From mboxrd@z Thu Jan 1 00:00:00 1970 Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.remote-ip=100.103.45.18 ARC-Seal: i=1; d=subspace.kernel.org; s=arc20261015; a=rsa-sha256; cv=none; t=1793873046; b=FRbAG4CFV/ccfjBvUE+MoklW2PSz2xuO9VY64otouK8YoEd7KPYpGZvk6RjSusivhy6J u4Q/oM8PZHQS6Ki5lHTr+JhO7ybxIXeTXUF4OECS1UIEVFN0H8m44oHvqf494VgaXM8B0 hFH2aijoFXg9DnUYp5JC1eQQzyAMqeTR/OrygzcGIsveTeIfEexuXLld+28tDv6hLvMV2 A0SJpnhTOTNUb0vD+60fF7LnTy0Pw2iw2Oo9lR5R9WKkLnsamwSfbcNiGiGHOTbv0AhR+ mtG5SUj/Clfr8yFn/TbLsk+AgekpL1Ed0RHapCyJAJ6Yq1o5b+Gr58ul6VHqWYYvGdg== ARC-Message-Signature: i=1; d=subspace.kernel.org; s=arc20261015; a=rsa-sha256; c=relaxed/relaxed; t=1793873046; h=Received:DKIM-Signature:Message-ID:Date:MIME-Version:User-Agent: Subject:To:Cc:References:From:Content-Language:Autocrypt:In-Reply-To: Content-Type:Content-Transfer-Encoding; bh=4sAOTQijm0unkHuug1Z8C+1HeeNtpwsDrUWnMkqB5cE=; b=pmf851fd1vL52UeTKsSmr4kB4KpwRHuBOOPuO+IhXzv7Lih7QRK5Is/viivL75MFJl8+ OF15B0RH6/dQCLWgBaliHgC1UERc9IgJLLqUIwZmNo4uohXEPpu2TzXWpCmvz3mI+zkvq pyeX+VbCFGbFOmd3a12RehUc6SDWKu/Ci5xVXDZLse50Td+dC1qx5/X5BVJZZjGSZACTg bmlDbJc8dh6dL7uj2tWve7DQ1f/EmeCIffeLLKM6LScanbi+ABVyWnFLDaXOritblUcyv BOFJrRlfgqgNwCa5t7QnIVzX7ydqd4moe13czzw8mLh85bEAouDit9YTR1dAxgw4Ggw== ARC-Authentication-Results: i=1; smtp.subspace.kernel.org; dkim=pass header.d=kernel.org header.i=@kernel.org header.a=rsa-sha256 header.s=k20260515 header.b=PPeTuCJW; arc=none smtp.remote-ip=100.103.45.18 Received: from smtp.kernel.org (quasi.space.kernel.org [100.103.45.18]) (using TLSv1.3 with cipher TLS_AES_256_GCM_SHA384 (256/256 bits) key-exchange X25519 server-signature ECDSA (prime256v1)) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id 7BA5B606A333; Thu, 5 Nov 2026 10:04:06 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; dkim=pass (2048-bit key, unprotected) header.d=kernel.org header.i=@kernel.org header.a=rsa-sha256 header.s=k20260515 header.b=PPeTuCJW Received: by smtp.kernel.org (Postfix) with ESMTPSA id E8F641F000C1; Thu, 5 Nov 2026 10:04:02 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=kernel.org; s=k20260515; t=1793873046; bh=4sAOTQijm0unkHuug1Z8C+1HeeNtpwsDrUWnMkqB5cE=; h=Date:Subject:To:Cc:References:From:In-Reply-To; b=PPeTuCJWAkflarn1lLbfkXeQH5EsCZzh76zu/pXKzFV0qNLrDhBw8QqE06mMTRHRo JrqaKJNfFY7/gsK6h6b3b9FKcjZT+YlwLE/NgP+j6rdiLpDelAc+uHd6Kirj9HuHS9 6fVjp63JPPJPND6a7fbz2kTI9UW2H4IubtgH1r+NmWpihjoSGHC7FKhBqDx2QDukQ3 GFWdGH9dXCuRr30RK60LfnSAga1t4i19RY6AMwXddsbHmZmnQy7wjGJf6xPbXla3H0 /CnCdkTVPV00PxMk+PHxR66JPJf86k20NCVwcgBYGNIRnggGZbxvgbtuEG2tCvo7iH /2LABFxL2v/cA== Message-ID: Date: Thu, 5 Nov 2026 11:03:59 +0100 X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Post: List-Help: List-Subscribe: List-Unsubscribe: List-Owner: Precedence: list MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH v2 1/2] mm/ksm: align smart scan skips with the scan number To: Qiliang Yuan , Andrew Morton , Xu Xin , Chengming Zhou , Stefan Roesch , Lorenzo Stoakes , "Liam R. Howlett" , Vlastimil Babka , Mike Rapoport , Suren Baghdasaryan , Michal Hocko , Shuah Khan Cc: linux-mm@kvack.org, linux-kernel@vger.kernel.org, linux-kselftest@vger.kernel.org