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 7F94EC56208 for ; Thu, 6 Aug 2026 11:22:10 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; q=dns/txt; c=relaxed/relaxed; d=lists.infradead.org; s=bombadil.20210309; h=Sender:Cc:List-Subscribe: List-Help:List-Post:List-Archive:List-Unsubscribe:List-Id:In-Reply-To: Content-Type:MIME-Version:References:Message-ID:Subject:To:From:Date:Reply-To :Content-Transfer-Encoding:Content-ID:Content-Description:Resent-Date: Resent-From:Resent-Sender:Resent-To:Resent-Cc:Resent-Message-ID:List-Owner; bh=63QAIwnchL8Oj8xhlzOBO2I3PZCelkaEyl/+cgVci/I=; b=DJBmRVELO136suzTOGhQo47p4Y taXMPp1LdTsGa41s1hHLgYI63WXP1aTEpLDhHg+0nbfC1MSHqpW/5uTHcGVT+usVv9IYho+iuz2It MtQTIvC1CPapopETkZ7517O1eRr3EtiXSb6jeT6pONkGBOQOQHQg4EsahHhUyHiMisC4jjB/nfUFR 9J45cCpy0acKXdbbAWZ1sYk13p79JQ8rOqBSm5GLJHhRUAkJI1Q3T9YpOmAtjUlMuVgAmGD0m0B4t o5mbLRU46WnvsQNpiZqYSfpX1ZjkqmCz4PFVZ9IsLoGuIxT0NhTh12jISNsBV1VrmSGl+CsnAWo22 B65yzI7w==; Received: from localhost ([::1] helo=bombadil.infradead.org) by bombadil.infradead.org with esmtp (Exim 4.99.1 #2 (Red Hat Linux)) id 1wrwAl-00000005dQN-1CxS; Thu, 06 Aug 2026 11:22:03 +0000 Received: from foss.arm.com ([217.140.110.172]) by bombadil.infradead.org with esmtp (Exim 4.99.1 #2 (Red Hat Linux)) id 1wrwAh-00000005dOi-07L6 for linux-arm-kernel@lists.infradead.org; Thu, 06 Aug 2026 11:22:00 +0000 Received: from usa-sjc-imap-foss1.foss.arm.com (unknown [10.121.207.14]) by usa-sjc-mx-foss1.foss.arm.com (Postfix) with ESMTP id 90784153B; Thu, 6 Aug 2026 04:21:52 -0700 (PDT) Received: from J2N7QTR9R3 (usa-sjc-imap-foss1.foss.arm.com [10.121.207.14]) by usa-sjc-imap-foss1.foss.arm.com (Postfix) with ESMTPSA id 0E4753F66F; Thu, 6 Aug 2026 04:21:53 -0700 (PDT) DKIM-Signature: v=1; a=rsa-sha256; c=simple/simple; d=arm.com; s=foss; t=1786015316; bh=NxvDQNUaxLdXEBOORglSp9h3tEgNpMWW3m9cpoWPstw=; h=Date:From:To:Cc:Subject:References:In-Reply-To:From; b=ge6pnxV1Fubr8Qf/BlaxjwFA9FcvmWmwAOcMjbmy/xhWZznGrFdl2G8X+S9wNbJzm TNGnyKVyombig3MpytVxqOG7/D4ybXCWFfUgY1RZ4RgxViO9f3/Vt8Gtql6RWD9nSR E9svhYLwwPHhMkHcaOLXXIZEoJNpmf86rg5JYf6Y= Date: Thu, 6 Aug 2026 12:21:51 +0100 From: Mark Rutland To: "David Hildenbrand (Arm)" Subject: Re: [PATCH v2 13/20] arm64: percpu: Add infrastructure for preemptible this_cpu_*() ops Message-ID: References: <20260804170503.3513916-1-mark.rutland@arm.com> <20260804170503.3513916-14-mark.rutland@arm.com> <7df18d5e-886e-43d1-b53d-52a1d08aba49@kernel.org> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: X-CRM114-Version: 20100106-BlameMichelson ( TRE 0.9.0 (BSD) ) MR-646709E3 X-CRM114-CacheID: sfid-20260806_042159_165236_B5E932AD X-CRM114-Status: GOOD ( 19.30 ) 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: , Cc: vladimir.murzin@arm.com, ryan.roberts@arm.com, peterz@infradead.org, catalin.marinas@arm.com, david.laight.linux@gmail.com, stable@vger.kernel.org, ruanjinjie@huawei.com, james.morse@arm.com, yang@os.amperecomputing.com, cl@gentwo.org, maz@kernel.org, ljs@kernel.org, will@kernel.org, ardb@kernel.org, linux-arm-kernel@lists.infradead.org Sender: "linux-arm-kernel" Errors-To: linux-arm-kernel-bounces+linux-arm-kernel=archiver.kernel.org@lists.infradead.org On Wed, Aug 05, 2026 at 08:47:08AM +0200, David Hildenbrand (Arm) wrote: > On 8/5/26 08:45, David Hildenbrand (Arm) wrote: > >> Atop v7.2-rc4, with GCC 15.2.0 and defconfig, this is compiled as: > >> > >> | : > >> | paciasp > >> | stp x29, x30, [sp, #-16]! > >> | mrs x2, sp_el0 > >> | mov x29, sp > >> | ldr w3, [x2, #8] > >> | add w3, w3, #0x1 > >> | str w3, [x2, #8] > >> | mrs x3, tpidr_el1 > >> | add x0, x0, x3 > >> | 1: ldxr x5, [x0] > >> | add x5, x5, x1 > >> | stxr w4, x5, [x0] > >> | cbnz w4, 1b > >> | ldr x0, [x2, #8] > >> | sub x0, x0, #0x1 > >> | str w0, [x2, #8] > >> | cbz x0, 2f > >> | ldr x0, [x2, #8] > >> | cbnz x0, 3f > >> | 2: bl preempt_schedule_notrace > >> | 3: ldp x29, x30, [sp], #16 > >> | autiasp > >> | ret > > > > FWIW, in a recent discussion on some prototype hacking [1] we saw some overhead > > in micro-benchmarks that would really hammer on a path that would now do a > > preempt_disable()+preempt_enable(). > > > > Switching from preempt_disable() to preempt_enable_no_resched() made it turn to > > noise. Of course, that has other undesirable impacts, and I am not sure if we > > are in the territory of code layout changes affecting the numbers. > > > > Just mentioning it as some data point. > > [1] https://lore.kernel.org/linux-mm/20260630174852-mutt-send-email-mst@kernel.org/ Thanks for the pointer. IIUC in those cases you're using preempt_disable() .. preempt_enable() directly, not this_cpu_*(), right? If so, patches 5 and 6 of this series [2,3] might have an impact, but I wouldn't expect a significant change unless you're calling preempt_enable a lot. Please beware that it's not safe to use preempt_enable_no_resched() UNLESS it is immediately followed by a call to schedule(). That's not documented today (and I couldn't find a good reference), so more folk are likely to be tempted to use it... I'll send a patch to clarify that (or at least start the discussion with scheduler/preempt folk). Mark. [2] https://lore.kernel.org/linux-arm-kernel/20260804170503.3513916-6-mark.rutland@arm.com/ [3] https://lore.kernel.org/linux-arm-kernel/20260804170503.3513916-7-mark.rutland@arm.com/