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=-0.8 required=3.0 tests=HEADER_FROM_DIFFERENT_DOMAINS, MAILING_LIST_MULTI,SPF_PASS 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 2D82BC43A1D for ; Thu, 12 Jul 2018 11:58:23 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id D1827213A2 for ; Thu, 12 Jul 2018 11:58:22 +0000 (UTC) DMARC-Filter: OpenDMARC Filter v1.3.2 mail.kernel.org D1827213A2 Authentication-Results: mail.kernel.org; dmarc=fail (p=none dis=none) header.from=de.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 S1732516AbeGLMHe (ORCPT ); Thu, 12 Jul 2018 08:07:34 -0400 Received: from mx0a-001b2d01.pphosted.com ([148.163.156.1]:55052 "EHLO mx0a-001b2d01.pphosted.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1726695AbeGLMHd (ORCPT ); Thu, 12 Jul 2018 08:07:33 -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 w6CBpjAp044868 for ; Thu, 12 Jul 2018 07:58:19 -0400 Received: from e06smtp07.uk.ibm.com (e06smtp07.uk.ibm.com [195.75.94.103]) by mx0a-001b2d01.pphosted.com with ESMTP id 2k65k044bt-1 (version=TLSv1.2 cipher=AES256-GCM-SHA384 bits=256 verify=NOT) for ; Thu, 12 Jul 2018 07:58:19 -0400 Received: from localhost by e06smtp07.uk.ibm.com with IBM ESMTP SMTP Gateway: Authorized Use Only! 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Violators will be prosecuted; (version=TLSv1/SSLv3 cipher=AES256-GCM-SHA384 bits=256/256) Thu, 12 Jul 2018 12:58:14 +0100 Received: from d06av25.portsmouth.uk.ibm.com (d06av25.portsmouth.uk.ibm.com [9.149.105.61]) by b06cxnps4074.portsmouth.uk.ibm.com (8.14.9/8.14.9/NCO v10.0) with ESMTP id w6CBwDrw37683202 (version=TLSv1/SSLv3 cipher=DHE-RSA-AES256-GCM-SHA384 bits=256 verify=FAIL); Thu, 12 Jul 2018 11:58:13 GMT Received: from d06av25.portsmouth.uk.ibm.com (unknown [127.0.0.1]) by IMSVA (Postfix) with ESMTP id 6DEA411C054; Thu, 12 Jul 2018 14:58:34 +0100 (BST) Received: from d06av25.portsmouth.uk.ibm.com (unknown [127.0.0.1]) by IMSVA (Postfix) with ESMTP id 27ED011C04C; Thu, 12 Jul 2018 14:58:34 +0100 (BST) Received: from oc7330422307.ibm.com (unknown [9.152.99.116]) by d06av25.portsmouth.uk.ibm.com (Postfix) with ESMTP; Thu, 12 Jul 2018 14:58:34 +0100 (BST) Subject: Re: [PATCH v2] kvm/x86: Inform RCU of quiescent state when entering guest mode To: David Woodhouse , paulmck@linux.vnet.ibm.com Cc: peterz@infradead.org, mhillenb@amazon.de, linux-kernel@vger.kernel.org, kvm@vger.kernel.org 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> <20180711234720.GJ3593@linux.vnet.ibm.com> <1531384288.8759.101.camel@infradead.org> <1531393828.8759.116.camel@infradead.org> From: Christian Borntraeger Openpgp: preference=signencrypt Autocrypt: addr=borntraeger@de.ibm.com; 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Linux x86_64; rv:52.0) Gecko/20100101 Thunderbird/52.8.0 MIME-Version: 1.0 In-Reply-To: <1531393828.8759.116.camel@infradead.org> Content-Language: en-US X-TM-AS-GCONF: 00 x-cbid: 18071211-0028-0000-0000-000002DAE06B X-IBM-AV-DETECTION: SAVI=unused REMOTE=unused XFE=unused x-cbparentid: 18071211-0029-0000-0000-000023928C38 Message-Id: <2997e5b4-57b4-dba5-2487-6908ca1b9500@de.ibm.com> Content-Type: text/plain; charset=utf-8 Content-Transfer-Encoding: 8bit X-Proofpoint-Virus-Version: vendor=fsecure engine=2.50.10434:,, definitions=2018-07-12_05:,, 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=961 adultscore=0 classifier=spam adjust=0 reason=mlx scancount=1 engine=8.0.1-1806210000 definitions=main-1807120124 Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 07/12/2018 01:10 PM, David Woodhouse wrote: > On Thu, 2018-07-12 at 13:00 +0200, Christian Borntraeger wrote: >> >> On s390 this seems to add about 10ns (~3%) for a guest exit/rentry >> microbenchmark mostly due to  rcu_eqs_enter and rcu_eqs_exit now being >> visible in perf samples. The older interface was cheaper. > > Well, the older interface wasn't actually working, which made it > moderately suboptimal :) > > But that is fixed by the first patch of the two, so perhaps the second > ought to be conditional on CONFIG_NO_HZ_FULL since it's only really > fixing a real bug there? > > Unless we can contrive some way to do the rcu_eqs_enter/exit only on > the paths to/from userspace, and *not* when we loop in the kernel to > handle interrupts (etc.) and immediately re-enter the guest without > returning? That's somewhat non-trivial though... Assuming that patch1 really fixes the issue (and all KVM hot loops are supposed to have something like if (need_resched()) schedule(); this could work out given that patch1 does what it is supposed to do. So you are thinking of something like diff --git a/include/linux/rcutree.h b/include/linux/rcutree.h index d8ee9f949ce3..cac5de8a887c 100644 --- a/include/linux/rcutree.h +++ b/include/linux/rcutree.h @@ -72,8 +72,17 @@ void cond_synchronize_sched(unsigned long oldstate); void rcu_idle_enter(void); void rcu_idle_exit(void); +#ifdef CONFIG_NO_HZ_FULL void rcu_kvm_enter(void); void rcu_kvm_exit(void); +#else +static inline void rcu_kvm_enter(void) +{ +} +static inline void rcu_kvm_exit(void) +{ +} +#endif void rcu_irq_enter(void); void rcu_irq_exit(void); void rcu_irq_enter_irqson(void); ?