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=-1.0 required=3.0 tests=HEADER_FROM_DIFFERENT_DOMAINS, MAILING_LIST_MULTI,SPF_PASS,URIBL_BLOCKED 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 64DD9C46464 for ; Fri, 10 Aug 2018 11:26:49 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id B12BD22404 for ; Fri, 10 Aug 2018 11:26:48 +0000 (UTC) DMARC-Filter: OpenDMARC Filter v1.3.2 mail.kernel.org B12BD22404 Authentication-Results: mail.kernel.org; dmarc=none (p=none dis=none) header.from=arm.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 S1727542AbeHJN4Q (ORCPT ); Fri, 10 Aug 2018 09:56:16 -0400 Received: from usa-sjc-mx-foss1.foss.arm.com ([217.140.101.70]:36854 "EHLO foss.arm.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1727209AbeHJN4Q (ORCPT ); Fri, 10 Aug 2018 09:56:16 -0400 Received: from usa-sjc-imap-foss1.foss.arm.com (unknown [10.72.51.249]) by usa-sjc-mx-foss1.foss.arm.com (Postfix) with ESMTP id BF20715A2; Fri, 10 Aug 2018 04:26:45 -0700 (PDT) Received: from [10.37.10.95] (unknown [10.37.10.95]) by usa-sjc-imap-foss1.foss.arm.com (Postfix) with ESMTPSA id 401413F5D4; Fri, 10 Aug 2018 04:26:44 -0700 (PDT) Subject: Re: [PATCH] KVM: arm/arm64: Skip updating page table entry if no change To: Punit Agrawal , kvmarm@lists.cs.columbia.edu Cc: linux-arm-kernel@lists.infradead.org, linux-kernel@vger.kernel.org, Marc Zyngier , Christoffer Dall References: <20180810111300.17144-1-punit.agrawal@arm.com> From: Suzuki K Poulose Message-ID: <23d3c76f-e839-dbab-227c-f6bc068d3ba0@arm.com> Date: Fri, 10 Aug 2018 12:27:24 +0100 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:52.0) Gecko/20100101 Thunderbird/52.7.0 MIME-Version: 1.0 In-Reply-To: <20180810111300.17144-1-punit.agrawal@arm.com> Content-Type: text/plain; charset=utf-8; format=flowed Content-Language: en-US Content-Transfer-Encoding: 7bit Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 08/10/2018 12:13 PM, Punit Agrawal wrote: > Contention on updating a page table entry by a large number of vcpus > can lead to duplicate work when handling stage 2 page faults. As the > page table update follows the break-before-make requirement of the > architecture, it can lead to repeated refaults due to clearing the > entry and flushing the tlbs. > > This problem is more likely when - > > * there are large number of vcpus > * the mapping is large block mapping > > such as when using PMD hugepages (512MB) with 64k pages. > > Fix this by skipping the page table update if there is no change in > the entry being updated. > > Signed-off-by: Punit Agrawal > Cc: Marc Zyngier > Cc: Christoffer Dall > Cc: Suzuki Poulose > -- > virt/kvm/arm/mmu.c | 16 ++++++++++++++++ > 1 file changed, 16 insertions(+) > > diff --git a/virt/kvm/arm/mmu.c b/virt/kvm/arm/mmu.c > index 1d90d79706bd..a66a5441ca2f 100644 > --- a/virt/kvm/arm/mmu.c > +++ b/virt/kvm/arm/mmu.c > @@ -1027,6 +1027,18 @@ static int stage2_set_pmd_huge(struct kvm *kvm, struct kvm_mmu_memory_cache > VM_BUG_ON(pmd_present(*pmd) && pmd_pfn(*pmd) != pmd_pfn(*new_pmd)); > > old_pmd = *pmd; > + /* > + * Multiple vcpus faulting on the same PMD entry, can lead to > + * them sequentially updating the PMD with the same > + * value. Following the break-before-make (pmd_clear() > + * followed by tlb_flush()) process can hinder forward > + * progress due to refaults generated on missing translations. > + * > + * Skip updating the page table if the entry is unchanged. > + */ > + if (pmd_val(old_pmd) == pmd_val(*new_pmd)) > + return 0; > + > if (pmd_present(old_pmd)) { > pmd_clear(pmd); > kvm_tlb_flush_vmid_ipa(kvm, addr); > @@ -1101,6 +1113,10 @@ static int stage2_set_pte(struct kvm *kvm, struct kvm_mmu_memory_cache *cache, > > /* Create 2nd stage page table mapping - Level 3 */ > old_pte = *pte; > + /* Skip page table update if there is no change */ > + if (pte_val(old_pte) == pte_val(*new_pte)) > + return 0; > + > if (pte_present(old_pte)) { > kvm_set_pte(pte, __pte(0)); > kvm_tlb_flush_vmid_ipa(kvm, addr); > Reviewed-by: Suzuki K Poulose