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=1793874233; b=TTLdqjJXOcKZm5LeLhW2stzcG8EGlEQOmh3BYgeuWTmKUTuc4IC6ywMbA0BynZW4cLKO nET9vkQIIAWlOhGf38mDRHErqL86iAYaCpIIfKF9N90yqcxn6Y3zhJJU9y6X5T3sr3dJC vyCTKaTkfKKHSRM3h7bvCC730od5sJgViA1z6XlrOCxu44O/ko0lQzNfA2V5gjapLt2uH EnCacCpbi2DhyaDwvxQ0VkmRSSHucyUmBCP3QKxLjtY0ePb4/na3Xk2j/zSTU18fv6pbW mtXDEZe9SSGu8MWjv1MkrIKWz9B6lqhN2H2EzH2JKz6bFRCgN2rgXQOq+sPfXx0OOYQ== ARC-Message-Signature: i=1; d=subspace.kernel.org; s=arc20261015; a=rsa-sha256; c=relaxed/relaxed; t=1793874233; 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=iF+E3A4lrYTQexomXs4BQ8l6HqAmssOEMa18CwE0L+c=; b=PY5ppJsZRhzKvNgvYHqOb8fbXs0Ogweet0ubzIbhP6kTTTVimWkhnUqRT04fXUa1EWEJ W9a44Io0RmAILQ/aO6rqh+LkNO+Z/R4TrVFpSlmExSUc9kTI4b53pBpE6xqAAYSIoIiNy xIltasH/wzO78vW0ef8CzcSWRfd3zWhRThSfj7ST7CbMkbIXDJ8Xs9da5DJqzo1LoBofT B2qIvNjk+mZZbEchqtZTpNabz/4cPDEmRsRfCFwbmdTjBgh996OA37LIv/TCBPXSp8P1I DHzpF4S/NRX/fRmxkyaF7BsLqrJY9EMoVlhQtUXsVSvwTg1Hj7iZRfavUY8gIZh1gWg== 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=NbrR9cYL; 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 AA454606A31B; Thu, 5 Nov 2026 10:23:52 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; dkim=pass (2048-bit key, unprotected) header.d=kernel.org header.i=@kernel.org header.a=rsa-sha256 header.s=k20260515 header.b=NbrR9cYL Received: by smtp.kernel.org (Postfix) with ESMTPSA id 85B221F00919; Thu, 5 Nov 2026 10:23:44 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=kernel.org; s=k20260515; t=1793874232; bh=iF+E3A4lrYTQexomXs4BQ8l6HqAmssOEMa18CwE0L+c=; h=Date:Subject:To:Cc:References:From:In-Reply-To; b=NbrR9cYLZhNHNpuDXvfPn4BsboYHzSp7gAlLtvj5WDDUdGPrsoxzVo27FDAU1Kstv jsbVLoX8hc7ZnvctVffNI8bZIiEpgNqujSQknPFncZPbV/h8pmUuKi8aCfULwX6J6y NGFZMnr+P5VIMf078k3sa1mT17JuiFg7c/AVxeg3bJyBt2BGE4BW35N2f73065VeAc ndogGL2WjONv92vt8T7F4zyf0xhw2M/Zp266ns4P2ZIFKTKHq1pJYJ0XA0VoHYQUD3 YmUWAE9qU+8F7uvo2FFyvasZ4KYNXyMPkF6uNZ/Kc/hMd3hlTB2FzWhFYESx+LiuCe oma2cgh+v5YMQ== Message-ID: Date: Thu, 5 Nov 2026 11:23:41 +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 2/5] mm/khugepaged: scan mm's more efficiently under mmap_lock contention 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 , Peter Zijlstra , Ingo Molnar , Will Deacon , Boqun Feng , Waiman Long , Suren Baghdasaryan , Vlastimil Babka , Shakeel Butt , Yonghong Song , Alexei Starovoitov , Daniel Borkmann , Andrii Nakryiko , Eduard Zingerman , Kumar Kartikeya Dwivedi , Martin KaFai Lau , Song Liu , Jiri Olsa , Emil Tsalapatis , Ihor Solodrai Cc: linux-mm@kvack.org, linux-kernel@vger.kernel.org, bpf@vger.kernel.org References: <20261103-khugepaged-latency-fix-v1-0-a07ab16ac860@kernel.org> <20261103-khugepaged-latency-fix-v1-2-a07ab16ac860@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: <20261103-khugepaged-latency-fix-v1-2-a07ab16ac860@kernel.org> Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 7bit On 11/3/26 09:27, Lorenzo Stoakes (ARM) wrote: > Ostensibly khugepaged collapse traverses eligible mm's, scanning up to "Ostensibly", now I need a dictionary to read commit messages :P > /sys/kernel/mm/transparent_hugepage/khugepaged/pages_to_scan eligible pages > at a time, sleeping for /sys/.../scan_sleep_millisecs between each > traversal. > > mm's are scanned in collapse_scan_mm_slot() under the mmap read lock, > obtained using mmap_read_trylock(), skipping the mm if there is lock > contention, and dropping the lock before allocation. > > khugepaged will keep scanning the same mm until it is about to be torn > down, prctl(PR_SET_THP_DISABLE) disables THP for it, mm->mmap_lock > contention