References: <20261105-bug-mm-ksm-smart-scan-align-skips-v2-0-8a9780b6844e@gmail.com> <20261105-bug-mm-ksm-smart-scan-align-skips-v2-1-8a9780b6844e@gmail.com> From: "David Hildenbrand (Arm)" Content-Language: en-US Autocrypt: addr=david@kernel.org; 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 B3JfpUqcWwE7vUaYrHG1nw54pLUoPG6sAA7Mehl3nd4pZUALHwARAQABzS5EYXZpZCBIaWxk ZW5icmFuZCAoQ3VycmVudCkgPGRhdmlkQGtlcm5lbC5vcmc+wsGQBBMBCAA6AhsDBQkmWAik AgsJBBUKCQgCFgICHgUCF4AWIQQb2cqtc1xMOkYN/MpN3hD3AP+DWgUCaYJt/AIZAQAKCRBN 3hD3AP+DWriiD/9BLGEKG+N8L2AXhikJg6YmXom9ytRwPqDgpHpVg2xdhopoWdMRXjzOrIKD g4LSnFaKneQD0hZhoArEeamG5tyo32xoRsPwkbpIzL0OKSZ8G6mVbFGpjmyDLQCAxteXCLXz ZI0VbsuJKelYnKcXWOIndOrNRvE5eoOfTt2XfBnAapxMYY2IsV+qaUXlO63GgfIOg8RBaj7x 3NxkI3rV0SHhI4GU9K6jCvGghxeS1QX6L/XI9mfAYaIwGy5B68kF26piAVYv/QZDEVIpo3t7 /fjSpxKT8plJH6rhhR0epy8dWRHk3qT5tk2P85twasdloWtkMZ7FsCJRKWscm1BLpsDn6EQ4 jeMHECiY9kGKKi8dQpv3FRyo2QApZ49NNDbwcR0ZndK0XFo15iH708H5Qja/8TuXCwnPWAcJ DQoNIDFyaxe26Rx3ZwUkRALa3iPcVjE0//TrQ4KnFf+lMBSrS33xDDBfevW9+Dk6IISmDH1R HFq2jpkN+FX/PE8eVhV68B2DsAPZ5rUwyCKUXPTJ/irrCCmAAb5Jpv11S7hUSpqtM/6oVESC 3z/7CzrVtRODzLtNgV4r5EI+wAv/3PgJLlMwgJM90Fb3CB2IgbxhjvmB1WNdvXACVydx55V7 LPPKodSTF29rlnQAf9HLgCphuuSrrPn5VQDaYZl4N/7zc2wcWM7BTQRVy5+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: <20261105-bug-mm-ksm-smart-scan-align-skips-v2-1-8a9780b6844e@gmail.com> Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 7bit On 11/5/26 10:36, Qiliang Yuan wrote: > Smart scan skips pages that KSM has failed to de-duplicate for a while. > Once a page is three scans old, its rmap_item counts down its own number > of scans to skip, 1, 2, 4 and then 8 as it ages, so an old page is looked > at every ninth scan. > > The unstable tree is rebuilt on every scan, so two identical pages that > are not in the stable tree yet can only be merged in a scan that looks > at both. Each countdown runs from when its own page became mergeable, so > two pages that became mergeable at different times end up on different > phases and, once both are old, may never be looked at in the same scan > again. They then stay unmerged for as long as they live. Making a page > mergeable, waiting for 0 to 20 full scans before doing the same with an > identical page, 20 times for each wait, and checking the pair after > another 64 full scans shows this, on two boots per kernel: > > before after > pairs merged (of 420) 329 / 317 420 / 420 > 64M of unique pages skipped 89.0% / 89.1% 87.5% / 87.5% > ksmd CPU time per scan 6.25 / 6.25 ms 6.88 / 7.03 ms > > Pick the skipped scans by the global scan number instead, with periods > of 2, 4 and 8 scans. The periods being powers of two, every old page is > looked at on each scan that is a multiple of 8, whatever its age, so any > two pages meet at least every eighth scan. The longest period goes from > nine scans to eight, which costs 10-12% more ksmd time on memory that > never merges. > > Fixes: 5e924ff54d08 ("mm/ksm: add "smart" page scanning mode") > Signed-off-by: Qiliang Yuan > --- Makes sense to me, without looking to closely into the details. Maybe Stefan can give some input? -- Cheers, David