From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from us-smtp-delivery-124.mimecast.com (us-smtp-delivery-124.mimecast.com [170.10.129.124]) (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 746D625DAE6 for ; Fri, 28 Feb 2025 15:18:10 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=170.10.129.124 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1740755892; cv=none; b=P6SUzdJjNA1xF4/ByPyYmXk77xYPicVq7mFxXLnO/4ZxbPTYnfNYl+ZCw8cErvF//g9sUyFAMhbRV3Rb99w+RkwmMG51AB1FO35F4/s5y2sB0pv4usnTvh/IonfFA4Xn2IB6ZHv/nByQPL8kSnqynBaOIDVAiHzNW9MKSLTdkB8= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1740755892; c=relaxed/simple; bh=XhIt/KoZqwDvvkIfHWRz0q4gswKfJ2Xd7V1fYHDEERM=; h=Message-ID:Date:MIME-Version:Subject:To:Cc:References:From: In-Reply-To:Content-Type; b=QNmlEbMZNiEgZ7IgOmdyGGeORUNE0y83gI2VRn0zXCNIpV3i3j3fJbMnH2TuoT7O1P1dEp/+xj2DUtB+HTtbpBiwCv7ur494UOM2h0ZIEX9mgdWL9S1pAWzux7qCfF9n8btqaOMLI+RfOJAol/xCBaRF4R+Hk7zJokI3TUGIElo= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dmarc=pass (p=none dis=none) header.from=redhat.com; spf=pass smtp.mailfrom=redhat.com; dkim=pass (1024-bit key) header.d=redhat.com header.i=@redhat.com header.b=Y8Bcfo4P; arc=none smtp.client-ip=170.10.129.124 Authentication-Results: smtp.subspace.kernel.org; dmarc=pass (p=none dis=none) header.from=redhat.com Authentication-Results: smtp.subspace.kernel.org; spf=pass smtp.mailfrom=redhat.com Authentication-Results: smtp.subspace.kernel.org; dkim=pass (1024-bit key) header.d=redhat.com header.i=@redhat.com header.b="Y8Bcfo4P" DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=redhat.com; s=mimecast20190719; t=1740755889; h=from:from:reply-to:subject:subject:date:date:message-id:message-id: to:to:cc:cc:mime-version:mime-version:content-type:content-type: content-transfer-encoding:content-transfer-encoding: in-reply-to:in-reply-to:references:references:autocrypt:autocrypt; bh=d3RzxgVOKGkYis1oh2Ggnk5D21zUMvhHIGAVWotSvO4=; b=Y8Bcfo4PGp1r1/SZaaVi2SC/E2JX2PPnJwi7VGJRVgXE9AfQw2i6DpD5JbYnTKoA9eopz5 IeXp9fBRtzuuXEMRt5gSI7cJvTE2M8wwjQayF13Dxf1eY6411Fr4VcvgaPaNL0n7yDd8Zw GpfhfbPARhqPNL1IyzSI66ujy4vwOZo= Received: from mail-wr1-f72.google.com (mail-wr1-f72.google.com [209.85.221.72]) by relay.mimecast.com with ESMTP with STARTTLS (version=TLSv1.3, cipher=TLS_AES_256_GCM_SHA384) id us-mta-363-eyLpiShzMfq3-xcfYiwRIw-1; Fri, 28 Feb 2025 10:18:07 -0500 X-MC-Unique: eyLpiShzMfq3-xcfYiwRIw-1 X-Mimecast-MFC-AGG-ID: eyLpiShzMfq3-xcfYiwRIw_1740755887 Received: by mail-wr1-f72.google.com