by e15.ny.us.ibm.com with IBM ESMTP SMTP Gateway: Authorized Use Only! Violators will be prosecuted for from ; Wed, 11 Jul 2018 19:45:04 -0400 Received: from b01cxnp23034.gho.pok.ibm.com (9.57.198.29) by e15.ny.us.ibm.com (146.89.104.202) 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 19:45:02 -0400 Received: from b01ledav003.gho.pok.ibm.com (b01ledav003.gho.pok.ibm.com [9.57.199.108]) by b01cxnp23034.gho.pok.ibm.com (8.14.9/8.14.9/NCO v10.0) with ESMTP id w6BNj1F460293160 (version=TLSv1/SSLv3 cipher=DHE-RSA-AES256-GCM-SHA384 bits=256 verify=FAIL); Wed, 11 Jul 2018 23:45:01 GMT Received: from b01ledav003.gho.pok.ibm.com (unknown [127.0.0.1]) by IMSVA (Postfix) with ESMTP id 1B3B6B2065; Wed, 11 Jul 2018 19:45:00 -0400 (EDT) Received: from b01ledav003.gho.pok.ibm.com (unknown [127.0.0.1]) by IMSVA (Postfix) with ESMTP id DE864B2064; Wed, 11 Jul 2018 19:44:59 -0400 (EDT) Received: from paulmck-ThinkPad-W541 (unknown [9.70.82.159]) by b01ledav003.gho.pok.ibm.com (Postfix) with ESMTP; Wed, 11 Jul 2018 19:44:59 -0400 (EDT) Received: by paulmck-ThinkPad-W541 (Postfix, from userid 1000) id 6E2EE16C41DD; Wed, 11 Jul 2018 16:47:20 -0700 (PDT) Date: Wed, 11 Jul 2018 16:47:20 -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> <18288874-e778-4f7f-2fd5-f03624717e9c@de.ibm.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <18288874-e778-4f7f-2fd5-f03624717e9c@de.ibm.com> User-Agent: Mutt/1.5.21 (2010-09-15) X-TM-AS-GCONF: 00 x-cbid: 18071123-0068-0000-0000-00000316ABC4 X-IBM-SpamModules-Scores: X-IBM-SpamModules-Versions: BY=3.00009353; HX=3.00000241; KW=3.00000007; PH=3.00000004; SC=3.00000266; SDB=6.01060000; UDB=6.00544068; IPR=6.00837921; MB=3.00022108; MTD=3.00000008; XFM=3.00000015; UTC=2018-07-11 23:45:04 X-IBM-AV-DETECTION: SAVI=unused REMOTE=unused XFE=unused x-cbparentid: 18071123-0069-0000-0000-000044FF76D3 Message-Id: <20180711234720.GJ3593@linux.vnet.ibm.com> X-Proofpoint-Virus-Version: vendor=fsecure engine=2.50.10434:,, definitions=2018-07-11_06:,, 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-1807110249 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 11:39:10PM +0200, Christian Borntraeger wrote: > > > On 07/11/2018 11:32 PM, 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. > > You talk about > commit bcbfdd01dce5556a952fae84ef16fd0f12525e7b > rcu: Make non-preemptive schedule be Tasks RCU quiescent state > > correct? In fact, then whatever (properly sent) patch comes up should contain > a fixes tag. Not that one, but this one is at least part of the "team": 28053bc72c0e5 ("rcu: Add long-term CPU kicking"). I might need to use "git bisect" to find the most relevant commit... :-/ > > 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! ;-) > > WIth that explanation it makes perfect sense to replace > rcu_virt_note_context_switch with rcu_kvm_enter/exit from an rcu performance > perspective. I assume that rcu_kvm_enter is not much slower than > rcu_virt_note_context_switch? Because we do call it on every guest entry/exit > which we might have plenty for ping pong I/O workload. But is there any way for a guest OS to sneak back out to the hypervisor without executing one of the rcu_kvm_exit() calls? If there is, RCU is broken. Thanx, Paul