From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from smtp.kernel.org (aws-us-west-2-korg-mail-1.web.codeaurora.org [10.30.226.201]) (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 8E2A53CF02B for ; Mon, 18 May 2026 08:43:32 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=10.30.226.201 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1779093812; cv=none; b=UreLQ9vdueFqnzJa2RGvRUZ5+zDUCyUO6ZQ4e2O9+g4XSbGnWtVV5KzpOz9OunS2y0gDEsIJqicqJBxnoZG53TiHWaR9EOAI2XvLj+1MHVP1v48+Ci9WkauOIbZFCVuCBtPRDO+dpf3x3U6FMtPOWTtU9rNBkfFc3F3np0lQ+vU= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1779093812; c=relaxed/simple; bh=Hh5WO9rSYoc4MeLyVuH9muGrehc2Jj8o8p7T0hzyPL0=; h=Message-ID:Date:MIME-Version:Subject:To:Cc:References:From: In-Reply-To:Content-Type; b=oi11Vi02H7iFj0rQO5iAclfSH6+heyS00IQm5mJAjk76HZlorktiXBdv5tB+rdy0V8gD5HRGRbWLXRO8P9/tAGxd2I3fKMiVBfp+lu7kAp6Q7aS+7sQFkSXhn8sfT3pLAL/hIjzebJ/9GNN914DmOe3VhGLGrENFlT5CzpWn26I= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b=iNGUBgh7; arc=none smtp.client-ip=10.30.226.201 Authentication-Results: smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b="iNGUBgh7" Received: by smtp.kernel.org (Postfix) with ESMTPSA id B0156C2BCB7; Mon, 18 May 2026 08:43:24 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=k20201202; t=1779093812; bh=Hh5WO9rSYoc4MeLyVuH9muGrehc2Jj8o8p7T0hzyPL0=; h=Date:Subject:To:Cc:References:From:In-Reply-To:From; b=iNGUBgh7SPTuSW/PxOmGzyuSjMWm+OpaCa4g/vudm0BacYg9QFqNqeEMIx5SQ4hua XAo3w/dQqQjbfMVBXisS2RVHFoxWDHUpeErwY9o1j7zxWdBdwQ4VXoaO0LGgIOUPaY kyiuYYZah2bgPkIBpjQDknxGNrDI9S+SarAthYDF9KPjvtWxQ9vsdNp8/kgGzh1Z/z zwEMBezoJIYWidOue1LIy7vzsPdAFUdZCNm05FLDdWUfSo3cYDVPd4KvE2176x5LRH gm6rjr+EtG74dGmgmQh1fqiLFnMeL8QP+Ma0fGnXpnPPqB8LWwV+gvGWIwqHdL2EF3 UNFbbACsPfjGg== Message-ID: <60fb497d-a35a-42b9-8bf1-7f9ccbde8ade@kernel.org> Date: Mon, 18 May 2026 10:43:22 +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: [RFC PATCH 1/1] mm: batch page copies in folio_copy() and folio_mc_copy() To: "Garg, Shivank" , Andrew Morton , linux-mm@kvack.org, linux-kernel@vger.kernel.org, x86@kernel.org Cc: Lorenzo Stoakes , "Liam R . Howlett" , Vlastimil Babka , Mike Rapoport , Suren Baghdasaryan , Michal Hocko , Thomas Gleixner , Ingo Molnar , Borislav Petkov , Dave Hansen , "H . Peter Anvin" , Ankur Arora , Bharata B Rao , Hrushikesh Salunke , David Rientjes , sandipan.das@amd.com References: <20260427142036.111940-2-shivankg@amd.com> <20260427142036.111940-4-shivankg@amd.com> <073e5e2c-7102-4141-b0d7-fa5635f811f5@kernel.org> <6a5e794a-a608-4126-9abe-0d512a57dd67@amd.