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=-2.5 required=3.0 tests=HEADER_FROM_DIFFERENT_DOMAINS, MAILING_LIST_MULTI,SPF_PASS,URIBL_BLOCKED,USER_AGENT_MUTT 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 40D90C46464 for ; Mon, 13 Aug 2018 14:27:17 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id 039C1217CB for ; Mon, 13 Aug 2018 14:27:17 +0000 (UTC) DMARC-Filter: OpenDMARC Filter v1.3.2 mail.kernel.org 039C1217CB 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 S1729864AbeHMRJo (ORCPT ); Mon, 13 Aug 2018 13:09:44 -0400 Received: from foss.arm.com ([217.140.101.70]:59460 "EHLO foss.arm.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1728293AbeHMRJo (ORCPT ); Mon, 13 Aug 2018 13:09:44 -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 2F76C80D; Mon, 13 Aug 2018 07:27:14 -0700 (PDT) Received: from localhost (e113682-lin.copenhagen.arm.com [10.32.144.41]) by usa-sjc-imap-foss1.foss.arm.com (Postfix) with ESMTPSA id 87C4D3F5D0; Mon, 13 Aug 2018 07:27:13 -0700 (PDT) Date: Mon, 13 Aug 2018 16:27:11 +0200 From: Christoffer Dall To: Punit Agrawal Cc: kvmarm@lists.cs.columbia.edu, linux-arm-kernel@lists.infradead.org, linux-kernel@vger.kernel.org, Marc Zyngier , Suzuki Poulose Subject: Re: [PATCH] KVM: arm/arm64: Skip updating page table entry if no change Message-ID: <20180813142711.GJ20606@e113682-lin.lund.arm.com> References: <20180810111300.17144-1-punit.agrawal@arm.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20180810111300.17144-1-punit.agrawal@arm.com> User-Agent: Mutt/1.5.24 (2015-08-30) Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Fri, Aug 10, 2018 at 12:13:00PM +0100, 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 > -- > Hi, > > This problem was reported by a user when testing PUD hugepages. During > VM restore when all threads are running cpu intensive workload, the > refauting was causing the VM to not make any forward progress. > > This patch fixes the problem for PMD and PTE page fault handling. > > Thanks, > Punit > > Change-Id: I04c9aa8b9fbada47deb1a171c9959f400a0d2a21 > --- > 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); > -- > 2.18.0 > Acked-by: Christoffer Dall