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[2003:cb:c70a:300:3ae1:c3c0:cef:8413]) by smtp.gmail.com with ESMTPSA id 5b1f17b1804b1-438dcc2ef08sm221424315e9.22.2025.02.04.01.59.55 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Tue, 04 Feb 2025 01:59:57 -0800 (PST) Message-ID: <3e5cfe8b-9ca8-4625-b6ff-7f170f1579ff@redhat.com> Date: Tue, 4 Feb 2025 10:59:55 +0100 MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [LSF/MM/BPF TOPIC] Restricting or migrating unmovable kernel allocations from slow tier To: Hyeonggon Yoo <42.hyeyoo@gmail.com>, lsf-pc@lists.linux-foundation.org, linux-mm@kvack.org Cc: linux-cxl@vger.kernel.org, Byungchul Park , Honggyu Kim References: From: David Hildenbrand 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: X-Mimecast-Spam-Score: 0 X-Mimecast-MFC-PROC-ID: 0xMHLDVq9yLKLLm-Hxt0GIQMZmeZj6i00VjEmv20vqM_1738663199 X-Mimecast-Originator: redhat.com Content-Language: en-US Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit X-Stat-Signature: nm3htk8di67e5frnp8eyt49g94cmjjmr X-Rspamd-Queue-Id: D7F221C000A X-Rspam-User: X-Rspamd-Server: rspam06 X-HE-Tag: 1738663201-245527 X-HE-Meta: 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 ouqjKJTm 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 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: List-Subscribe: List-Unsubscribe: On 01.02.25 14:29, Hyeonggon Yoo wrote: > Hi, > > Byungchul and I would like to suggest a topic about the performance impact of > kernel allocations on CXL memory. > > As CXL-enabled servers and memory devices are being developed, CXL-supported > hardware is expected to continue emerging in the coming years. > > The Linux kernel supports hot-plugging CXL memory via dax/kmem functionality. > The hot-plugged memory allows either unmovable kernel allocations > (ZONE_NORMAL), or restricts them to movable allocations (ZONE_MOVABLE) > depending on the hot-plug policy. > > Recently, Byungchul and I observed a measurable performance degradation with > memhp_default_state=online compared to memhp_default_state=online_movable > on a server where the ratio of memory capacity between DRAM and CXL is 1:2 > when running the llama.cpp workload with the default mempolicy. > The workload performs LLM inference and pressures the memory subsystem > due to its large working set size. > > Obviously, allowing kernel allocations from CXL memory degrades performance > because kernel memory like page tables, kernel stacks, and slab allocations, > is accessed frequently and may reside in physical memory with significantly > higher access latency. > > However, as far as I can tell there are at least two reasons why we need to > support ZONE_NORMAL for CXL memory (please add if there are more): > 1. When hot-plugging a huge amount of CXL memory, the size of > the struct page array might not fit into DRAM > -> This could be relaxed with memmap_on_memory There are some others, although most are less significant, and I tried documenting them here: https://www.kernel.org/doc/html/latest/admin-guide/mm/memory-hotplug.html#zone-movable-sizing-considerations E.g., a 4 KiB page requires a single PTE (8 bytes) to be mapped into user space, corresponding to 0.2 %. At least for anonymous memory, PMD-sized THPs don't help, because we still have to allocate the page table to be prepared for a PMD->PTE remapping. In the worst case, the directmap requires another 0.2 % (but usually, we rely on PMD mappings). So that usage depends on how you are intending to use the CXL memory (e.g., pagecache vs. anonymous memory). > 2. To hot-unplug CXL memory, pages in CXL memory should be migrated to DRAM, > which means sometimes some portion of CXL memory should be ZONE_NORMAL. I don't quite understand that argument for ZONE_NORMAL. -- Cheers, David / dhildenb