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=-4.0 required=3.0 tests=BAYES_00,MAILING_LIST_MULTI, SPF_HELO_NONE,SPF_PASS autolearn=no 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 7962AC43381 for ; Wed, 10 Mar 2021 17:33:00 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 5C9D9600EF for ; Wed, 10 Mar 2021 17:33:00 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S233249AbhCJRc1 (ORCPT ); Wed, 10 Mar 2021 12:32:27 -0500 Received: from mail.kernel.org ([198.145.29.99]:49358 "EHLO mail.kernel.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S232697AbhCJRcH (ORCPT ); Wed, 10 Mar 2021 12:32:07 -0500 Received: from disco-boy.misterjones.org (disco-boy.misterjones.org [51.254.78.96]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by mail.kernel.org (Postfix) with ESMTPSA id 8038E600EF; Wed, 10 Mar 2021 17:32:06 +0000 (UTC) Received: from 78.163-31-62.static.virginmediabusiness.co.uk ([62.31.163.78] helo=why.misterjones.org) by disco-boy.misterjones.org with esmtpsa (TLS1.3) tls TLS_ECDHE_RSA_WITH_AES_256_GCM_SHA384 (Exim 4.94) (envelope-from ) id 1lK2gm-000oge-8P; Wed, 10 Mar 2021 17:32:04 +0000 Date: Wed, 10 Mar 2021 17:31:56 +0000 Message-ID: <871rcmhq43.wl-maz@kernel.org> From: Marc Zyngier To: Paolo Bonzini Cc: Jing Zhang , KVM , KVM ARM , Linux MIPS , KVM PPC , Linux S390 , Linux kselftest , James Morse , Julien Thierry , Suzuki K Poulose , Will Deacon , Huacai Chen , Aleksandar Markovic , Thomas Bogendoerfer , Paul Mackerras , Christian Borntraeger , Janosch Frank , David Hildenbrand , Cornelia Huck , Claudio Imbrenda , Sean Christopherson , Vitaly Kuznetsov , Jim Mattson , Peter Shier , Oliver Upton , David Rientjes , Emanuele Giuseppe Esposito Subject: Re: [RFC PATCH 3/4] KVM: stats: Add ioctl commands to pull statistics in binary format In-Reply-To: <2749fe68-acbb-8f4d-dc76-4cb23edb9b35@redhat.com> References: <20210310003024.2026253-1-jingzhangos@google.com> <20210310003024.2026253-4-jingzhangos@google.com> <875z1zxb11.wl-maz@kernel.org> <8735x3x7lu.wl-maz@kernel.org> <2749fe68-acbb-8f4d-dc76-4cb23edb9b35@redhat.com> User-Agent: Wanderlust/2.15.9 (Almost Unreal) SEMI-EPG/1.14.7 (Harue) FLIM-LB/1.14.9 (=?UTF-8?B?R29qxY0=?=) APEL-LB/10.8 EasyPG/1.0.0 Emacs/27.1 (x86_64-pc-linux-gnu) MULE/6.0 (HANACHIRUSATO) MIME-Version: 1.0 (generated by SEMI-EPG 1.14.7 - "Harue") Content-Type: text/plain; charset=US-ASCII X-SA-Exim-Connect-IP: 62.31.163.78 X-SA-Exim-Rcpt-To: pbonzini@redhat.com, jingzhangos@google.com, kvm@vger.kernel.org, kvmarm@lists.cs.columbia.edu, linux-mips@vger.kernel.org, kvm-ppc@vger.kernel.org, linux-s390@vger.kernel.org, linux-kselftest@vger.kernel.org, james.morse@arm.com, julien.thierry.kdev@gmail.com, suzuki.poulose@arm.com, will@kernel.org, chenhuacai@kernel.org, aleksandar.qemu.devel@gmail.com, tsbogend@alpha.franken.de, paulus@ozlabs.org, borntraeger@de.ibm.com, frankja@linux.ibm.com, david@redhat.com, cohuck@redhat.com, imbrenda@linux.ibm.com, seanjc@google.com, vkuznets@redhat.com, jmattson@google.com, pshier@google.com, oupton@google.com, rientjes@google.com, eesposit@redhat.com X-SA-Exim-Mail-From: maz@kernel.org X-SA-Exim-Scanned: No (on disco-boy.misterjones.org); SAEximRunCond expanded to false Precedence: bulk List-ID: X-Mailing-List: kvm@vger.kernel.org On Wed, 10 Mar 2021 17:11:47 +0000, Paolo Bonzini wrote: > > On 10/03/21 18:05, Marc Zyngier wrote: > > On Wed, 10 Mar 2021 16:03:42 +0000, > > Paolo Bonzini wrote: > >> > >> On 10/03/21 16:51, Marc Zyngier wrote: > >>>> + kvm_for_each_vcpu(j, vcpu, kvm) { > >>>> + pdata = data + VM_STAT_COUNT; > >>>> + for (i = 0; i < VCPU_STAT_COUNT; i++, pdata++) > >>>> + *pdata += *((u64 *)&vcpu->stat + i); > >>> Do you really need the in-kernel copy? Why not directly organise the > >>> data structures in a way that would allow a bulk copy using > >>> copy_to_user()? > >> > >> The result is built by summing per-vCPU counters, so that the counter > >> updates are fast and do not require a lock. So consistency basically > >> cannot be guaranteed. > > > > Sure, but I wonder whether there is scope for VM-global counters to be > > maintained in parallel with per-vCPU counters if speed/efficiency is > > of the essence (and this seems to be how it is sold in the cover > > letter). > > Maintaining VM-global counters would require an atomic instruction and > would suffer lots of cacheline bouncing even on architectures that > have relaxed atomic memory operations. Which is why we have per-cpu counters already. Making use of them doesn't seem that outlandish. > Speed/efficiency of retrieving statistics is important, but let's keep > in mind that the baseline for comparison is hundreds of syscalls and > filesystem lookups. Having that baseline in the cover letter would be a good start, as well as an indication of the frequency this is used at. M. -- Without deviation from the norm, progress is not possible.