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[2003:cb:c705:4200:ce79:acf6:d832:60df]) by smtp.gmail.com with ESMTPSA id 5b1f17b1804b1-433b46426d7sm19481465e9.36.2024.11.20.05.46.10 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Wed, 20 Nov 2024 05:46:11 -0800 (PST) Message-ID: <01b0a528-bec0-41d7-80f6-8afe213bd56b@redhat.com> Date: Wed, 20 Nov 2024 14:46:09 +0100 Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [RFC PATCH 0/4] KVM: ioctl for populating guest_memfd To: kalyazin@amazon.com, pbonzini@redhat.com, corbet@lwn.net, kvm@vger.kernel.org, linux-doc@vger.kernel.org, linux-kernel@vger.kernel.org Cc: jthoughton@google.com, brijesh.singh@amd.com, michael.roth@amd.com, graf@amazon.de, jgowans@amazon.com, roypat@amazon.co.uk, derekmn@amazon.com, nsaenz@amazon.es, xmarcalx@amazon.com, Sean Christopherson , linux-mm@kvack.org References: <20241024095429.54052-1-kalyazin@amazon.com> <08aeaf6e-dc89-413a-86a6-b9772c9b2faf@amazon.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: <08aeaf6e-dc89-413a-86a6-b9772c9b2faf@amazon.com> Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit On 20.11.24 13:09, Nikita Kalyazin wrote: > On 24/10/2024 10:54, Nikita Kalyazin wrote: >> [2] proposes an alternative to >> UserfaultFD for intercepting stage-2 faults, while this series >> conceptually compliments it with the ability to populate guest memory >> backed by guest_memfd for `KVM_X86_SW_PROTECTED_VM` VMs. > > +David > +Sean > +mm Hi! > > While measuring memory population performance of guest_memfd using this > series, I noticed that guest_memfd population takes longer than my > baseline, which is filling anonymous private memory via UFFDIO_COPY. > > I am using x86_64 for my measurements and 3 GiB memory region: > - anon/private UFFDIO_COPY: 940 ms > - guest_memfd: 1371 ms (+46%) > > It turns out that the effect is observable not only for guest_memfd, but > also for any type of shared memory, eg memfd or anonymous memory mapped > as shared. > Below are measurements of a plain mmap(MAP_POPULATE) operation:> > mmap(NULL, 3ll * (1 << 30), PROT_READ | PROT_WRITE, MAP_PRIVATE | > MAP_ANONYMOUS | MAP_POPULATE, -1, 0); > vs > mmap(NULL, 3ll * (1 << 30), PROT_READ | PROT_WRITE, MAP_SHARED | > MAP_ANONYMOUS | MAP_POPULATE, -1, 0); > > Results: > - MAP_PRIVATE: 968 ms > - MAP_SHARED: 1646 ms At least here it is expected to some degree: as soon as the page cache is involved map/unmap gets slower, because we are effectively maintaining two datastructures (page tables + page cache) instead of only a single one (page cache) Can you make sure that THP/large folios don't interfere in your experiments (e.g., madvise(MADV_NOHUGEPAGE))? > > I am seeing this effect on a range of kernels. The oldest I used was > 5.10, the newest is the current kvm-next (for-linus-2590-gd96c77bd4eeb). > > When profiling with perf, I observe the following hottest operations > (kvm-next). Attaching full distributions at the end of the email. > > MAP_PRIVATE: > - 19.72% clear_page_erms, rep stos %al,%es:(%rdi) > > MAP_SHARED: > - 43.94% shmem_get_folio_gfp, lock orb $0x8,(%rdi), which is atomic > setting of the PG_uptodate bit > - 10.98% clear_page_erms, rep stos %al,%es:(%rdi) Interesting. > > Note that MAP_PRIVATE/do_anonymous_page calls __folio_mark_uptodate that > sets the PG_uptodate bit regularly. > , while MAP_SHARED/shmem_get_folio_gfp calls folio_mark_uptodate that > sets the PG_uptodate bit atomically. > > While this logic is intuitive, its performance effect is more > significant that I would expect. Yes. How much of the performance difference would remain if you hack out the atomic op just to play with it? I suspect there will still be some difference. > > The questions are: > - Is this a well-known behaviour? > - Is there a way to mitigate that, ie make shared memory (including > guest_memfd) population faster/comparable to private memory? Likely. But your experiment measures above something different than what guest_memfd vs. anon does: guest_memfd doesn't update page tables, so I would assume guest_memfd will be faster than MAP_POPULATE. How do you end up allocating memory for guest_memfd? Using simple fallocate()? Note that we might improve allocation times with guest_memfd when allocating larger folios. -- Cheers, David / dhildenb