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=1793969081; b=XJzZuiT3zr0WqQw8w5hg4q4w8f+AA7LrSh/ZX5dtCf/4qN3mWIgpjpNGtf1GXoZNL2E5 +W/4Z5Wgu81nQWVnLxtoixwL85VRPwkrsHb1BIffr+gJpKBXiUASD8XuLKvbzg8AbozwZ 4R9EDCURY3H/7xUwWI5NQwG9zfJvjkUU+8ZCwUiz16zUp3a+/us0eEUzbEI0XcpTfbyVD oIGpI8KdLlDIYS9lpAX9AX3JMhFia5SxVs6QryiKu2t6IN1iDEAVXbhrmg0TBZrnI3RPa /O5eDq3O9HUAUz3ahS3DkXyu7PPW4QQHetslR9xZM7ztzfj/zp9UzrceyVxEVHlsJUQ== ARC-Message-Signature: i=1; d=subspace.kernel.org; s=arc20261015; a=rsa-sha256; c=relaxed/relaxed; t=1793969081; 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=m22QOupz/Gj7ONgc3YhQfUsN4LYhFgGjhiGRN+k82cU=; b=x9LeiCWHdgDPj1mosDQHsw47DC1LNPkqp6xGomBy6TZBuC+PACq0sCF9djCqP2GYBxJL v6WC+++kNzH4/c42HrFQ0Krz39Pnm1N53OcMETBkNmVMLRIr/nMJYsx2PPD27hi0PvtpN CuWIIV/KBpGn/M0jnMIqJaoYTpHyl2VoTLyJLfLDlmvsJANjoRqf9+2VNfGeweAQX5h5X zAeKyZzQ8nvjoqOWduPbuZeamcfWP8cSf8mby06P8m2LAm2XlcEC7uFJGLXXvFEKE95mp KnbvSyy/QTHLJPU8RuAqUE9SY5tK6dbULXMRn4ES/ZJ/kBzGLYjkncKb78ArF1U2rEA== 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=Qjafa3YK; 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 751E0606A329; Fri, 6 Nov 2026 12:44:41 +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=Qjafa3YK Received: by smtp.kernel.org (Postfix) with ESMTPSA id 955491F0089F; Fri, 6 Nov 2026 12:44:37 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=kernel.org; s=k20260515; t=1793969081; bh=m22QOupz/Gj7ONgc3YhQfUsN4LYhFgGjhiGRN+k82cU=; h=Date:Subject:To:Cc:References:From:In-Reply-To; b=Qjafa3YK0xGgJNwxjXBJ7J3iCGKQxc2IYZGgEozspFNho4P1EXhyZFN4k2QWmwYge OjcpjE59HMF5JRRv0Ac+GeQ6HLCrHuniwhH1XqL5ivv446zS9LjJIeAL8uuqRmHy5i ZphYsXU5efvyXSK7hgqogdscMcskcgHhn3v8ODQZTQgRPTbfvnijGmVN1p8/3BPFxS fcwreGWBHb2/Yc36hhSGbLEMFS5qErUE3pajBMBDCPYDiICwFn97gVjDHYCVn4cGeK qSk8+HwOD4/28nl2XCjhm1QPaBrAOnuT9k6KbAjfU8qfTTEEAC1iAj7qpdLRluF91/ QybBGkhgjrMBQ== Message-ID: Date: Fri, 6 Nov 2026 13:44:31 +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: "Xu Xin (vivo)" , Qiliang Yuan Cc: Andrew Morton , Chengming Zhou , Stefan Roesch , Lorenzo Stoakes , "Liam R. Howlett" , Vlastimil Babka , Mike Rapoport , Suren Baghdasaryan , Michal Hocko , Shuah Khan , 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: Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 8bit On 11/6/26 10:45, Xu Xin (vivo) wrote: > On Thu, Nov 05, 2026 at 04:36:14AM -0500, 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 >> --- >> mm/ksm.c | 24 ++++++++++-------------- >> 1 file changed, 10 insertions(+), 14 deletions(-) > > Brilliant design. > By using the global ksm_scan.seqnr as a unified time base, this patch eliminates > the out-of-sync problem of the original per-page skip counters. > > It guarantees that any two pages will be processed in the same scan at least once > every 8 scans — the least common multiple of all scan periods — which completely > resolves the permanent de-duplication failure. > > Admittedly, this comes at the cost of shrinking the longest scan interval from 9 > to 8 scans, raising the scan frequency for aged pages by roughly 12.5%. But I > believe this is a very reasonable trade-off for correctness. So > > Reviewed-by: Xu Xin (vivo) > > > Besides, can we add a extra description about new smart_scan algorithm that we > missed in Documentation/mm/ksm.rst? Agreed, that would be nice. Thanks! -- Cheers, David