occurs, or all its VMAs have been scanned. > > When any of these occur, khugepaged doesn't keep track of where it left > off, as the current address is tracked in > khugepaged_scan.address (essentially a global value), which is reset. > > This is a problem when lock contention occurs, as when the mm is next > scanned, it starts from address 0 again. > > If there's a lot of mmap_lock contention this can significantly slow down > collapse later in the mm, since the /sys/.../pages_to_scan budget can run > out before the end of the mm is reached. > > The budget is incremented (as tracked in cc->progress) at a rate of 1 page > per ineligible VMA, but the budget is only checked once eligible VMAs are > encountered. > > Then, it only takes the combined total of ineligible VMAs and eligible but > non-collapsible pages to exceed the /sys/.../pages_to_scan budget for the > rest of the mm to be skipped. > > For a system with 4 KiB page size and default sysctl settings this only > requires 4096 ineligible VMAs/uncollapsible pages to be skipped before this > happens. > > Examples of eligible but uncollapsible ranges are those containing a large > number of forked PTE entries (more than /sys/.../max_ptes_shared), swapped > out PTE entries (more than /sys/.../max_ptes_swap) or none or zero-page PTE > entries (more than /sys/.../max_ptes_none). > > The latter might seem unlikely to happen given /sys/.../max_ptes_none > defaults to HPAGE_PMD_NR - 1 and CONFIG_PT_RECLAIM frees PTE page tables, > however CONFIG_PT_RECLAIM only removes the page table if the zapped range > spans at least a PMD size (see pte_table_reclaim_possible()). > > Thus, userland software that applies madvise(..., MADV_DONTNEED) across > sub-PMD ranges can thwart this, which is quite feasible for memory > allocators and GC heaps, for instance. > > Resolve the issue by simply tracking the last examined address in the > khugepaged_mm_slot object, so when the mm is resumed, the scan starts from > where it left off. > > Additionally, skip to the next candidate if a VMA cannot be collapsed. > > This issue was noticed during some work in which an mm was instantiated > with a large number of small VMAs under significant mmap_lock contention, > during which collapse was observed to take 10.8s. With this patch applied > it took 3.6s to collapse the same mm. > > Also, while here, update a couple VM_BUG_ON()'s to VM_WARN_ON_ONCE()'s. > > This patch is also a pre-requisite for a later patch in this series. > > Signed-off-by: Lorenzo Stoakes (ARM) > --- > mm/khugepaged.c | 38 ++++++++++++++++++++------------------ > 1 file changed, 20 insertions(+), 18 deletions(-) > > diff --git a/mm/khugepaged.c b/mm/khugepaged.c > index aad8797f8d5a..216c4b8d28c4 100644 > --- a/mm/khugepaged.c > +++ b/mm/khugepaged.c > @@ -69,23 +69,23 @@ static struct kmem_cache *mm_slot_cache __ro_after_init; > /** > * struct khugepaged_mm_slot - khugepaged information per mm that is being scanned > * @slot: hash lookup from mm to mm_slot > + * @address: the next address inside that to be scanned > */ > struct khugepaged_mm_slot { > struct mm_slot slot; > + unsigned long address; > }; I could have sworn I saw a similar patch from Nico at some point ... maybe it was in a private discussion. Am I correct or is the short version of this change "Remember the next scan address per MM instead of starting from address 0 whenever we continue scanning a MM after a skip; and we might skip under mmap lock contention" ? I think a lot of the commit message above can be simplified. Reviewed-by: David Hildenbrand (Arm) -- Cheers, David