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 8D2CA4AEE2 for ; Thu, 7 Aug 2025 19:33:36 +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=1754595221; cv=none; b=e922NSnTT/Rtrkn0ViWqe05Y5Fub/sfrJ8qpWuV3Yw53XV6VNdSFsdcTfLitLfN7noEo8GDQ0ieo7O1MIisXMpEv61nVu4e+tZHIZ+hKvHVolGhqCIPrkUkQGv96p/r/EGFE9w7VilivdJfMPO2pnaL6xNsN2vnprKH8KIBmU3s= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1754595221; c=relaxed/simple; bh=0PpTd1ChISQ71JxJkunjCFPnT9Z5REI9zAzFmUaURfo=; h=Message-ID:Date:MIME-Version:Subject:To:Cc:References:From: In-Reply-To:Content-Type; b=Ke+dfEbnaDp2TYjZNA01r5ul1vstX/WdcEMU8Aca0od+33EGoSbSkODjWo/M5WVFDvRvoGKi7ALU6QM36n6knxVzx7P2f8gG94z9jRedh31BwkiENIikI3hx+abukqQyfypzc+gELoIP7ronM2K8LNH6L/UjOWGwvfsV2DqU4gA= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dmarc=pass (p=quarantine 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=Xi9eMMs0; arc=none smtp.client-ip=170.10.129.124 Authentication-Results: smtp.subspace.kernel.org; dmarc=pass (p=quarantine 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="Xi9eMMs0" DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=redhat.com; s=mimecast20190719; t=1754595215; 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=L3sfW+BuUN8qg0raSUf+jpJ80763tcAVtLgdzo6FODg=; b=Xi9eMMs0QN3dnKyuDFhVRN/u+u6ey/77miMTF3kJIeTniJdBWOdkXUlPvHpJfnAP9gEV7O /kqgIwy/+OPnuJmtIlFJro7T8S4FMdhade1RGVbFboo6f4iQunyWmi5cAO1d4nNh9xdego /BPk+sYk2MnqB9IZeqi21mnVXAAnS9I= Received: from mail-wm1-f71.google.com (mail-wm1-f71.google.com [209.85.128.71]) by relay.mimecast.com with ESMTP with STARTTLS (version=TLSv1.3, cipher=TLS_AES_256_GCM_SHA384) id us-mta-261-DgPzpR74Nsi3bCtDxZ-x4w-1; Thu, 07 Aug 2025 15:33:34 -0400 X-MC-Unique: DgPzpR74Nsi3bCtDxZ-x4w-1 X-Mimecast-MFC-AGG-ID: DgPzpR74Nsi3bCtDxZ-x4w_1754595213 Received: by mail-wm1-f71.google.com with SMTP id 5b1f17b1804b1-459db6e35c3so10833545e9.2 for ; Thu, 07 Aug 2025 12:33:33 -0700 (PDT) X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20230601; t=1754595213; x=1755200013; 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=L3sfW+BuUN8qg0raSUf+jpJ80763tcAVtLgdzo6FODg=; b=l0AooEVJH7wx2wbjbKs61IaewnOLd5qRT2S5Z0fkIMD4YZB1FGQsk6ImuK17o3nnNf LVryOww7CnN/5eSi8UUwJ91E2As3ieieaNtFch2xxH2Aoj+hS4l67pnrIIXLMGQpBAe5 EmFAiMDhPMnWp2la4+mmutQMNVGEHczzjahq0airJb18CHTiuwRUUCTMoDrlHMMPBN0k 4H/gul64JJm+NIcbAJ3g1cySH0OdlrNxPQGIpvZXb2OAFFDlpG2FPY2ZINGXvCz4TmBE i0XZoaYuI6o/PYst4d4yns0LprjLqPqwLoG+rviOtlrj3J0b3dBAM5Pw6/+fV+zOtFG7 we/w== X-Forwarded-Encrypted: i=1; AJvYcCXP7eY0qe3XxnOMe5oZ20lIgfpT3Eqs+uIpcMBC+K5jqDHaWzMPNnJnv7TMt06520WScW0keXuwC5q4XHU=@vger.kernel.org X-Gm-Message-State: AOJu0YzB4mCxWzy/njSK+Cx+vhEl+oMztto2k9IpTZm1TsdSVa7cM+Qr +PaaENUhe+L3FvvAI8nqobkOf+kwiFo7oFUeODTyj/6ZPG7FiaIiHw+f3jzoRGDhBlbSsrKzdoD CIsoWOhlAdHf3bRlDK/OGwGBN+2ipLY70pX+yuBiDrNFQ4UeEdX6M3/8c6LfCTdmlzg== X-Gm-Gg: ASbGncv0rqsE78t8wUQrGkOsinsXuhHrAwAENnAWVx1+Fc94BHkKcXWJROM/xC8N8Pv tUKgLAFq7RLjrIRk8t9umT5y4i1PDIwvUr2RLUVUn58GUDpjFu8cUOlUYl7aD/1mx+HJ93yt6HL