From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org Received: from bombadil.infradead.org (bombadil.infradead.org [198.137.202.133]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by smtp.lore.kernel.org (Postfix) with ESMTPS id E9A1AC55822 for ; Tue, 4 Aug 2026 21:47:26 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; q=dns/txt; c=relaxed/relaxed; d=lists.infradead.org; s=bombadil.20210309; h=Sender:List-Subscribe:List-Help :List-Post:List-Archive:List-Unsubscribe:List-Id:Content-Transfer-Encoding: Content-Type:In-Reply-To:From:References:Cc:To:Subject:MIME-Version:Date: Message-ID:Reply-To:Content-ID:Content-Description:Resent-Date:Resent-From: Resent-Sender:Resent-To:Resent-Cc:Resent-Message-ID:List-Owner; bh=b4Tb1oIjWwUxjFhTBFD/zWgNtoh6N0RIyHfctUTX0zA=; b=C/1bz4iVVqlZzSP/FSLmYlFfrX aWEE5DGiIvgOErxYLdxQ+hzmjZFwc4/lZxPVmQgHoiz7gwhc7o5JOIKvUJ4zIupWXlMRZK20Rv+c3 iseeLy0L/kkgLX371hAyaDqweSib0WlMudydYb8hpDjxjvY6TA99XXF3l0VFXGRiT5V9DFeZaAMgJ FOmuWATFf3fFYLYcqwjWYHmhOK3vQYt1AwEo8HPX2h9wvkPMbkCPcPLYwLSebXjwQVHh6zRmikBPF E/YYn7rCZLT47SE3sCxbSSb+r9aFhD7lqgt6DjN1K6e4UgNNnFUdH4qyvg55VPYiZP1pkE4sYkGoN 3X10pHxg==; Received: from localhost ([::1] helo=bombadil.infradead.org) by bombadil.infradead.org with esmtp (Exim 4.99.1 #2 (Red Hat Linux)) id 1wrMyg-00000002ppM-0JtM; Tue, 04 Aug 2026 21:47:14 +0000 Received: from tor.source.kernel.org ([2600:3c04:e001:324:0:1991:8:25]) by bombadil.infradead.org with esmtps (Exim 4.99.1 #2 (Red Hat Linux)) id 1wrMyf-00000002ppF-07o5 for linux-arm-kernel@lists.infradead.org; Tue, 04 Aug 2026 21:47:13 +0000 Received: from smtp.kernel.org (quasi.space.kernel.org [100.103.45.18]) by tor.source.kernel.org (Postfix) with ESMTP id C770E600AA; Tue, 4 Aug 2026 21:47:11 +0000 (UTC) Received: by smtp.kernel.org (Postfix) with ESMTPSA id 82C651F000E9; Tue, 4 Aug 2026 21:47:05 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=kernel.org; s=k20260515; t=1785880031; bh=b4Tb1oIjWwUxjFhTBFD/zWgNtoh6N0RIyHfctUTX0zA=; h=Date:Subject:To:Cc:References:From:In-Reply-To; b=anIpDvCbLz1al5lEwizeQ9TRqV0GfEwS3ElIBSUJbK9mwo6IQiwKGEfx9O3lKpIe5 6rLdNm+ZejMW8uukRRb/U3GnkcSRU5N4Qgrcrbhej0DmvEgRPRShipVaDF+ifEO29O un+yx37gRX/Ir3fQ5Occp8CBb7sXLKEvdaBZCGGb0EBsAiEBMJtcJ2rPre27JqQaow j5dDnMSADfw7+CXmEYORzsAS3nPPS4nYt4Cs/K35WAbVCEZAfOOtMTVDYKH/DNuyCz zcBZ3GzUGKJAdAqEAhdP1CCEk3DSYInSPWkW6cpK9Ae6V+Fn5ShxZOAaJZfpi5l3cq q6FXjyJt6QDtQ== Message-ID: Date: Tue, 4 Aug 2026 23:47:03 +0200 MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [RFC v2 PATCH 0/16] Optimize this_cpu_*() ops for non-x86 (ARM64 for this series) To: "Christoph Lameter (Ampere)" Cc: "Lorenzo Stoakes (ARM)" , Mark Rutland , Yang Shi , Ryan Roberts , dennis@kernel.org, tj@kernel.org, urezki@gmail.com, catalin.marinas@arm.com, will@kernel.org, akpm@linux-foundation.org, hca@linux.ibm.com, gor@linux.ibm.com, agordeev@linux.ibm.com, linux-mm@kvack.org, linux-arm-kernel@lists.infradead.org, linux-kernel@vger.kernel.org, Linus Torvalds , Jason Gunthorpe References: <20260715180455.515692-1-yang@os.amperecomputing.com> <0344c559-1959-4531-9265-d5a5180eb7cd@arm.com> <25d1e09b-53e4-7cd5-87db-b58437e4e690@gentwo.org> <4887267b-dc26-4c33-96ca-8dff054a0d1f@kernel.org> <63ea8156-a109-b2a2-6d35-1c17ad7f1a9d@gentwo.org> <84b31836-1d10-4bec-a469-468b91fd4645@kernel.org> <9f146ce0-4adc-0c00-a2d8-c01b47edc9d1@gentwo.org> From: "David Hildenbrand (Arm)" Content-Language: en-US Autocrypt: addr=david@kernel.org; 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charset=UTF-8 Content-Transfer-Encoding: 7bit X-BeenThere: linux-arm-kernel@lists.infradead.org X-Mailman-Version: 2.1.34 Precedence: list List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Sender: "linux-arm-kernel" Errors-To: linux-arm-kernel-bounces+linux-arm-kernel=archiver.kernel.org@lists.infradead.org On 8/4/26 23:25, Christoph Lameter (Ampere) wrote: > On Tue, 4 Aug 2026, David Hildenbrand (Arm) wrote: > >>> rather hacky restart logic. It relies on a long preemable and postscript >>> to each per cpu operations. > >> But I am sure you did you homework and have data to back up your claims. Please >> share that data, because I am very curious. > > The comparison here is a single instruction on one side and abnd the per > cpu operation prefixed with a preeamble and another instruction / set of > instructions on the other side. > > Even if its only one before and one after then we are still looking at 3 > instructions instead of one. This is a frequently used operation in the > VM. > > You need data to show that one instruction is faster than 3??? I want you to show that the difference *matters* in *practice* enough that we should *care* about accepting the additional *complexity*. No hand-waving about "3 > 1". IIRC people were asking that previously but received no results back. So to be clear: claiming that the additional instructions are unacceptable I'd expect something minimal like: "removing these 3 instructions speeds up micro-benchmark (a, b, c) by X% and macro-benchmarks (d, e, f) by Y% compared to Mark's approach". Maybe there really is a big difference. > >> Mark's solution is the obvious improvement to the problem, doing it just like >> s390 already does. > > Well there is interest by the S390 folks to move to what we proposed from > what I can tell. Heiko said that as reply to v1, bit he's been replying a lot on Mark's patches. Only Heiko can tell. (on CC) > > >> If the proposed solution has a significant benefit over Mark's solution it could >> be discussed as a next step; and it must be backed with sufficient data to >> warrant all this complexity here. > > You cannot accept that one instruction is faster than the same > instruction prefixed with another and something else at its tail? I am pretty sure that you got exactly what I meant, you likely just don't have the data to back it up. Proof me wrong! -- Cheers, David