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(p200300cbc7156600a077c7da33626896.dip0.t-ipconnect.de. [2003:cb:c715:6600:a077:c7da:3362:6896]) by smtp.gmail.com with ESMTPSA id 5b1f17b1804b1-432dab721d7sm33653205e9.9.2024.11.14.13.16.42 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Thu, 14 Nov 2024 13:16:43 -0800 (PST) Message-ID: <5f04a1dc-ca0a-488b-812e-7cebf393f59f@redhat.com> Date: Thu, 14 Nov 2024 22:16:41 +0100 MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH v1 2/2] migration: Do not perform RAMBlock dirty sync during the first iteration To: Peter Xu Cc: Yong Huang , qemu-devel@nongnu.org, Fabiano Rosas , Wei Wang , "Michael S . Tsirkin" References: <43700d36-b9f8-42da-ba72-b0ec6580032d@redhat.com> <3049bc19-2556-4fbf-9d34-578db523b63b@redhat.com> <382461ab-d620-4d2e-becd-720daadf3c55@redhat.com> <8ee7d398-0139-4628-9276-f6a89fa35245@redhat.com> From: David Hildenbrand Content-Language: en-US Autocrypt: addr=david@redhat.com; keydata= xsFNBFXLn5EBEAC+zYvAFJxCBY9Tr1xZgcESmxVNI/0ffzE/ZQOiHJl6mGkmA1R7/uUpiCjJ dBrn+lhhOYjjNefFQou6478faXE6o2AhmebqT4KiQoUQFV4R7y1KMEKoSyy8hQaK1umALTdL QZLQMzNE74ap+GDK0wnacPQFpcG1AE9RMq3aeErY5tujekBS32jfC/7AnH7I0v1v1TbbK3Gp XNeiN4QroO+5qaSr0ID2sz5jtBLRb15RMre27E1ImpaIv2Jw8NJgW0k/D1RyKCwaTsgRdwuK Kx/Y91XuSBdz0uOyU/S8kM1+ag0wvsGlpBVxRR/xw/E8M7TEwuCZQArqqTCmkG6HGcXFT0V9 PXFNNgV5jXMQRwU0O/ztJIQqsE5LsUomE//bLwzj9IVsaQpKDqW6TAPjcdBDPLHvriq7kGjt WhVhdl0qEYB8lkBEU7V2Yb+SYhmhpDrti9Fq1EsmhiHSkxJcGREoMK/63r9WLZYI3+4W2rAc UucZa4OT27U5ZISjNg3Ev0rxU5UH2/pT4wJCfxwocmqaRr6UYmrtZmND89X0KigoFD/XSeVv jwBRNjPAubK9/k5NoRrYqztM9W6sJqrH8+UWZ1Idd/DdmogJh0gNC0+N42Za9yBRURfIdKSb B3JfpUqcWwE7vUaYrHG1nw54pLUoPG6sAA7Mehl3nd4pZUALHwARAQABzSREYXZpZCBIaWxk ZW5icmFuZCA8ZGF2aWRAcmVkaGF0LmNvbT7CwZgEEwEIAEICGwMGCwkIBwMCBhUIAgkKCwQW AgMBAh4BAheAAhkBFiEEG9nKrXNcTDpGDfzKTd4Q9wD/g1oFAl8Ox4kFCRKpKXgACgkQTd4Q 9wD/g1oHcA//a6Tj7SBNjFNM1iNhWUo1lxAja0lpSodSnB2g4FCZ4R61SBR4l/psBL73xktp rDHrx4aSpwkRP6Epu6mLvhlfjmkRG4OynJ5HG1gfv7RJJfnUdUM1z5kdS8JBrOhMJS2c/gPf wv1TGRq2XdMPnfY2o0CxRqpcLkx4vBODvJGl2mQyJF/gPepdDfcT8/PY9BJ7FL6Hrq1gnAo4 3Iv9qV0JiT2wmZciNyYQhmA1V6dyTRiQ4YAc31zOo2IM+xisPzeSHgw3ONY/XhYvfZ9r7W1l pNQdc2G+o4Di9NPFHQQhDw3YTRR1opJaTlRDzxYxzU6ZnUUBghxt9cwUWTpfCktkMZiPSDGd KgQBjnweV2jw9UOTxjb4LXqDjmSNkjDdQUOU69jGMUXgihvo4zhYcMX8F5gWdRtMR7DzW/YE BgVcyxNkMIXoY1aYj6npHYiNQesQlqjU6azjbH70/SXKM5tNRplgW8TNprMDuntdvV9wNkFs 9TyM02V5aWxFfI42+aivc4KEw69SE9KXwC7FSf5wXzuTot97N9Phj/Z3+jx443jo2NR34XgF 89cct7wJMjOF7bBefo0fPPZQuIma0Zym71cP61OP/i11ahNye6HGKfxGCOcs5wW9kRQEk8P9 M/k2wt3mt/fCQnuP/mWutNPt95w9wSsUyATLmtNrwccz63XOwU0EVcufkQEQAOfX3n0g0fZz Bgm/S2zF/kxQKCEKP8ID+Vz8sy2GpDvveBq4H2Y34XWsT1zLJdvqPI4af4ZSMxuerWjXbVWb T6d4odQIG0fKx4F8NccDqbgHeZRNajXeeJ3R7gAzvWvQNLz4piHrO/B4tf8svmRBL0ZB5P5A 2uhdwLU3NZuK22zpNn4is87BPWF8HhY0L5fafgDMOqnf4guJVJPYNPhUFzXUbPqOKOkL8ojk CXxkOFHAbjstSK5Ca3fKquY3rdX3DNo+EL7FvAiw1mUtS+5GeYE+RMnDCsVFm/C7kY8c2d0G