From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from kanga.kvack.org (kanga.kvack.org [205.233.56.17]) by smtp.subspace.kernel.org (Postfix) with ESMTP id B10F5606A32C for ; Thu, 5 Nov 2026 10:23:56 +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 kanga.kvack.org (Postfix) id D86896B05B2; Thu, 5 Nov 2026 05:23:55 -0500 (EST) Received: by kanga.kvack.org (Postfix, from userid 40) id D2FF46B05B5; Thu, 5 Nov 2026 05:23:55 -0500 (EST) X-Delivered-To: int-list-linux-mm@kvack.org Received: by kanga.kvack.org (Postfix, from userid 63042) id AC6F06B05B2; Thu, 5 Nov 2026 05:23:55 -0500 (EST) X-Delivered-To: linux-mm@kvack.org Received: from relay.hostedemail.com (smtprelay0017.hostedemail.com [216.40.44.17]) by kanga.kvack.org (Postfix) with ESMTP id 823A56B05B2 for ; Thu, 5 Nov 2026 05:23:55 -0500 (EST) Received: from smtpin29.hostedemail.com (lb01a-stub [10.200.18.249]) by unirelay07.hostedemail.com (Postfix) with ESMTP id 0ACEE1608F5 for ; Thu, 5 Nov 2026 10:23:55 +0000 (UTC) X-FDA: 85400679630.29.DF20B22 Received: from sea.source.kernel.org (sea.source.kernel.org [172.234.252.31]) by imf19.hostedemail.com (Postfix) with ESMTP id 2EAD71A0007 for ; Thu, 5 Nov 2026 10:23:53 +0000 (UTC) Authentication-Results: imf19.hostedemail.com; dkim=pass header.d=kernel.org header.s=k20260515 header.b=NbrR9cYL; spf=pass (imf19.hostedemail.com: domain of david@kernel.org designates 172.234.252.31 as permitted sender) smtp.mailfrom=david@kernel.org; dmarc=pass (policy=quarantine) header.from=kernel.org ARC-Seal: i=1; a=rsa-sha256; d=hostedemail.com; s=arc-20220608; cv=none; t=1793874233; b=x36B4fBPeLb8qT6nnCFbZhalXnHpacGF6gnrIXg6/R14OjDx2qJjRzpzw6bbemGnyhu3aw t0ejLuiPTuhvtf+mU6APqnD/2eysm4aue+kcsBXS7qNSNdlm9bMbzVjyL61BmGXF7uaKkN rv3Z+IoD/TC73Dh2giuWQTygeSv+Pjs= ARC-Authentication-Results: i=1; imf19.hostedemail.com; dkim=pass header.d=kernel.org header.s=k20260515 header.b=NbrR9cYL; spf=pass (imf19.hostedemail.com: domain of david@kernel.org designates 172.234.252.31 as permitted sender) smtp.mailfrom=david@kernel.org; dmarc=pass (policy=quarantine) header.from=kernel.org ARC-Message-Signature: i=1; a=rsa-sha256; c=relaxed/relaxed; d=hostedemail.com; s=arc-20220608; t=1793874233; h=from:from:sender: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:dkim-signature; bh=iF+E3A4lrYTQexomXs4BQ8l6HqAmssOEMa18CwE0L+c=; b=cP1VexRJrIZDC4miLNpcevSOtiL4XnGLIKVO1pOvnv81RQajdYYXZCxBp/5teU9agjubmz jvlBxieEJGYBvTwZG9D/qVhkOichxLQGY3bGNjp8graHc/ltrXgWGUvZIGTHC4P4/VU8Se TpZC8v2e4fsarXHzTYWIEboE9Y+Pd8A= Received: from smtp.kernel.org (quasi.space.kernel.org [100.103.45.18]) by sea.source.kernel.org (Postfix) with ESMTP id 683754091C; Thu, 5 Nov 2026 10:23:52 +0000 (UTC) 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-Archived-By: 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 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<20261103-khugepaged-latency-fix-v1-2-a07ab16ac860@kernel.org> Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 7bit X-Spam-Status: No, score=-17.3 required=3.0 tests=BAYES_00,DKIMWL_WL_HIGH, DKIM_SIGNED,DKIM_VALID,DKIM_VALID_AU,HEADER_FROM_DIFFERENT_DOMAINS, INCLUDES_CR_TRAILER,INCLUDES_PATCH,MAILING_LIST_MULTI,SPF_HELO_PASS, SPF_PASS,USER_AGENT_SANE_1 autolearn=ham autolearn_force=no version=4.0.1 X-Rspamd-Server: rspam07 X-Rspamd-Queue-Id: 2EAD71A0007 X-Stat-Signature: spfbuunb95aufhykskxppcgbz3hx5np6 X-Rspam-User: X-HE-Tag: 1793874233-560857 X-HE-Meta: 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 Fxfu4GY1 fHtMSihTgS+Tk4JPqOTcm9r1zZ7uQK4JZHNjwSOs6OxzI7UkUhzdM/8F/K9ogTkENEz7a7aX7eoEEJKzp7UXaj6NZ7HJCeUTzvpKm+sThA0sKSRnc0eewTpgsZjXI0xeWVs90HmuUo3hgT/dsa9OEoJawmyom+QJ6QYqsDW0+XupWdOAfVaa7W8cVqei4wIhMFsxroPaKv/zFHZ8hCmQXwUbhxAHZgs7j1SRQ2pg2akQ0yYdqRlpU7M1VTi6Bnwlgu+XUXWNWob3+0bhd5Mj8MlWS5hLreOrcxJ1L5KG48Ua+YfKS/jnf2LUZU3OqhMKjaEiz7U2RXzFuBMU= Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: X-Spam-Checker-Version: SpamAssassin 4.0.1 (2024-03-26) on aws-us-west-2-korg-listserv-alma10-1 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