with SMTP id ffacd0b85a97d-38f4e3e9c5bso977074f8f.1 for ; Fri, 28 Feb 2025 07:18:07 -0800 (PST) X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20230601; t=1740755886; x=1741360686; h=content-transfer-encoding:in-reply-to:organization:autocrypt :content-language:from:references:cc:to:subject:user-agent :mime-version:date:message-id:x-gm-message-state:from:to:cc:subject :date:message-id:reply-to; bh=d3RzxgVOKGkYis1oh2Ggnk5D21zUMvhHIGAVWotSvO4=; b=PfimNP2zjnMSzNgDNv8rRsK57szhoKHfOBTTFI8c5g/iY6MUIxprx3MdiPU8dxIMPT nY4nMKkR6VWMKgfAJj1ZVqtPGUWKuGCLmwI8maeWXd73Svwh74n0j6zLYcdWsOZLJXXj jPsTEGOxat+rGbfujEFQ+BEGykFx/8sWwTFKf1JeOWB4V+3Xo/AzOPApMDqhJRAVy2n3 URraQ5oIZcj4nEEQKhS3heL0q/9KyQqhf5uLamlymGqnUkihzYZUBc2WmZWbY0kJfMda 9mcMS7YtXYEDBUNYfaFs/CGz9Xw7qv3KA9zIGcfr2sKZsPQNVRB6Rf3pQuqOGJfVPtW2 LNjQ== X-Forwarded-Encrypted: i=1; AJvYcCVCEEt2EhxtydrLsJJq4MZZi4lgyYLLaW3i42CGKEM46/Lj5ilf5sTPSc3Nl1sg8jbX6QkvkLz8eLFEawY=@vger.kernel.org X-Gm-Message-State: AOJu0Yy30T5NApKeWrhIHpxAdJPDFzKYq4uyuUHTJC/2XWAvrYipz1ya 4vJA6VgTAtiaBKsex/1oWLTO7qoeaSVBVbC+/lFaCKzmL+jiUaT+9VupMNp8S06q/ZZYcFQph4Q J1l0mGrIuVwoksiI/oYhZC24fsqV+9szl7cviPaT8lhyEQXYKDkDZdXbsZOiUlw== X-Gm-Gg: ASbGncvLGT6gA0Z5lQqDWwIhanTfRhH+SbyF3tTLx5/t2m6bof2H3JJmoeMhqDpNuuD kAJHajsYfG6z4kNX/G7X18n3ayYhkLg8+8Q8UENg3Fg+5lysQ8sRXhrjdliry3Z930vxCKcvsZf 7tXZl/IAGzAITAAgA37Mee3vmHpNhauPE0eiTXwQmXFFhkLeS464Uxq8u4O9OvyJDFihLkHa8ME BPyS/kYb1dJvzEuXldG80oeEuvCPsfMiP6sI+w1QRGVKJSVzg1Xh9/0B8hdJAMOPfLU48N68UVw a3r//nsyD4ZNIA99JUqeYYDF X-Received: by 2002:a05:6000:1f88:b0:38b:f44b:8663 with SMTP id ffacd0b85a97d-390eca38bfdmr3863877f8f.55.1740755885152; Fri, 28 Feb 2025 07:18:05 -0800 (PST) X-Google-Smtp-Source: AGHT+IE+h8QzgQEysGgPFIp2qJ/GwMRCcXRRRrDEODCU9GYT3OUmcARrVeMuokjbfP7Emvy7x9Abdw== X-Received: by 2002:a05:6000:1f88:b0:38b:f44b:8663 with SMTP id ffacd0b85a97d-390eca38bfdmr3863654f8f.55.1740755883253; Fri, 28 Feb 2025 07:18:03 -0800 (PST) Received: from ?IPV6:2a02:8070:4688:86a0::a40b? ([2a02:8070:4688:86a0::a40b]) by smtp.gmail.com with ESMTPSA id 5b1f17b1804b1-43aba5870e7sm95040575e9.35.2025.02.28.07.18.02 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Fri, 28 Feb 2025 07:18:02 -0800 (PST) Message-ID: <7c5cc18e-71e0-4ebb-bb0d-5765a9467753@redhat.com> Date: Fri, 28 Feb 2025 16:18:01 +0100 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 0/2] SKSM: Synchronous Kernel Samepage Merging To: Mathieu Desnoyers , Linus Torvalds Cc: Andrew Morton , linux-kernel@vger.kernel.org, Matthew Wilcox , Olivier Dion , linux-mm@kvack.org References: <20250228023043.83726-1-mathieu.desnoyers@efficios.com> <8524caa9-e1f6-4411-b86b-d9457ddb8007@efficios.com> <029759d4-f7b2-4ec6-b5d0-7c8a1c0fbd80@redhat.com> <78ebaad9-c7ba-423a-a824-c2b1a499aea6@efficios.com> 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 ZW5icmFuZCA8ZGF2aWRAcmVkaGF0LmNvbT7CwZgEEwEIAEICGwMGCwkIBwMCBhUIAgkKCwQW AgMBAh4BAheAAhkBFiEEG9nKrXNcTDpGDfzKTd4Q9wD/g1oFAl8Ox4kFCRKpKXgACgkQTd4Q 9wD/g1oHcA//a6Tj7SBNjFNM1iNhWUo1lxAja0lpSodSnB2g4FCZ4R61SBR4l/psBL73xktp rDHrx4aSpwkRP6Epu6mLvhlfjmkRG4OynJ5HG1gfv7RJJfnUdUM1z5kdS8JBrOhMJS2c/gPf wv1TGRq2XdMPnfY2o0CxRqpcLkx4vBODvJGl2mQyJF/gPepdDfcT8/PY9BJ7FL6Hrq1gnAo4 3Iv9qV0JiT2wmZciNyYQhmA1V6dyTRiQ4YAc31zOo2IM+xisPzeSHgw3ONY/XhYvfZ9r7W1l pNQdc2G+o4Di9NPFHQQhDw3YTRR1opJaTlRDzxYxzU6ZnUUBghxt9cwUWTpfCktkMZiPSDGd KgQBjnweV2jw9UOTxjb4LXqDjmSNkjDdQUOU69jGMUXgihvo4zhYcMX8F5gWdRtMR7DzW/YE BgVcyxNkMIXoY1aYj6npHYiNQesQlqjU6azjbH70/SXKM5tNRplgW8TNprMDuntdvV9wNkFs 9TyM02V5aWxFfI42+aivc4KEw69SE9KXwC7FSf5wXzuTot97N9Phj/Z3+jx443jo2NR34XgF 89cct7wJMjOF7bBefo0fPPZQuIma0Zym71cP61OP/i11ahNye6HGKfxGCOcs5wW9kRQEk8P9 M/k2wt3mt/fCQnuP/mWutNPt95w9wSsUyATLmtNrwccz63XOwU0EVcufkQEQAOfX3n0g0fZz Bgm/S2zF/kxQKCEKP8ID+Vz8sy2GpDvveBq4H2Y34XWsT1zLJdvqPI4af4ZSMxuerWjXbVWb T6d4odQIG0fKx4F8NccDqbgHeZRNajXeeJ3R7gAzvWvQNLz4piHrO/B4tf8svmRBL0ZB5P5A 2uhdwLU3NZuK22zpNn4is87BPWF8HhY0L5fafgDMOqnf4guJVJPYNPhUFzXUbPqOKOkL8ojk CXxkOFHAbjstSK5Ca3fKquY3rdX3DNo+EL7FvAiw1mUtS+5GeYE+RMnDCsVFm/C7kY8c2d0G NWkB9pJM5+mnIoFNxy7YBcldYATVeOHoY4LyaUWNnAvFYWp08dHWfZo9WCiJMuTfgtH9tc75 7QanMVdPt6fDK8UUXIBLQ2TWr/sQKE9xtFuEmoQGlE1l6bGaDnnMLcYu+Asp3kDT0w4zYGsx 5r6XQVRH4+5N6eHZiaeYtFOujp5n+pjBaQK7wUUjDilPQ5QMzIuCL4YjVoylWiBNknvQWBXS lQCWmavOT9sttGQXdPCC5ynI+1ymZC1ORZKANLnRAb0NH/UCzcsstw2TAkFnMEbo9Zu9w7Kv AxBQXWeXhJI9XQssfrf4Gusdqx8nPEpfOqCtbbwJMATbHyqLt7/oz/5deGuwxgb65pWIzufa N7eop7uh+6bezi+rugUI+w6DABEBAAHCwXwEGAEIACYCGwwWIQQb2cqtc1xMOkYN/MpN3hD3 AP+DWgUCXw7HsgUJEqkpoQAKCRBN3hD3AP+DWrrpD/4qS3dyVRxDcDHIlmguXjC1Q5tZTwNB boaBTPHSy/Nksu0eY7x6HfQJ3xajVH32Ms6t1trDQmPx2iP5+7iDsb7OKAb5eOS8h+BEBDeq 3ecsQDv0fFJOA9ag5O3LLNk+3x3q7e0uo06XMaY7UHS341ozXUUI7wC7iKfoUTv03iO9El5f XpNMx/YrIMduZ2+nd9Di7o5+KIwlb2mAB9sTNHdMrXesX8eBL6T9b+MZJk+mZuPxKNVfEQMQ a5SxUEADIPQTPNvBewdeI80yeOCrN+Zzwy/Mrx9EPeu59Y5vSJOx/z6OUImD/GhX7Xvkt3kq Er5KTrJz3++B6SH9pum9PuoE/k+nntJkNMmQpR4MCBaV/J9gIOPGodDKnjdng+mXliF3Ptu6 3oxc2RCyGzTlxyMwuc2U5Q7KtUNTdDe8T0uE+9b8BLMVQDDfJjqY0VVqSUwImzTDLX9S4g/8 kC4HRcclk8hpyhY2jKGluZO0awwTIMgVEzmTyBphDg/Gx7dZU1Xf8HFuE+UZ5UDHDTnwgv7E