NWkB9pJM5+mnIoFNxy7YBcldYATVeOHoY4LyaUWNnAvFYWp08dHWfZo9WCiJMuTfgtH9tc75 7QanMVdPt6fDK8UUXIBLQ2TWr/sQKE9xtFuEmoQGlE1l6bGaDnnMLcYu+Asp3kDT0w4zYGsx 5r6XQVRH4+5N6eHZiaeYtFOujp5n+pjBaQK7wUUjDilPQ5QMzIuCL4YjVoylWiBNknvQWBXS lQCWmavOT9sttGQXdPCC5ynI+1ymZC1ORZKANLnRAb0NH/UCzcsstw2TAkFnMEbo9Zu9w7Kv AxBQXWeXhJI9XQssfrf4Gusdqx8nPEpfOqCtbbwJMATbHyqLt7/oz/5deGuwxgb65pWIzufa N7eop7uh+6bezi+rugUI+w6DABEBAAHCwXwEGAEIACYCGwwWIQQb2cqtc1xMOkYN/MpN3hD3 AP+DWgUCXw7HsgUJEqkpoQAKCRBN3hD3AP+DWrrpD/4qS3dyVRxDcDHIlmguXjC1Q5tZTwNB boaBTPHSy/Nksu0eY7x6HfQJ3xajVH32Ms6t1trDQmPx2iP5+7iDsb7OKAb5eOS8h+BEBDeq 3ecsQDv0fFJOA9ag5O3LLNk+3x3q7e0uo06XMaY7UHS341ozXUUI7wC7iKfoUTv03iO9El5f XpNMx/YrIMduZ2+nd9Di7o5+KIwlb2mAB9sTNHdMrXesX8eBL6T9b+MZJk+mZuPxKNVfEQMQ a5SxUEADIPQTPNvBewdeI80yeOCrN+Zzwy/Mrx9EPeu59Y5vSJOx/z6OUImD/GhX7Xvkt3kq Er5KTrJz3++B6SH9pum9PuoE/k+nntJkNMmQpR4MCBaV/J9gIOPGodDKnjdng+mXliF3Ptu6 3oxc2RCyGzTlxyMwuc2U5Q7KtUNTdDe8T0uE+9b8BLMVQDDfJjqY0VVqSUwImzTDLX9S4g/8 kC4HRcclk8hpyhY2jKGluZO0awwTIMgVEzmTyBphDg/Gx7dZU1Xf8HFuE+UZ5UDHDTnwgv7E th6RC9+WrhDNspZ9fJjKWRbveQgUFCpe1sa77LAw+XFrKmBHXp9ZVIe90RMe2tRL06BGiRZr jPrnvUsUUsjRoRNJjKKA/REq+sAnhkNPPZ/NNMjaZ5b8Tovi8C0tmxiCHaQYqj7G2rgnT0kt WNyWQQ== Organization: Red Hat In-Reply-To: Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit Received-SPF: pass client-ip=170.10.133.124; envelope-from=david@redhat.com; helo=us-smtp-delivery-124.mimecast.com X-Spam_score_int: -21 X-Spam_score: -2.2 X-Spam_bar: -- X-Spam_report: (-2.2 / 5.0 requ) BAYES_00=-1.9, DKIMWL_WL_HIGH=-0.122, DKIM_SIGNED=0.1, DKIM_VALID=-0.1, DKIM_VALID_AU=-0.1, DKIM_VALID_EF=-0.1, RCVD_IN_DNSWL_NONE=-0.0001, RCVD_IN_MSPIKE_H3=0.001, RCVD_IN_MSPIKE_WL=0.001, RCVD_IN_VALIDITY_RPBL_BLOCKED=0.001, RCVD_IN_VALIDITY_SAFE_BLOCKED=0.001, SPF_HELO_NONE=0.001, SPF_PASS=-0.001 autolearn=ham autolearn_force=no X-Spam_action: no action X-BeenThere: qemu-devel@nongnu.org X-Mailman-Version: 2.1.29 Precedence: list List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Errors-To: qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org Sender: qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org On 14.11.24 20:28, Peter Xu wrote: > On Thu, Nov 14, 2024 at 10:02:37AM +0100, David Hildenbrand wrote: >> On 13.11.24 21:12, Peter Xu wrote: >>> On Wed, Nov 13, 2024 at 07:49:44PM +0100, David Hildenbrand wrote: >>>> I think I had precisely that, and I recall you suggested to have it only >>>> after the initial sync. Would work for me, but I'd still like to understand >>>> why essentially none of the "discard" was effective -- all of guest RAM got >>>> touched. >>> >>> Yes it'll be interesting to know.. >>> >>> One thing I'm wildly guessing is dirty_memory_extend(), so maybe after the >>> ramblock is created nobody yet to clear the "1"s there for each of the >>> client, including DIRTY_MEMORY_MIGRATION. Then it'll be synced to ramblock >>> bmap only in the initial sync, once for each qemu lifecycle. >> >> >> In ram_block_add() we do the >> >> cpu_physical_memory_set_dirty_range(new_block->offset, >> new_block->used_length, >> DIRTY_CLIENTS_ALL); >> >> ramblock_dirty_bitmap_clear_discarded_pages()->...