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(p200300cbc7218100177e1983547864ec.dip0.t-ipconnect.de. [2003:cb:c721:8100:177e:1983:5478:64ec]) by smtp.gmail.com with ESMTPSA id 5b1f17b1804b1-432da244924sm61346855e9.7.2024.11.15.07.49.56 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Fri, 15 Nov 2024 07:49:57 -0800 (PST) Message-ID: <91a7fcc0-f4b6-4129-bde2-c776c1e09859@redhat.com> Date: Fri, 15 Nov 2024 16:49:56 +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: <3049bc19-2556-4fbf-9d34-578db523b63b@redhat.com> <382461ab-d620-4d2e-becd-720daadf3c55@redhat.com> <8ee7d398-0139-4628-9276-f6a89fa35245@redhat.com> <5f04a1dc-ca0a-488b-812e-7cebf393f59f@redhat.com> <5577f4cc-63f9-4f4a-b857-48a9405075c1@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=-0.12, 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 >> We better double check and document that, because it must be guaranteed, not >> "let's cross fingers". > > Yes, we should double check on at least known good use cases, maybe not > all. > > E.g., I see nvmm_log_sync() and whpx_log_sync() unconditionally set dirty > to all mem always. I actually don't know how some of these trackers work > for live migration, or if it works at all. > > If by accident this change breaks something, I also wonder whether we > should fix the patch that breaks it, or fix the assumption that the 1st > sync must happen at setup. That's simply wrong to me.. and not all dirty > track users have such either. E.g. vga tracking won't even have a SETUP > phase at all. > > The simplest solution for any potential breakage is to provide a > log_global_start() and sync once there, that's exactly what SETUP should > do. But I hope it won't happen at all.. > > This is legacy tech debt to me. The earlier we understand it the better, > so I'm personally ok if someone would try to do this as long as major archs > will be safe. > >> >> Also, I'm not 100% sure how KVM internals implement that (I recall some >> s390x oddities, but might be wrong ...). >> >>> >>>> >>>>> >>>>>> >>>>>> >>>>>> 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. >>> >>> Yes, if it does what I mentioned below, IIUC it'll clear all dirty bits >>> across the whole stack. Only the ram_list bitmap is missing. IIUC it >>> could mean it could stop working for at least tcg, as tcg sololy uses >>> it.. even with kvm some MRs may use it. Maybe we want to fix it >>> separately. >> >> Yes, that definitely needs care. >> >> [...] >> >>>> (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 ...) >>> >>> So IMHO most of the bitmaps should be initialized with zeros, not >>> ones.. like I mentioned above. >>> >>> Migration bitmap is special, because it's not about dirty tracking >>> capability / reporting but that we know we need to migrate the first round. >>> So setting all ones makes sense for migration only, not a reporting >>> facility. While KVM_DIRTY_LOG_INITIALLY_SET existed for its own reasoning >>> on speeding up migration starts.. >>> >>> So, now I am thinking whether we should not set all ones in ram_list bitmap >>> at all, corresponds to this change: >>> >>> ===8<=== >>> diff --git a/system/physmem.c b/system/physmem.c >>> index dc1db3a384..10966fa68c 100644 >>> --- a/system/physmem.c >>> +++ b/system/physmem.c >>> @@ -1913,10 +1913,6 @@ static void ram_block_add(RAMBlock *new_block, Error **errp) >>> ram_list.version++; >>> qemu_mutex_unlock_ramlist(); >>> - cpu_physical_memory_set_dirty_range(new_block->offset, >>> - new_block->used_length, >>> - DIRTY_CLIENTS_ALL); >>> - >>> if (new_block->host) { >>> qemu_ram_setup_dump(new_block->host, new_block->max_length); >>> qemu_madvise(new_block->host, new_block->max_length, QEMU_MADV_HUGEPAGE); >>> ===8<=== >>> >> >> It will leave things in an inconsistent state with regards of >> qemu_ram_resize() though. So we'd want to do the same thing there as well. >> >> I don't know about DIRTY_CLIENTS_ALL, though ... no expert on that, which >> side effects it has. >> >> Because saying "initially, all memory is dirty" can make sense, depending on >> from which angle you look at it. > > Migration definitely doesn't need it, so to be safe we could also make > CLIENT_ALL to CODE|VGA. > > And if we agree virtio-mem will need to punch holes on all the bitmaps, > then this patch is even not needed. For that, more below. Yes, it's fine for me as a fix. It's not optimal, but I don't really care as long as it works :) > >> >>> I'm guessing whether above could fix the virtio-mem regression after >>> Hyman's current patch applied. >>> >>> Said that, IMHO virtio-mem should still use the same helper just like >>> virtio-balloon as I discussed previously, so as to reset bitmap for the >>> whole stack (which seems to always be the right thing to do to not miss one >>> layer of them)? >> >> Well, the difference is that virtio-mem really only gets called exactly once >> initially. If we can just reset all bitmaps initially, then virtio-mem >> really only has to zap the rb->bmap. > > I think virtio-mem should better always punch through, regardless of > whether we can have above change. Consider cases like KVM "initial-all-set" > feature I mentioned above, when that feature bit is set by QEMU, KVM sets > all ones to the bitmap initially. So that may be required for virtio-mem > to clear 1s in KVM at least. Yes. Anyhow, I'm happy as long as we don't break virtio-mem. Clearing all bitmaps will work. Getting virtio-mem to clear discarded after every bitmap sync would be even more robust, for example, if we'd had some stupid sync() implementation that simply always sets all memory dirty ... -- Cheers, David / dhildenb