th6RC9+WrhDNspZ9fJjKWRbveQgUFCpe1sa77LAw+XFrKmBHXp9ZVIe90RMe2tRL06BGiRZr jPrnvUsUUsjRoRNJjKKA/REq+sAnhkNPPZ/NNMjaZ5b8Tovi8C0tmxiCHaQYqj7G2rgnT0kt WNyWQQ== Organization: Red Hat In-Reply-To: <78ebaad9-c7ba-423a-a824-c2b1a499aea6@efficios.com> Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit On 28.02.25 16:01, Mathieu Desnoyers wrote: > On 2025-02-28 08:59, David Hildenbrand wrote: >> On 28.02.25 06:17, Linus Torvalds wrote: >>> On Thu, 27 Feb 2025 at 19:03, Mathieu Desnoyers >>> wrote: >>>> >>>> I'd be fine with SKSM replacing KSM entirely. However, I don't >>>> think we should try to re-implement the existing KSM userspace ABIs >>>> over SKSM. >>> >>> No, absolutely. The only point (for me) for your new synchronous one >>> would be if it replaced the kernel thread async scanning, which would >>> make the old user space interface basically pointless. >>> >>> But I don't actually know who uses KSM right now. My reaction really >>> comes from a "it's not nice code in the kernel", not from any actual >>> knowledge of the users. >>> >>> Maybe it works really well in some cloud VM environment, and we're >>> stuck with it forever. >> >> Exactly that; and besides the VM use-case, lately people stated using it >> in the context of interpreters (IIRC inside Meta) quite successfully as >> well. >> > > I suspect that SKSM is a better fit for JIT and code patching than KSM, > because user-space knows better when a set of pages is going to become > invariant for a long time and thus benefit from merging. This removes > the background scanning from the picture. > > Does the interpreter use-case require background scanning, or does > it know when a set of pages are meant to become invariant for a long > time ? To make the JIT/interpreter use case happy, people wanted ways to *force* KSM on for *the whole process*, not just individual VMAs like the traditional VM use case would have done. I recall one of the reasons being that you don't really want to modify your JIT/interpreter to just make KSM work. See [1] "KSM at Meta" for some details, and in general, optimization work to adapt KSM to new use cases. Regarding some concerns you raised, Stefan did a lot of optimization work like "smart scanning" (slide "Optimization - Smart Scan (6.7)") to reduce the scanning overhead and make it much more efficient. So people started optimizing for that already and got pretty good results. [1] https://lpc.events/event/17/contributions/1625/attachments/1320/2649/KSM.pdf -- Cheers, David / dhildenb