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: <6a5e794a-a608-4126-9abe-0d512a57dd67@amd.com> Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 8bit On 5/14/26 07:17, Garg, Shivank wrote: > > > On 5/12/2026 3:01 PM, David Hildenbrand (Arm) wrote: >> On 4/27/26 16:20, Shivank Garg wrote: >>> Rewrite folio_copy() and folio_mc_copy() as thin wrappers around new >>> batched helpers copy_highpages() and copy_mc_highpages(). >>> >>> The current implementations iterate copy_highpage() (or its #MC-aware >>> variant) per 4 KB page. For a single 2 MB folio that loop runs 512 >>> times and pays, per page: >>> >>> - kmap_local_page() / kunmap_local() >>> - cond_resched() >>> - one invocation of the architecture copy_page()/memcpy() primitive >>> >>> The new helpers issue a single copy_mc_to_kernel()/memcpy() over >>> the whole contiguous range when CONFIG_HIGHMEM is off and no >>> architecture overrides (__HAVE_ARCH_COPY_HIGHPAGE) copy_highpage(). >>> HIGHMEM and arch overrides keep the existing per-page path. >>> >>> Tested on dual-socket AMD EPYC 9655 (Zen 5) with a CXL.mem node. >>> In-kernel folio_mc_copy() microbenchmark on 2 MB folios, source >>> evicted from cache before each iteration and measured throughput: >>> >>> direction baseline GB/s optimized GB/s speedup >>> DRAM0 -> DRAM1 18.65 ± 1.37 38.03 ± 3.21 2.04x >>> DRAM0 -> CXL 25.46 ± 2.89 39.29 ± 1.17 1.54x >>> CXL -> DRAM0 20.61 ± 3.95 35.07 ± 0.62 1.70x >>> >>> End-to-end move_pages(2) throughput on anonymous 2 MB mTHP folios, >>> 1 GB migrated per run: >>> >>> direction baseline GB/s optimized GB/s speedup >>> DRAM0 -> DRAM1 7.20 ± 0.03 8.01 ± 0.02 1.11x >>> DRAM0 -> CXL 11.12 ± 0.15 13.07 ± 0.03 1.18x >>> DRAM1 -> DRAM0 7.21 ± 0.02 7.95 ± 0.02 1.10x >>> CXL -> DRAM0 9.10 ± 0.05 9.49 ± 0.01 1.04x >>> >>> On AMD EPYC 7713 (Zen 3 / Milan, REP_GOOD without FSRM/ERMS) the >>> folio_copy() bulk path regresses because memcpy() falls through to >>> memcpy_orig (an unrolled movq loop), which is slower than the >>> per-page copy_page() (microcoded rep movsq) it replaces. >> >> Do you know what the reason for that fallback is? Could it be fixed (e.g., when >> we detect page alignment or sth like that?) >> > > The fallback is gated on X86_FEATURE_FSRM in arch/x86/lib/memcpy_64.S: > > SYM_TYPED_FUNC_START(__memcpy) > ALTERNATIVE "jmp memcpy_orig", "", X86_FEATURE_FSRM > movq %rdi, %rax > movq %rdx, %rcx > rep movsb > RET > > AMD Zen 3 does not have FSRM, so it jmp to memcpy_orig (unrolled movq loop). > > > On v7.1.0-rc3, I measured these primitives and the kernel's actual memcpy() > across three CPUs, using a kernel module that vmallocs 16MB src/dst buffer > and times each primitive for comparison. > Numbers are mean (in GB/s) ± SD% (= SD as percent of mean). > > > 1.) AMD EPYC 7713 (Zen 3), Flags: rep_good only, no ERMS/FSRM: > > size unrolled_movq GB/s±SD% rep_movsq GB/s±SD% kernel_memcpy GB/s±SD% > ------------------------------------------------------------------------------ > 16B 0.38± 8.73% 0.41± 0.43% 0.43± 0.31% > 32B 0.85± 0.19% 0.80± 8.37% 0.84± 0.07% > 64B 1.68± 0.35% 1.60± 0.03% 1.59± 9.37% > 128B 3.23± 0.22% 3.04± 0.62% 3.19± 0.03% > 256B 5.99± 5.78% 5.62± 4.15% 5.93± 0.42% > 512B 10.07± 1.36% 10.49± 2.60% 10.02± 0.21% > 1K 14.49± 0.09% 18.19± 0.37% 