jGTK09M61NId7aKVlY0cCYEyauxhwj0bJlXRC/ksUd2JBkM2JwJFq6OY1YgmAemFeP/FF1mQBhg j+Oc0E/aU2HEk2+hsX3wXnR3DhFef23hOX32FArYA98QzJJwO6WuZe5BZLJI4ODwPxpaxjZHPMB yUvvDxIqE50/p8FUet6oe5ie6pbYjJoQuF6HNSbNVs5YVHVQYyoJck97Oo2tE0GdCnfqnHMJQa/ jOLs316XT7OAvehjIjtzEnB9IyaA4V149uUAycAOWZBIg5vt9I1Xy4w8UnR3TSli8tMo= X-Received: by 2002:a05:600c:474a:b0:455:fc16:9eb3 with SMTP id 5b1f17b1804b1-459f4f3e33bmr1378065e9.33.1754595212778; Thu, 07 Aug 2025 12:33:32 -0700 (PDT) X-Google-Smtp-Source: AGHT+IFKzAaiLE4wWUDzv/kHmD0X6RwWzUT4LbWdsvRdTyvoc2LKZ8O7shwZOIhexGhHBHA6I/MKBg== X-Received: by 2002:a05:600c:474a:b0:455:fc16:9eb3 with SMTP id 5b1f17b1804b1-459f4f3e33bmr1377855e9.33.1754595212308; Thu, 07 Aug 2025 12:33:32 -0700 (PDT) Received: from ?IPV6:2003:d8:2f49:bc00:12fa:1681:c754:1630? (p200300d82f49bc0012fa1681c7541630.dip0.t-ipconnect.de. [2003:d8:2f49:bc00:12fa:1681:c754:1630]) by smtp.gmail.com with ESMTPSA id 5b1f17b1804b1-459e587d378sm103146285e9.23.2025.08.07.12.33.31 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Thu, 07 Aug 2025 12:33:31 -0700 (PDT) Message-ID: Date: Thu, 7 Aug 2025 21:33:30 +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 HOTFIX 6.17] mm/mremap: avoid expensive folio lookup on mremap folio pte batch To: Lorenzo Stoakes , Pedro Falcato Cc: Andrew Morton , "Liam R . Howlett" , Vlastimil Babka , Jann Horn , Barry Song , Dev Jain , linux-mm@kvack.org, linux-kernel@vger.kernel.org References: <20250807185819.199865-1-lorenzo.stoakes@oracle.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/g1oFAmgsLPQFCRvGjuMACgkQTd4Q 9wD/g1o0bxAAqYC7gTyGj5rZwvy1VesF6YoQncH0yI79lvXUYOX+Nngko4v4dTlOQvrd/vhb 02e9FtpA1CxgwdgIPFKIuXvdSyXAp0xXuIuRPQYbgNriQFkaBlHe9mSf8O09J3SCVa/5ezKM OLW/OONSV/Fr2VI1wxAYj3/Rb+U6rpzqIQ3Uh/5Rjmla6pTl7Z9/o1zKlVOX1SxVGSrlXhqt kwdbjdj/csSzoAbUF/duDuhyEl11/xStm/lBMzVuf3ZhV5SSgLAflLBo4l6mR5RolpPv5wad GpYS/hm7HsmEA0PBAPNb5DvZQ7vNaX23FlgylSXyv72UVsObHsu6pT4sfoxvJ5nJxvzGi69U s1uryvlAfS6E+D5ULrV35taTwSpcBAh0/RqRbV0mTc57vvAoXofBDcs3Z30IReFS34QSpjvl Hxbe7itHGuuhEVM1qmq2U72ezOQ7MzADbwCtn+yGeISQqeFn9QMAZVAkXsc9Wp0SW/WQKb76 FkSRalBZcc2vXM0VqhFVzTb6iNqYXqVKyuPKwhBunhTt6XnIfhpRgqveCPNIasSX05VQR6/a OBHZX3seTikp7A1z9iZIsdtJxB88dGkpeMj6qJ5RLzUsPUVPodEcz1B5aTEbYK6428H8MeLq NFPwmknOlDzQNC6RND8Ez7YEhzqvw7263MojcmmPcLelYbfOwU0EVcufkQEQAOfX3n0g0fZz 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+DWgUCaCwtJQUJG8aPFAAKCRBN3hD3AP+DWlDnD/4k2TW+HyOOOePVm23F5HOhNNd7nNv3 Vq2cLcW1DteHUdxMO0X+zqrKDHI5hgnE/E2QH9jyV8mB8l/ndElobciaJcbl1cM43vVzPIWn 01vW62oxUNtEvzLLxGLPTrnMxWdZgxr7ACCWKUnMGE2E8eca0cT2pnIJoQRz242xqe/nYxBB /BAK+dsxHIfcQzl88G83oaO7vb7s/cWMYRKOg+WIgp0MJ8DO2IU5JmUtyJB+V3YzzM4cMic3 bNn8nHjTWw/9+QQ5vg3TXHZ5XMu9mtfw2La3bHJ6AybL0DvEkdGxk6YHqJVEukciLMWDWqQQ RtbBhqcprgUxipNvdn9KwNpGciM+hNtM9kf9gt0fjv79l/FiSw6KbCPX9b636GzgNy0Ev2UV m00EtcpRXXMlEpbP4V947ufWVK2Mz7RFUfU4+ETDd1scMQDHzrXItryHLZWhopPI4Z+ps0rB CQHfSpl+wG4XbJJu1D8/Ww3FsO42TMFrNr2/cmqwuUZ0a0uxrpkNYrsGjkEu7a+9MheyTzcm