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=-21.7 required=3.0 tests=BAYES_00, HEADER_FROM_DIFFERENT_DOMAINS,INCLUDES_CR_TRAILER,INCLUDES_PATCH, MAILING_LIST_MULTI,MENTIONS_GIT_HOSTING,NICE_REPLY_A,SPF_HELO_NONE,SPF_PASS, USER_AGENT_SANE_1 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 E3B30C07E99 for ; Mon, 12 Jul 2021 15:44:16 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id CC6876120A for ; Mon, 12 Jul 2021 15:44:16 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S235657AbhGLPrE (ORCPT ); Mon, 12 Jul 2021 11:47:04 -0400 Received: from foss.arm.com ([217.140.110.172]:57294 "EHLO foss.arm.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S233648AbhGLPrD (ORCPT ); Mon, 12 Jul 2021 11:47:03 -0400 Received: from usa-sjc-imap-foss1.foss.arm.com (unknown [10.121.207.14]) by usa-sjc-mx-foss1.foss.arm.com (Postfix) with ESMTP id 0E1141FB; Mon, 12 Jul 2021 08:44:15 -0700 (PDT) Received: from [10.57.36.240] (unknown [10.57.36.240]) by usa-sjc-imap-foss1.foss.arm.com (Postfix) with ESMTPSA id 580EE3F774; Mon, 12 Jul 2021 08:44:13 -0700 (PDT) Subject: Re: [PATCH v2] KVM: arm64: Disabling disabled PMU counters wastes a lot of time To: Alexandre Chartre , maz@kernel.org, will@kernel.org, catalin.marinas@arm.com, alexandru.elisei@arm.com, james.morse@arm.com, suzuki.poulose@arm.com, linux-arm-kernel@lists.infradead.org, kvmarm@lists.cs.columbia.edu, kvm@vger.kernel.org Cc: konrad.wilk@oracle.com References: <20210712151700.654819-1-alexandre.chartre@oracle.com> From: Robin Murphy Message-ID: Date: Mon, 12 Jul 2021 16:44:06 +0100 User-Agent: Mozilla/5.0 (Windows NT 10.0; rv:78.0) Gecko/20100101 Thunderbird/78.11.0 MIME-Version: 1.0 In-Reply-To: <20210712151700.654819-1-alexandre.chartre@oracle.com> Content-Type: text/plain; charset=utf-8; format=flowed Content-Language: en-GB Content-Transfer-Encoding: 7bit Precedence: bulk List-ID: X-Mailing-List: kvm@vger.kernel.org On 2021-07-12 16:17, Alexandre Chartre wrote: > In a KVM guest on arm64, performance counters interrupts have an > unnecessary overhead which slows down execution when using the "perf > record" command and limits the "perf record" sampling period. > > The problem is that when a guest VM disables counters by clearing the > PMCR_EL0.E bit (bit 0), KVM will disable all counters defined in > PMCR_EL0 even if they are not enabled in PMCNTENSET_EL0. > > KVM disables a counter by calling into the perf framework, in particular > by calling perf_event_create_kernel_counter() which is a time consuming > operation. So, for example, with a Neoverse N1 CPU core which has 6 event > counters and one cycle counter, KVM will always disable all 7 counters > even if only one is enabled. > > This typically happens when using the "perf record" command in a guest > VM: perf will disable all event counters with PMCNTENTSET_EL0 and only > uses the cycle counter. And when using the "perf record" -F option with > a high profiling frequency, the overhead of KVM disabling all counters > instead of one on every counter interrupt becomes very noticeable. > > The problem is fixed by having KVM disable only counters which are > enabled in PMCNTENSET_EL0. If a counter is not enabled in PMCNTENSET_EL0 > then KVM will not enable it when setting PMCR_EL0.E and it will remain > disabled as long as it is not enabled in PMCNTENSET_EL0. So there is > effectively no need to disable a counter when clearing PMCR_EL0.E if it > is not enabled PMCNTENSET_EL0. > > Signed-off-by: Alexandre Chartre > --- > The patch is based on https://git.kernel.org/pub/scm/linux/kernel/git/maz/arm-platforms.git/log/?h=kvm-arm64/pmu/reset-values > > arch/arm64/kvm/pmu-emul.c | 8 +++++--- > 1 file changed, 5 insertions(+), 3 deletions(-) > > diff --git a/arch/arm64/kvm/pmu-emul.c b/arch/arm64/kvm/pmu-emul.c > index fae4e95b586c..1f317c3dac61 100644 > --- a/arch/arm64/kvm/pmu-emul.c > +++ b/arch/arm64/kvm/pmu-emul.c > @@ -563,21 +563,23 @@ void kvm_pmu_software_increment(struct kvm_vcpu *vcpu, u64 val) > */ > void kvm_pmu_handle_pmcr(struct kvm_vcpu *vcpu, u64 val) > { > - unsigned long mask = kvm_pmu_valid_counter_mask(vcpu); > + unsigned long mask; > int i; > > if (val & ARMV8_PMU_PMCR_E) { > kvm_pmu_enable_counter_mask(vcpu, > __vcpu_sys_reg(vcpu, PMCNTENSET_EL0)); > } else { > - kvm_pmu_disable_counter_mask(vcpu, mask); > + kvm_pmu_disable_counter_mask(vcpu, > + __vcpu_sys_reg(vcpu, PMCNTENSET_EL0)); > } > > if (val & ARMV8_PMU_PMCR_C) > kvm_pmu_set_counter_value(vcpu, ARMV8_PMU_CYCLE_IDX, 0); > > if (val & ARMV8_PMU_PMCR_P) { > - mask &= ~BIT(ARMV8_PMU_CYCLE_IDX); > + mask = kvm_pmu_valid_counter_mask(vcpu) > + & BIT(ARMV8_PMU_CYCLE_IDX); This looks suspiciously opposite of what it replaces; however did we even need to do a bitwise operation here in the first place? Couldn't we skip the cycle counter by just limiting the for_each_set_bit iteration below to 31 bits? Robin. > for_each_set_bit(i, &mask, 32) > kvm_pmu_set_counter_value(vcpu, i, 0); > } > > base-commit: 83f870a663592797c576846db3611e0a1664eda2 >