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Aug 2023 10:17:20 +0000 Date: Thu, 10 Aug 2023 17:50:06 +0800 From: Yan Zhao To: David Hildenbrand CC: , , , , , , , , , , Subject: Re: [RFC PATCH v2 0/5] Reduce NUMA balance caused TLB-shootdowns in a VM Message-ID: Reply-To: Yan Zhao References: <20230810085636.25914-1-yan.y.zhao@intel.com> <41a893e1-f2e7-23f4-cad2-d5c353a336a3@redhat.com> Content-Type: text/plain; charset="us-ascii" Content-Disposition: inline In-Reply-To: <41a893e1-f2e7-23f4-cad2-d5c353a336a3@redhat.com> X-ClientProxiedBy: CP6P284CA0105.BRAP284.PROD.OUTLOOK.COM (2603:10d6:103:1a8::17) To DS7PR11MB5966.namprd11.prod.outlook.com (2603:10b6:8:71::6) MIME-Version: 1.0 X-MS-PublicTrafficType: Email X-MS-TrafficTypeDiagnostic: DS7PR11MB5966:EE_|PH7PR11MB7147:EE_ X-MS-Office365-Filtering-Correlation-Id: 1978d3aa-b3d3-4bd8-0560-08db998afb83 X-LD-Processed: 46c98d88-e344-4ed4-8496-4ed7712e255d,ExtAddr X-MS-Exchange-SenderADCheck: 1 X-MS-Exchange-AntiSpam-Relay: 0 X-Microsoft-Antispam: BCL:0; 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J93iwCT8M++FDy8F0X43OpABpKoC6MXDZpr0mY4oIMcOnN9GcAafmU+2UY1642xyEMOz0tgprjkV/qJXQ0kZduLON5j87vD75kRsi4TgrOA4Ut8kjwzl7FqyddcbF+9LExCJR+X1s3tKGCH5eAzg+WetbIA+ffa7EMDbUUriqfk46md6EPdmj60pO9xqIrKFpeuYOAgu+UHQ9zbb77PddHjVonyhq6vKJRzhhlZMyPvJdRLzXWe1rQHXUKDwd1y0ucVLxiEGAMFC270C5f7RefXOcOSsP8pD9qpN52ildEUsJgozmq1szwYwmv/vFg7u8j9XSPHYURfpn+oIgrFA61opM1EvVfjKGgJeECIinw4sLGBWqj9VuCCFpx1OudzQGucUk X-Bogosity: Ham, tests=bogofilter, spamicity=0.000000, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: On Thu, Aug 10, 2023 at 11:34:07AM +0200, David Hildenbrand wrote: > > This series first introduces a new flag MMU_NOTIFIER_RANGE_NUMA in patch 1 > > to work with mmu notifier event type MMU_NOTIFY_PROTECTION_VMA, so that > > the subscriber (e.g.KVM) of the mmu notifier can know that an invalidation > > event is sent for NUMA migration purpose in specific. > > > > Patch 2 skips setting PROT_NONE to long-term pinned pages in the primary > > MMU to avoid NUMA protection introduced page faults and restoration of old > > huge PMDs/PTEs in primary MMU. > > > > Patch 3 introduces a new mmu notifier callback .numa_protect(), which > > will be called in patch 4 when a page is ensured to be PROT_NONE protected. > > > > Then in patch 5, KVM can recognize a .invalidate_range_start() notification > > is for NUMA balancing specific and do not do the page unmap in secondary > > MMU until .numa_protect() comes. > > > > Why do we need all that, when we should simply not be applying PROT_NONE to > pinned pages? > > In change_pte_range() we already have: > > if (is_cow_mapping(vma->vm_flags) && > page_count(page) != 1) > > Which includes both, shared and pinned pages. Ah, right, currently in my side, I don't see any pinned pages are outside of this condition. But I have a question regarding to is_cow_mapping(vma->vm_flags), do we need to allow pinned pages in !is_cow_mapping(vma->vm_flags)? > Staring at page #2, are we still missing something similar for THPs? Yes. > Why is that MMU notifier thingy and touching KVM code required? Because NUMA balancing code will firstly send .invalidate_range_start() with event type MMU_NOTIFY_PROTECTION_VMA to KVM in change_pmd_range() unconditionally, before it goes down into change_pte_range() and change_huge_pmd() to check each page count and apply PROT_NONE. Then current KVM will unmap all notified pages from secondary MMU in .invalidate_range_start(), which could include pages that finally not set to PROT_NONE in primary MMU. For VMs with pass-through devices, though all guest pages are pinned, KVM still periodically unmap pages in response to the .invalidate_range_start() notification from auto NUMA balancing, which is a waste. So, if there's a new callback sent when pages is set to PROT_NONE for NUMA migrate only, KVM can unmap only those pages. As KVM still needs to unmap pages for other type of event in its handler of .invalidate_range_start() (.i.e. kvm_mmu_notifier_invalidate_range_start()), and MMU_NOTIFY_PROTECTION_VMA also include other reasons, so patch 1 added a range flag to help KVM not to do a blind unmap in .invalidate_range_start(), but do it in the new .numa_protect() handler. > > -- > Cheers, > > David / dhildenb > >