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 9A09430B508 for ; Tue, 23 Jun 2026 09:19:43 +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=1782206384; cv=none; b=l8QwJtIgMeJkLZU37bcOrLtL9ppkqKKYx/+zoXuGHBVIpkEnOWaNQjD1pXltENuA6MONuhZ5fwzoUEllA+GxBZmNuLxOEt3nvfFDtI/udn4W8BL2r8THjpKYVuDQURuwbQJo3OiZs0fDJ09qzDZlsLYgckBJOSynwWAFc1mlGQA= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1782206384; c=relaxed/simple; bh=MEMJ0XdS+82WuqtiLAsgLlSrWdfwY0Dwf0E0hCvfA9Q=; h=Message-ID:Date:MIME-Version:Subject:To:Cc:References:From: In-Reply-To:Content-Type; b=ug35SxsRmtEmkTJGFxIIch87j4TtYEcAcCFGgbjrbO4UPX7igVqVXTmqc8Bh9sPQwZRQZGp9WmO7k1cVkIbe0uWvLGEx/a0erfwKS2JAuQ/bE627EDx7rEtDdFQj67tHq8ddAbG0V0RbPDSG7jwE8usC8zxe5Mle1fUwUC3pxzY= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b=nFVtQfvT; 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="nFVtQfvT" Received: by smtp.kernel.org (Postfix) with ESMTPSA id B209F1F000E9; Tue, 23 Jun 2026 09:19:34 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=kernel.org; s=k20260515; t=1782206383; bh=7/ZDlIzWUndvoYE+PX4E3AjeijBVQXYalQoydvzOzPI=; h=Date:Subject:To:Cc:References:From:In-Reply-To; b=nFVtQfvT9AfvbFLnnzylgEGUQnr6IMpTPOSnvSKsEpUQRT3kmxRC/9cIH8RcoicFk FFqU0TxSxGw0AC734lZRcpPIckloggVgYsorxJRrlsS2F+yYMN+QASLxuKtyIpQZFJ ayUlZMOQRYP1y8NlaEymDArMwbwC17WLg8f489AltFPVDiIMel21JzVgzqjP2uFfIA Px5SHNV/GEBqQzro9+d4LuGKuH18tWjK2s6Q6lGt6BpxD7dYIewlVNXN0LVmoNbdKe SU2nDDxT77JLiEg4l94NmZFddXYXm0Ks1tJtM96EOgZPtbo0ge5zH2lSnraykHTUTg sxVHwOIeo/7+A== Message-ID: <0b7df58d-c403-4bab-b703-1e40e64a2eee@kernel.org> Date: Tue, 23 Jun 2026 11:19:32 +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 v2 00/13] Dynamic Kernel Stacks To: David Laight Cc: Zach O'Keefe , Thomas Gleixner , Dave Hansen , "H. Peter Anvin" , David Stevens , Pasha Tatashin , Linus Walleij , Will Deacon , Quentin Perret , Ingo Molnar , Borislav Petkov , Dave Hansen , x86@kernel.org, Andy Lutomirski , Xin Li , Peter Zijlstra , Andrew Morton , Lorenzo Stoakes , "Liam R. Howlett" , Vlastimil Babka , Mike Rapoport , Suren Baghdasaryan , Michal Hocko , Uladzislau Rezki , Kees Cook , linux-kernel@vger.kernel.org, linux-mm@kvack.org, Matthew Wilcox References: <20260424191456.2679717-1-stevensd@google.com> <6369e5ce-74e3-4c68-8053-d7d7d21b6955@zytor.com> <87pl1md7h0.ffs@fw13> <87qzm2b39k.ffs@fw13> <87mrwon5uw.ffs@fw13> <20260623101018.789a4094@pumpkin> 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: <20260623101018.789a4094@pumpkin> Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 7bit On 6/23/26 11:10, David Laight wrote: > On Tue, 23 Jun 2026 09:50:00 +0200 > "David Hildenbrand (Arm)" wrote: > > ... >> There was some (hallway?) talk at LSF/MM about possibly removing direct reclaim, >> similar to how other operating systems handle it. Now, I don't know how feasible >> it is (I guess devil is in the detail ;) ), or any details how that would work, >> but direct reclaim was repeatedly called out as one of the main reasons we can >> get huge stacks. >> >> So I guess direct reclaim (incl. compaction) is one of the main problematic >> pieces. Are we aware of other scenarios where we (easily) trigger consumption of >> larger stacks? > > I suspect some printk in error paths could get problematic. > Especially if they involve direct writes to a non-trivial console > (does that happen any more??). Good question! > How much stack does dump_stack() need? > >> Wild idea: as a first step to test the waters, use smaller stacks on selected >> kernel threads and disallow direct reclaim/compaction if the stack for the >> thread is small? > > Or if there isn't much stack space left? > > Isn't reclaim one of the places that is actually likely to exceed the stack > limit because it is unusual and won't be exercised at most of the kmalloc() > call sites? > One you get to the slow bits (especially ones that do IO) it is unlikely > that a context-switch to a special worker thread will make much difference. > (You might need to boost the thread priority based on the priority of the > waiting process.) I think that was exactly the idea of removing direct reclaim: instead of doing reclaim yourself, you trigger another (reclaim) thread (like kswapd) and wait for it to finish. -- Cheers, David