->migration_clear_memory_region_dirty_bitmap_range()->migration_clear_memory_region_dirty_bitmap() >> won't end up clearing the bits in the dirty bitmap. >> >> First I thought because of: >> >> if (!rb->clear_bmap || !clear_bmap_test_and_clear(rb, page)) { >> return; >> } >> >> But then I realized that even memory_region_clear_dirty_bitmap() will not >> clear the ramblock_dirty_bitmap_ bits! It's only concerned about >> listener->log_clear() calls. >> >> Looking for DIRTY_MEMORY_BLOCK_SIZE users, only >> cpu_physical_memory_sync_dirty_bitmap() and >> cpu_physical_memory_clear_dirty_range() clear them, whereby the latter is >> only used when resizing RAMblocks. >> >> At first I wondered whether ramblock_dirty_bitmap_clear_discarded_pages() >> should also call cpu_physical_memory_clear_dirty_range(), but then I am not >> so sure if that is really the right approach. > > That sounds actually reasonable to me so far.. What's the concern in your > mind? I think what I had in mind was that for the initial bitmap sync, when we set the bmap to all-1s already, we could just clear the whole ramblock_dirty_bitmap_ + KVM ... bitmaps. So, instead of an "initial sync" we might just want to do an "initial clearing" of all bitmaps. > >> >> >> virtio-balloon() calls qemu_guest_free_page_hint() which calls >> >> migration_clear_memory_region_dirty_bitmap_range() >> bitmap_clear() >> >> So it would maybe have the same issue. > > Should virtio-balloon do the same? virtio-balloon is more interesting, because I assume here we could run after the "initial clearing" and would want to mark it clean everywhere. > > So I suppose the idea here is some module may want to say "we should ignore > these pages in the dirty bitmap", and so far that's only about migration. > > Then cpu_physical_memory_clear_dirty_range() does look like the right thing > to do, in which case the bmap in ram_list used to be overlooked.. it seems. > > But of course, cpu_physical_memory_clear_dirty_range() still doesn't cover > the migration bitmap itself, which is ramblock->bmap. So we'll need to > switch from migration_clear_memory_region_dirty_bitmap() to use things like > cpu_physical_memory_clear_dirty_range(), just to cover ram_list bitmaps. > Then keeping the rb->bmap operations like before.. For virtio-balloon likely yes. Regarding virtio-mem, maybe "initial clearing" + only modifying the rb->bmap when processing discards could work and would even be more efficient. (but I'm confused because we have way too many bitmaps, and maybe the KVM one could be problematic without an initial sync ... we'd want an initial clearing for that as well ...) -- Cheers, David / dhildenb