From mboxrd@z Thu Jan 1 00:00:00 1970 Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.remote-ip=100.103.45.18 ARC-Seal: i=1; d=subspace.kernel.org; s=arc20261015; a=rsa-sha256; cv=none; t=1794475613; b=HekMVrfC4l14LdTDUe3M08uc+3EPyUG6ztsXwdkbUa+gUhlgQkBZ2Rzir5EugewVTath +O5iO348QpVwmAsJFl5kg05lr6cinILOOnO8lCeGJ7fJJgNlLCAoWLgz92TEFbfFFdJF3 rpDp1T/s1bFylf8Xe4snAVbgHnloivwmhURRlPJxTm8FGAnV8bw7WY8I9bdX4TYyBis9D F/or3N/x17G8H9Im9M/TE2m+vDU1bSXoDTT89rgqyuml54vMlfT0XdpFDBM7o9S/y8zMW nRULb3zhEFNcAGYxaxdv7b99SIJfTbDHS4wI+qmokSZwO1aUJHpRIggcSJLfKiKK2Qg== ARC-Message-Signature: i=1; d=subspace.kernel.org; s=arc20261015; a=rsa-sha256; c=relaxed/relaxed; t=1794475613; h=Received:DKIM-Signature:Message-ID:Date:MIME-Version:User-Agent: Subject:To:Cc:References:From:Content-Language:Autocrypt:In-Reply-To: Content-Type:Content-Transfer-Encoding; bh=6llN3WGsJ9HH0olCgh6YbKzYaA4IOfRnHqvyyuyt2z4=; b=ipljsMgqKIq0+/H5X3aBuNOWhjJROom5GHIg6tE1cat6hPYMGXsyJ6XDBI8BqK/SN8Ua Gsymo6caUCOjaE7I5Mpem2Tz2n4ObELk8uoKGIH/r0Zk5B/XNZRmoJ6PX3exZFriUebow hdnquAyWJHAALVnZTt+6OybILmkDOYBhWoSU6KnNPoq82PiiEDj/EqGA7wDY9nB7ASDYZ HZStG/ZykAU7tGw5SmgLq8MOuY21KytZdcD6xhje5pmFO9r4/lKhejDRS/R3TJIcxPfAB epHIiALzyZG+jSXmR7FcpJCtZr5P7t5eo66yiYlVogbUz0EDc0iRBui1Dsyuich90Cg== ARC-Authentication-Results: i=1; smtp.subspace.kernel.org; dkim=pass header.d=kernel.org header.i=@kernel.org header.a=rsa-sha256 header.s=k20260515 header.b=PKhHwF3j; arc=none smtp.remote-ip=100.103.45.18 Received: from smtp.kernel.org (quasi.space.kernel.org [100.103.45.18]) (using TLSv1.3 with cipher TLS_AES_256_GCM_SHA384 (256/256 bits) key-exchange X25519 server-signature ECDSA (prime256v1)) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id 4571F606A331 for ; Thu, 12 Nov 2026 09:26:53 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.a=rsa-sha256 header.s=k20260515 header.b=PKhHwF3j Received: by smtp.kernel.org (Postfix) with ESMTPSA id 684BC1F00A0B; Thu, 12 Nov 2026 09:26:49 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=kernel.org; s=k20260515; t=1794475613; bh=6llN3WGsJ9HH0olCgh6YbKzYaA4IOfRnHqvyyuyt2z4=; h=Date:Subject:To:Cc:References:From:In-Reply-To; b=PKhHwF3jHeR+K40Zwx1KzkLdBPuVfLLkSwb9frVG7uXK6WFgawSqR3KQldi7tr7iY KGRKzEoZ4gk5fE1MYCq2lzljvS+G+b0KJXvygqEpJQmPx1/4BMj8ZAmZtgcMaI83iP CvhYFFin/kHUZ1Dkh6NK6G+SHd1+ycJ+OGkFWtG25ABHxh7vwI5CCXvsy8eK7oumKH yjW9qm7ytfFqu1YUCKn6EmUjibHUov6WIsDLm94DFRarZkSIJRK8EeBeqV3CbD/4Gh GwXtz8FOole5V0AyUvJb9FyIJ3IUJyWZwIuIbwzcAeCcOcKQmiTLyUTIru6Nawwyqi EM2tz8rbQ2YIQ== Message-ID: Date: Thu, 12 Nov 2026 10:26:46 +0100 X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Post: List-Help: List-Subscribe: List-Unsubscribe: List-Owner: Precedence: list MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH v3 3/3] mm/khugepaged: release contended mmap read lock after scanning enough VMAs To: "Lorenzo Stoakes (ARM)" , Zi Yan , Baolin Wang , "Liam R. Howlett" , Nico Pache , Ryan Roberts , Dev Jain , Barry Song , Lance Yang , Usama Arif , Kiryl Shutsemau , Andrew Morton Cc: linux-mm@kvack.org, linux-kernel@vger.kernel.org References: <20261110-khugepaged-latency-fix-v3-0-6eea743cf9a3@kernel.org> <20261110-khugepaged-latency-fix-v3-3-6eea743cf9a3@kernel.org> 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: Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 7bit On 11/10/26 20:09, Lorenzo Stoakes (ARM) wrote: > On Tue, Nov 10, 2026 at 05:19:13PM +0000, Lorenzo Stoakes (ARM) wrote: >> khugepaged walks an mm's VMAs under the mmap read lock in >> collapse_scan_mm_slot(), checking for ineligible VMAs first, and skipping >> them if encountered. >> >> It charges cc->progress 1 page against the /sys/.../pages_to_scan budget >> when these are encountered, however only checking this budget once an >> eligible PMD-spanning VMA is encountered. >> >> It can therefore hold the mmap read lock for a long time if there are a >> large number of ineligible VMAs present in an mm, over which time no mmap >> write locks (and thus no VMA write locks) can be acquired. >> >> Address this by checking whether the mmap lock is contended after 1-10 >> ms (depending on CONFIG_HZ), i.e. the granularity of jiffies. This is a >> long time for a lock to be held, so is a reasonable compromise for >> determining when best to do this. > > Oops I left this in from the previous approach :) > > Replace with: > > Address this by checking whether the mmap lock is contended after 1024 > VMAs scanned. This was measured to take around ~100 us on a fast > machine, and given VMA eligibility checking is a light operation, it is > likely this will remain at the same order of magnitude for slower > machines. > > This therefore establishes a reasonable upper bound on how long a > contended mmap lock is held. > > Thanks! :) Fixing it up as I am applying :) -- Cheers, David