14.31± 3.48% > 2K 17.11± 1.01% 28.04± 2.37% 18.14± 0.56% > 4K 18.36± 0.22% 39.15± 0.50% 19.57± 1.14% > > - kernel_memcpy is tracking unrolled_movq. > - rep_movsq is 1.4x-2x faster than the unrolled_movq fallback for >= 1 KiB. > > 2.) On Intel(R) Xeon(R) Platinum 8362 > Flags: rep_good, erms, fsrm > > size unrolled_movq GB/s±SD% rep_movsq GB/s±SD% rep_movsb GB/s±SD% kernel_memcpy GB/s±SD% > -------------------------------------------------------------------------------------------- > 16B 0.89± 0.93% 0.64± 0.10% 0.69± 0.57% 0.66± 3.52% > 32B 2.08± 2.46% 1.28± 0.15% 1.38± 6.21% 1.33± 4.28% > 64B 3.97± 2.26% 2.55± 0.24% 2.83± 0.22% 2.65± 4.48% > 128B 7.45± 0.09% 5.00± 2.53% 5.48± 5.04% 5.30± 1.60% > 256B 13.24± 0.01% 9.79± 0.57% 10.12± 0.37% 9.81± 0.34% > 512B 21.67± 0.03% 17.87± 0.02% 18.43± 0.79% 17.81± 0.25% > 1K 27.84± 1.96% 34.54± 1.24% 35.67± 1.88% 34.56± 2.49% > 2K 32.67± 2.35% 59.58± 0.01% 65.67± 0.18% 59.35± 1.12% > 4K 34.85± 0.64% 95.35± 0.00% 96.64± 0.69% 95.35± 0.00% > > - kernel_memcpy is using rep_movsb (FSRM in use). > - Below 512 B the unrolled movq loop is ~20-50% faster, >1 KiB FSRM wins. > > 3.) On AMD EPYC 9655 96-Core Processor (Zen 5) > Flags: rep_good, erms, fsrm > > size unrolled_movq GB/s±SD% rep_movsq GB/s±SD% rep_movsb GB/s±SD% kernel_memcpy GB/s±SD% > -------------------------------------------------------------------------------------------- > 16B 0.53± 0.39% 0.53± 0.21% 0.55± 0.13% 0.53± 0.14% > 32B 1.13± 1.49% 1.06± 0.07% 1.09± 0.16% 1.06± 0.09% > 64B 2.21± 0.12% 2.13± 0.07% 2.18± 0.14% 2.13± 0.09% > 128B 4.25± 0.12% 4.26± 0.10% 4.37± 0.12% 4.31± 0.14% > 256B 8.01± 0.19% 8.61± 0.27% 8.61± 0.18% 8.51± 0.10% > 512B 14.14± 0.18% 16.80± 0.24% 16.80± 0.23% 16.81± 0.24% > 1K 22.93± 0.73% 31.70± 0.48% 32.37± 0.28% 32.02± 0.22% > 2K 30.36± 0.27% 53.24± 1.01% 56.58± 0.22% 56.04± 0.22% > 4K 35.05± 0.65% 80.25± 0.41% 83.90± 0.20% 76.23± 0.37% > > - kernel_memcpy is using rep_movsb (FSRM in use). > - For smaller size, unrolled movq are close enough to be within noise. > > > Regarding the fix, > One option is to make memcpy() fall back to rep movsq instead of unrolled > movq loop when FSRM is absent. The data shows the benefit on Zen 3. For > the Intel, unrolled movq is faster for smaller sizes. > But, I'm not sure if adding these complexities to memcpy() is welcome. > Happy to work on this if it is helpful. I was wondering whether optimizing memcpy() further would be of value elsewhere. Of course, we wouldn't want to degrade it :) Some direction from x86 folks would be nice. > > Another option is to leave memcpy() untouched for this series and add > a new copy_pages() helper that the folio copy path can use. It would > use ALTERNATIVE_2 that picks rep movsb on ERMS/FSRM and rep movsq on > REP_GOOD and per-page copy_page() loop as the final fallback. That would fit the clear_pages() design we have. But if that's avoidable, that would be nice. -- Cheers, David