From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from smtp.kernel.org (aws-us-west-2-korg-mail-alma10-1.taild15c8.ts.net [100.103.45.18]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id 9A00F3B47DD for ; Tue, 16 Jun 2026 08:06:20 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=100.103.45.18 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1781597182; cv=none; b=KoRg5FGVQq2OsxZRX4zvOPAqPflmdrRZ90uZD0ahF33MX2ukt8gJV2CViTUdg59Jg6b50e7J0sSZiAuIg5qvX/b5qXkOwuKVfeFTs5JP7D4riZTg/H/AxJhZl08Rlw7oApmslimeXaxxnBWGWynZ2iBmupyN2xDMNH3TtgsPcSI= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1781597182; c=relaxed/simple; bh=jXmCQknLIlQiNpG5Dl7wffgbbd4G+wj4hvob5/6Xb0Q=; h=Message-ID:Date:MIME-Version:Subject:To:Cc:References:From: In-Reply-To:Content-Type; b=EM5fGF2+ggRPbW8HaRMGHbL+1CV4lzTzycMLEKcvlQEeY0L8e01FSzUKw4Fy6KHvmTsPzZ5glN6sitg+QhA2QrBW2akHckBaM+IERU6KqyEvNwSHyJR+SBVR2Sa6kx9qxLm2I2GooZGctn1uvMr1phRNu2i3Fjd5cg9u61JgZh0= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b=TzC+GBQ2; arc=none smtp.client-ip=100.103.45.18 Authentication-Results: smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b="TzC+GBQ2" Received: by smtp.kernel.org (Postfix) with ESMTPSA id 43A791F000E9; Tue, 16 Jun 2026 08:06:17 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=kernel.org; s=k20260515; t=1781597180; bh=luLefOEiC5+iAvMdCtPPmM37Spy4Cv00eehSR5VZHxA=; h=Date:Subject:To:Cc:References:From:In-Reply-To; b=TzC+GBQ2iHNrObWgFipEAv2M5fitN6tk2Ub+TGBqnifOu4gNvoqXQuShquM3RgH5B oHuRH2K1Wvda12JZBcQbRBUWTmn6XTLJWOcDZUaDNRr2t/NX4C+7ygY/VzhzHtr3AT WKN0VBWRWXql31YW5dnuD5M6eBjBaRRD5h5o6g18trA2crgrDcByp+5GAnxhc3+lda 2GsRSlvCFoDmJ3c3pGg9YckjDusyY3n/+LLwpGLFJA2KIzbOjk6yjXB9BHlbFVgwL2 qB1G/eXedjKjKeOyZplbljpkO6rxxCOcdtwzQiaiEFWkfFb2qAfxi7Gqj3s3YIYK7x o2Blz/4ZlukPg== Message-ID: <06fe0173-d8ea-4477-b043-81d5ad84af0c@kernel.org> Date: Tue, 16 Jun 2026 10:06:15 +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: [PATCH] mm/sparse: Optimize section number calculations using bit shifts To: Zhen Ni , Andrew Morton , Kairui Song , Qi Zheng , Shakeel Butt , Barry Song , Axel Rasmussen , Yuanchu Xie , Wei Xu , Lorenzo Stoakes , "Liam R. Howlett" , Vlastimil Babka , Mike Rapoport , Suren Baghdasaryan , Michal Hocko Cc: linux-mm@kvack.org, linux-kernel@vger.kernel.org References: <20260616025942.3572473-1-zhen.ni@easystack.cn> 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: <20260616025942.3572473-1-zhen.ni@easystack.cn> Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 7bit On 6/16/26 04:59, Zhen Ni wrote: > Add SECTIONS_PER_ROOT_SHIFT = ilog2(SECTIONS_PER_ROOT) with correctness > guaranteed by BUILD_BUG_ON in sparse_init(). Convert SECTION_NR_TO_ROOT > to use right shift instead of division for better performance. Add > SECTION_NR_IN_ROOT() macro to improve code readability. > > This improves code efficiency in hot paths where __nr_to_section() is > frequently called, such as sparse_init() and memory section management > operations. > > Performance verification in sparse_init() on ARM (8GB RAM, 4 NUMA nodes): > > sparse_init() > | > +----> memblocks_present() > | > +----> section initialization (sparse_init_nid loop) > > Time measurement points: > > [T1] sparse_init start > | > v > [T2] memblocks_present() complete > | > v > [T3] sparse_init_nid() loop complete / sparse_init end > > Measurement values: > memblocks_present_cycles = T2 - T1 > section_initialization_cycles = T3 - T2 > total_cycles = T3 - T1 > > Before (division): > [ 0.000000] sparse_init: total 7538 cycles > [ 0.000000] memblocks_present: 4232 cycles > [ 0.000000] section initialization: 3261 cycles > > After (bit shift): > [ 0.000000] sparse_init: total 5641 cycles > [ 0.000000] memblocks_present: 3562 cycles > [ 0.000000] section initialization: 2057 cycles > > Performance improvement: > Total: (7538-5641)/7538 = 25.2% faster > memblocks_present: (4232-3562)/4232 = 15.8% faster > section initialization: (3261-2057)/3261 = 36.9% faster > > Signed-off-by: Zhen Ni > --- > include/linux/mmzone.h | 7 +++++-- > 1 file changed, 5 insertions(+), 2 deletions(-) > > diff --git a/include/linux/mmzone.h b/include/linux/mmzone.h > index 9adb2ad21da5..5daf471f6823 100644 > --- a/include/linux/mmzone.h > +++ b/include/linux/mmzone.h > @@ -2035,11 +2035,14 @@ struct mem_section { > > #ifdef CONFIG_SPARSEMEM_EXTREME > #define SECTIONS_PER_ROOT (PAGE_SIZE / sizeof (struct mem_section)) > +#define SECTIONS_PER_ROOT_SHIFT ilog2(SECTIONS_PER_ROOT) > #else > #define SECTIONS_PER_ROOT 1 > +#define SECTIONS_PER_ROOT_SHIFT 0 > #endif > > -#define SECTION_NR_TO_ROOT(sec) ((sec) / SECTIONS_PER_ROOT) > +#define SECTION_NR_TO_ROOT(sec) ((sec) >> SECTIONS_PER_ROOT_SHIFT) > +#define SECTION_NR_IN_ROOT(sec) ((sec) & SECTION_ROOT_MASK) > #define NR_SECTION_ROOTS DIV_ROUND_UP(NR_MEM_SECTIONS, SECTIONS_PER_ROOT) > #define SECTION_ROOT_MASK (SECTIONS_PER_ROOT - 1) > >From a compiler POV, "/ SECTIONS_PER_ROOT" is exactly the same as >> ilog2(SECTIONS_PER_ROOT) *as long as* the variable we are processing is "unsigned long". The compiler should be smart enough to figure out that out with SECTIONS_PER_ROOT being known at compiletime. Can you compare the generated code __nr_to_section() to see if and why the compiler fails to optimize that? -- Cheers, David