vyU2knz5/stkTN2LKz5REqOe24oRnypjpAfaoxRYXs+F8wml519InWlwCra49IUSxD1hXPxO WBe5lqcozu9LpNDH/brVSzHCSb7vjNGvvSVESDuoiHK8gNlf0v+epy5WYd7CGAgODPvDShGN g3eXuA== Organization: Red Hat In-Reply-To: Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit On 07.08.25 21:22, Lorenzo Stoakes wrote: > On Thu, Aug 07, 2025 at 08:14:09PM +0100, Pedro Falcato wrote: >> On Thu, Aug 07, 2025 at 07:58:19PM +0100, Lorenzo Stoakes wrote: >>> It was discovered in the attached report that commit f822a9a81a31 ("mm: >>> optimize mremap() by PTE batching") introduced a significant performance >>> regression on a number of metrics on x86-64, most notably >>> stress-ng.bigheap.realloc_calls_per_sec - indicating a 37.3% regression in >>> number of mremap() calls per second. >>> >>> I was able to reproduce this locally on an intel x86-64 raptor lake system, >>> noting an average of 143,857 realloc calls/sec (with a stddev of 4,531 or >>> 3.1%) prior to this patch being applied, and 81,503 afterwards (stddev of >>> 2,131 or 2.6%) - a 43.3% regression. >>> >>> During testing I was able to determine that there was no meaningful >>> difference in efforts to optimise the folio_pte_batch() operation, nor >>> checking folio_test_large(). >>> >>> This is within expectation, as a regression this large is likely to >>> indicate we are accessing memory that is not yet in a cache line (and >>> perhaps may even cause a main memory fetch). >>> >>> The expectation by those discussing this from the start was that >>> vm_normal_folio() (invoked by mremap_folio_pte_batch()) would likely be the >>> culprit due to having to retrieve memory from the vmemmap (which mremap() >>> page table moves does not otherwise do, meaning this is inevitably cold >>> memory). >>> >>> I was able to definitively determine that this theory is indeed correct and >>> the cause of the issue. >>> >>> The solution is to restore part of an approach previously discarded on >>> review, that is to invoke pte_batch_hint() which explicitly determines, >>> through reference to the PTE alone (thus no vmemmap lookup), what the PTE >>> batch size may be. >>> >>> On platforms other than arm64 this is currently hardcoded to return 1, so >>> this naturally resolves the issue for x86-64, and for arm64 introduces >>> little to no overhead as the pte cache line will be hot. >>> >>> With this patch applied, we move from 81,503 realloc calls/sec to >>> 138,701 (stddev of 496.1 or 0.4%), which is a -3.6% regression, however >>> accounting for the variance in the original result, this is broadly >>> restoring performance to its prior state. >>> >> >> So, do we still have a regression then? If so, do we have any idea why? > > It's within 1 stddev of the original results, so I'd say it's possibly > noise. It's very likely noise. And even if it's not, even a simple code layout change by the compiler can provoke something like that. -- Cheers, David / dhildenb