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=1793874539; b=PzhZkZnB6dJKuAjA7Ns7eztjG2f6GtmfVSJ3++075D0DQ45DR+83cscZsxeJP7j9fOHD SEvEWiViLgTQJheCNUknZM87zKLDkIk0W95z8tPkmacvW0pGrwsy8AJOLbkNenn62bIzz gvUL4Srq+RCSBKjr9yV+t5WhmLD172rKRRbpQRN7OF/YOqVxWQKuNowJdk3Ep44XY3i4k cIakjoFifYC23zG/c5q+vPrmNIvaJaPPEwm0KPKoq/l196PX6giAgQ0vj1hOG2qZQ6gMj v39aJ+J5P4304MaBG1dxdL8VfMF8eriuD6OZC1J3oVTa2+OCvn4k0y1hxoyQScw2jEA== ARC-Message-Signature: i=1; d=subspace.kernel.org; s=arc20261015; a=rsa-sha256; c=relaxed/relaxed; t=1793874539; 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=DSkOHXuDBf1pgGwk7mqhtOIuvilX/GFlHMjt2QNkx6I=; b=wxPVwpPWcYfZdb+NP5OVTxOqQyHMxIL6G4Y9acLzXCODKrGAwC9ifz3ArduCFH/+gZFW fi3y/SCrgQ9MtKc1v/QZ+UDMLEGUi8dupFfqltQJ3IQB7/GybibQotCvSWqfLs9O3sTYA O3ZWP/3nsFg9uTOk1Sf0nG5lIH0NSBrOAuFUTYC9/pUSPlXWNd0Tvc3G+kHmZRgzCTqa4 Q7wkjacIXnK0GDJVPsU8fJCJ1p4RKQ+EDgdRuI6az1YsU1R2RyJFPMpd8m67Iqz2PnFU5 EePhiKP5Gx+OINlSbhYLtGsn54XIVPYYvL5zf/JaCQL3W5DmxkZUMwHAydCC78fEZ1g== 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="D5FX/qth"; 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 00F97606A31B; Thu, 5 Nov 2026 10:28:58 +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=D5FX/qth Received: by smtp.kernel.org (Postfix) with ESMTPSA id 9919D1F000C1; Thu, 5 Nov 2026 10:28:50 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=kernel.org; s=k20260515; t=1793874538; bh=DSkOHXuDBf1pgGwk7mqhtOIuvilX/GFlHMjt2QNkx6I=; h=Date:Subject:To:Cc:References:From:In-Reply-To; b=D5FX/qthyK0NqnE0cZ6gDihcGQqcR+80V0Pvt0dHUbgQZE6mFbffUCUREvE5DEvzS 99vhZZ6A/YabImxHJ8X34i2j8aeyZ24ncS/tjQ6Dghp3ahbNi7qRasKjsE4tb+0LDp 8rY2fNrgoGg+v6Mqr7rnbzY3H423iPxrkGBWXayxV5cpx3WLY3ITr67vMcSkW4eDlm PIU7REyUmeiVu7VELSVOleQQdjnEkae/F48HBht2qROXj+W0/yCABs32z49O8Ux/zl V1zSQNF0Brx8TEz+dnYPI0qYdNgOhltl+vaCGSQpG39O6T22vky+xsm0yM6ZTWDwns 2JypCrn1Hzzrw== Message-ID: Date: Thu, 5 Nov 2026 11:28:47 +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 5/5] mm/khugepaged: drop/reacquire mmap read lock if skipped 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 , 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-v1-5-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-v1-5-a07ab16ac860@kernel.org> Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 7bit On 11/3/26 09:27, 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 8 * HPAGE_PMD_NR 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 dropping + re-acquiring the mmap read lock after > KHUGEPAGED_MAX_SKIP_BATCH VMAs have been processed. Use the newly > introduced mmap_read_lock_nosteal() to ensure queued writers take > precedence when doing so. > > This was tested locally with ~1M ineligible VMAs mapped ahead of a > collapsible region of memory, and resulted in a reduced mmap_lock hold time > of 170us down from 95ms. > > This was also observed to reduce mmap_lock stalls in concurrent threads of > a similar duration. > > KHUGEPAGED_MAX_SKIP_BATCH was derived empirically to a conservative value > of 1024 VMAs, which should not interfere with mm's that have an ordinary > number of VMAs. > > Local measurements indicated no change for ordinary use cases, so this is > all upside. > > This work came out of investigations into mmap stalls when getting latency > numbers for an unrelated patch series. > > Signed-off-by: Lorenzo Stoakes (ARM) > --- > mm/khugepaged.c | 15 +++++++++++++++ > 1 file changed, 15 insertions(+) > > diff --git a/mm/khugepaged.c b/mm/khugepaged.c > index 216c4b8d28c4..b9c72ee8b421 100644 > --- a/mm/khugepaged.c > +++ b/mm/khugepaged.c > @@ -66,6 +66,12 @@ static DEFINE_READ_MOSTLY_HASHTABLE(mm_slots_hash, MM_SLOTS_HASH_BITS); > > static struct kmem_cache *mm_slot_cache __ro_after_init; > > +/* > + * The number of VMAs with nothing to scan to walk before dropping and > + * reacquiring the mmap_lock to relieve lock contention. > + */ > +#define KHUGEPAGED_MAX_SKIP_BATCH 1024 I don't quite enjoy this magic number. Besides it saying nothing about "VMAS", so the name it self is pretty unclear. I was wondering whether we can give the "cc->progress += 1" a different weight. Skipping VMAs seems to be more expensive than we thought. (Maybe we should also just say "skip after X" VMAs in general, whether we skipped them or not?) -- Cheers, David