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[80.187.83.189]) by smtp.gmail.com with ESMTPSA id 5b1f17b1804b1-46fb497af18sm198743305e9.3.2025.10.13.11.55.35 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Mon, 13 Oct 2025 11:55:37 -0700 (PDT) Message-ID: <80b153cd-8bba-4bcd-9b56-3b2ad3f295e1@redhat.com> Date: Mon, 13 Oct 2025 20:55:34 +0200 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: [Regerssion] [KSM] KSM CPU overhead in 6.16+ kernel compared to <=6.15 versions ("folio_walk_start" kernel object overhead) To: 423de7a3-1c62-4e72-8e79-19a6413e420c@redhat.com Cc: akpm@linux-foundation.org, chengming.zhou@linux.dev, craftfever@murena.io, linux-kernel@vger.kernel.org, linux-mm@kvack.org, regressions@lists.linux.dev, xu.xin16@zte.com.cn, Lorenzo Stoakes References: <423de7a3-1c62-4e72-8e79-19a6413e420c@redhat.com> <34d27471-80a4-49f8-b6cb-f2e51518d9ea@airmail.cc> 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 ZW5icmFuZCA8ZGF2aWRAcmVkaGF0LmNvbT7CwZoEEwEIAEQCGwMCF4ACGQEFCwkIBwICIgIG FQoJCAsCBBYCAwECHgcWIQQb2cqtc1xMOkYN/MpN3hD3AP+DWgUCaJzangUJJlgIpAAKCRBN 3hD3AP+DWhAxD/9wcL0A+2rtaAmutaKTfxhTP0b4AAp1r/eLxjrbfbCCmh4pqzBhmSX/4z11 opn2KqcOsueRF1t2ENLOWzQu3Roiny2HOU7DajqB4dm1BVMaXQya5ae2ghzlJN9SIoopTWlR 0Af3hPj5E2PYvQhlcqeoehKlBo9rROJv/rjmr2x0yOM8qeTroH/ZzNlCtJ56AsE6Tvl+r7cW 3x7/Jq5WvWeudKrhFh7/yQ7eRvHCjd9bBrZTlgAfiHmX9AnCCPRPpNGNedV9Yty2Jnxhfmbv Pw37LA/jef8zlCDyUh2KCU1xVEOWqg15o1RtTyGV1nXV2O/mfuQJud5vIgzBvHhypc3p6VZJ lEf8YmT+Ol5P7SfCs5/uGdWUYQEMqOlg6w9R4Pe8d+mk8KGvfE9/zTwGg0nRgKqlQXrWRERv cuEwQbridlPAoQHrFWtwpgYMXx2TaZ3sihcIPo9uU5eBs0rf4mOERY75SK+Ekayv2ucTfjxr Kf014py2aoRJHuvy85ee/zIyLmve5hngZTTe3Wg3TInT9UTFzTPhItam6dZ1xqdTGHZYGU0O otRHcwLGt470grdiob6PfVTXoHlBvkWRadMhSuG4RORCDpq89vu5QralFNIf3EysNohoFy2A LYg2/D53xbU/aa4DDzBb5b1Rkg/udO1gZocVQWrDh6I2K3+cCs7BTQRVy5+RARAA59fefSDR 9nMGCb9LbMX+TFAoIQo/wgP5XPyzLYakO+94GrgfZjfhdaxPXMsl2+o8jhp/hlIzG56taNdt VZtPp3ih1AgbR8rHgXw1xwOpuAd5lE1qNd54ndHuADO9a9A0vPimIes78Hi1/yy+ZEEvRkHk /kDa6F3AtTc1m4rbbOk2fiKzzsE9YXweFjQvl9p+AMw6qd/iC4lUk9g0+FQXNdRs+o4o6Qvy iOQJfGQ4UcBuOy1IrkJrd8qq5jet1fcM2j4QvsW8CLDWZS1L7kZ5gT5EycMKxUWb8LuRjxzZ 3QY1aQH2kkzn6acigU3HLtgFyV1gBNV44ehjgvJpRY2cC8VhanTx0dZ9mj1YKIky5N+C0f21 zvntBqcxV0+3p8MrxRRcgEtDZNav+xAoT3G0W4SahAaUTWXpsZoOecwtxi74CyneQNPTDjNg azHmvpdBVEfj7k3p4dmJp5i0U66Onmf6mMFpArvBRSMOKU9DlAzMi4IvhiNWjKVaIE2Se9BY FdKVAJaZq85P2y20ZBd08ILnKcj7XKZkLU5FkoA0udEBvQ0f9QLNyyy3DZMCQWcwRuj1m73D sq8DEFBdZ5eEkj1dCyx+t/ga6x2rHyc8Sl86oK1tvAkwBNsfKou3v+jP/l14a7DGBvrmlYjO 59o3t6inu6H7pt7OL6u6BQj7DoMAEQEAAcLBfAQYAQgAJgIbDBYhBBvZyq1zXEw6Rg38yk3e EPcA/4NaBQJonNqrBQkmWAihAAoJEE3eEPcA/4NaKtMQALAJ8PzprBEXbXcEXwDKQu+P/vts IfUb1UNMfMV76BicGa5NCZnJNQASDP/+bFg6O3gx5NbhHHPeaWz/VxlOmYHokHodOvtL0WCC 8A5PEP8tOk6029Z+J+xUcMrJClNVFpzVvOpb1lCbhjwAV465Hy+NUSbbUiRxdzNQtLtgZzOV Zw7jxUCs4UUZLQTCuBpFgb15bBxYZ/BL9MbzxPxvfUQIPbnzQMcqtpUs21CMK2PdfCh5c4gS sDci6D5/ZIBw94UQWmGpM/O1ilGXde2ZzzGYl64glmccD8e87OnEgKnH3FbnJnT4iJchtSvx yJNi1+t0+qDti4m88+/9IuPqCKb6Stl+s2dnLtJNrjXBGJtsQG/sRpqsJz5x1/2nPJSRMsx9 5YfqbdrJSOFXDzZ8/r82HgQEtUvlSXNaXCa95ez0UkOG7+bDm2b3s0XahBQeLVCH0mw3RAQg r7xDAYKIrAwfHHmMTnBQDPJwVqxJjVNr7yBic4yfzVWGCGNE4DnOW0vcIeoyhy9vnIa3w1uZ 3iyY2Nsd7JxfKu1PRhCGwXzRw5TlfEsoRI7V9A8isUCoqE2Dzh3FvYHVeX4Us+bRL/oqareJ CIFqgYMyvHj7Q06kTKmauOe4Nf0l0qEkIuIzfoLJ3qr5UyXc2hLtWyT9Ir+lYlX9efqh7mOY qIws/H2t In-Reply-To: <34d27471-80a4-49f8-b6cb-f2e51518d9ea@airmail.cc> Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit On 13.10.25 19:09, craftfever wrote: > > Looking again, no, that's not the case. We do a cond_resched() after > every page we looked up. > > > > Also, b1d3e9bbccb4 was introduced in v6.12 already. Regarding > folio_walk_start(), also nothing major changed ever since v6.12. > > > > Looking at scan_get_next_rmap_item(). I guess we might hold the mmap > lock for quite a long time (if we're iterating large areas where there > are no suitable pages mapped -- very large sparse areas). > > > > That would explain why we end up calling folio_walk_start() that > frequently. > > > > But nothing really changed in that regard lately in KSM code. > > > > What we probably should be doing, is give up the mmap lock after > scanning a certain size. Or better, switch to per-VMA locks if possible. > > > > Also, looking up each address is highly inefficient if we end up having > > large empty areas. A range-walk function would be much better suited > for that, so we can just jump over holes completely. > > > > But anyhow, nothing seems to have changed ever since 6.15 AFAIKT, so > I'm not really sure what's going on here. Likely it's unrelated to KSM > changes. > > > > -- Cheers > > > > David / dhildenb > > > > I have to make a correction, folio_start_walk is present in "perf top" > statistics on 6.12-6.15, it just consumes 0.5-1% of kernel time compared > to 11-14% on 6.16+, where it causes ksmd 100% cpu usage compared <=6.15 > kernels. I'm currently looking at the diff from 6.15 -> 6.16. In KSM code nothing changed, really. In folio_walk_start() itself nothing changed. In the functions it calls also nothing relevant should have changed. So the only explanation would be that it is simply called much more frequently. And that might be the case if we are now scanning much, much larger VMAs that are mostly empty, that would otherwise not be scanned. I now recall that we had a fix from Lorenzo: commit cf7e7a3503df0b71afd68ee84e9a09d4514cc2dd Author: Lorenzo Stoakes Date: Thu May 29 18:15:47 2025 +0100 mm: prevent KSM from breaking VMA merging for new VMAs If a user wishes to enable KSM mergeability for an entire process and all fork/exec'd processes that come after it, they use the prctl() PR_SET_MEMORY_MERGE operation. That went into 6.17. Assuming we merge more carefully now, we might no longer run into the if (!vma->anon_vma) ksm_scan.address = vma->vm_end; For the gigantic VMAs and possibly end up scanning these gigantic empty VMAs. Just a thought: A) Can you reproduce on 6.17? B) Does the 6.16 you are testing with contain a backport of that commit? Definitely, scan_get_next_rmap_item() must be optimized to walk a sparse page table more efficiently. > I understand, that something changed in linked function, that > affecting KSM behavior. Maybe, you can reproduce it with same settings, > especially it happens with Chromium apps, there is V8 sandbox with huge > VM size. Maybe, you could reproduce the problem with the same > MemoryKSM=yes in user@.service, that sets KSM processing for all user > processes, especially, when Chromium is running. KSM CPU usage really > differs between 6.12-6.15 and 6.16+. Maybe, it's related to your > explanation. I'm afraid I don't currently have time to reproduce. -- Cheers David / dhildenb