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 3EDAFECDFB8 for ; Mon, 23 Jul 2018 13:33:56 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id 0428120856 for ; Mon, 23 Jul 2018 13:33:56 +0000 (UTC) DMARC-Filter: OpenDMARC Filter v1.3.2 mail.kernel.org 0428120856 Authentication-Results: mail.kernel.org; dmarc=fail (p=none dis=none) header.from=redhat.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 S2388238AbeGWOfL (ORCPT ); Mon, 23 Jul 2018 10:35:11 -0400 Received: from mx3-rdu2.redhat.com ([66.187.233.73]:44692 "EHLO mx1.redhat.com" rhost-flags-OK-OK-OK-FAIL) by vger.kernel.org with ESMTP id S2387831AbeGWOfK (ORCPT ); Mon, 23 Jul 2018 10:35:10 -0400 Received: from smtp.corp.redhat.com (int-mx06.intmail.prod.int.rdu2.redhat.com [10.11.54.6]) (using TLSv1.2 with cipher AECDH-AES256-SHA (256/256 bits)) (No client certificate requested) by mx1.redhat.com (Postfix) with ESMTPS id EAE9C814F0A3; Mon, 23 Jul 2018 13:33:52 +0000 (UTC) Received: from llong.remote.csb (dhcp-17-175.bos.redhat.com [10.18.17.175]) by smtp.corp.redhat.com (Postfix) with ESMTP id C1CB62166BA0; Mon, 23 Jul 2018 13:33:51 +0000 (UTC) Subject: Re: [PATCH v2] xen/spinlock: Don't use pvqspinlock if only 1 vCPU To: Wanpeng Li , Paolo Bonzini , Radim Krcmar Cc: Davidlohr Bueso , Boris Ostrovsky , Juergen Gross , Thomas Gleixner , Ingo Molnar , "H. Peter Anvin" , the arch/x86 maintainers , xen-devel , LKML , Konrad Rzeszutek Wilk References: <1532036397-19449-1-git-send-email-longman@redhat.com> <20180719215456.5ho3udhfoqlkh75a@linux-r8p5> <00e98205-606a-a121-36c2-dedaeae1d0bb@redhat.com> From: Waiman Long Organization: Red Hat Message-ID: <29f7dd77-267a-0ba7-f4e8-6ef3b3de167c@redhat.com> Date: Mon, 23 Jul 2018 09:33:51 -0400 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:52.0) Gecko/20100101 Thunderbird/52.2.0 MIME-Version: 1.0 In-Reply-To: Content-Type: text/plain; charset=utf-8 Content-Transfer-Encoding: 7bit Content-Language: en-US X-Scanned-By: MIMEDefang 2.78 on 10.11.54.6 X-Greylist: Sender IP whitelisted, not delayed by milter-greylist-4.5.16 (mx1.redhat.com [10.11.55.8]); Mon, 23 Jul 2018 13:33:53 +0000 (UTC) X-Greylist: inspected by milter-greylist-4.5.16 (mx1.redhat.com [10.11.55.8]); Mon, 23 Jul 2018 13:33:53 +0000 (UTC) for IP:'10.11.54.6' DOMAIN:'int-mx06.intmail.prod.int.rdu2.redhat.com' HELO:'smtp.corp.redhat.com' FROM:'longman@redhat.com' RCPT:'' Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 07/22/2018 11:31 PM, Wanpeng Li wrote: > On Fri, 20 Jul 2018 at 06:03, Waiman Long wrote: >> On 07/19/2018 05:54 PM, Davidlohr Bueso wrote: >>> On Thu, 19 Jul 2018, Waiman Long wrote: >>> >>>> On a VM with only 1 vCPU, the locking fast paths will always be >>>> successful. In this case, there is no need to use the the PV qspinlock >>>> code which has higher overhead on the unlock side than the native >>>> qspinlock code. >>>> >>>> The xen_pvspin veriable is also turned off in this 1 vCPU case to >>>> eliminate unneeded pvqspinlock initialization in xen_init_lock_cpu() >>>> which is run after xen_init_spinlocks(). >>> Wouldn't kvm also want this? >>> >>> diff --git a/arch/x86/kernel/kvm.c b/arch/x86/kernel/kvm.c >>> index a37bda38d205..95aceb692010 100644 >>> --- a/arch/x86/kernel/kvm.c >>> +++ b/arch/x86/kernel/kvm.c >>> @@ -457,7 +457,8 @@ static void __init sev_map_percpu_data(void) >>> static void __init kvm_smp_prepare_cpus(unsigned int max_cpus) >>> { >>> native_smp_prepare_cpus(max_cpus); >>> - if (kvm_para_has_hint(KVM_HINTS_REALTIME)) >>> + if (num_possible_cpus() == 1 || >>> + kvm_para_has_hint(KVM_HINTS_REALTIME)) >>> static_branch_disable(&virt_spin_lock_key); >>> } >> That doesn't really matter as the slowpath will never get executed in >> the 1 vCPU case. > So this is not needed in kvm tree? > https://git.kernel.org/pub/scm/virt/kvm/kvm.git/commit/?h=queue&id=3a792199004ec335346cc607d62600a399a7ee02 > > Regards, > Wanpeng Li With only 1 vCPU, the slowpath will not be executed. It will be a deadlock if it happens. So we don't need to explicitly disable the static key here. -Longman