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 CDCBEC56208 for ; Thu, 6 Aug 2026 13:30:30 +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: Content-Transfer-Encoding:Content-Type:In-Reply-To:From:References: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=rwX9iK0odyquXbuoqMvw+Qag1xkTbpizu9Idecdr2L8=; b=ZTRo39JzT6TR6N jvN8a0t8sbg8t66u3ztH/tlPbOK9RowazJncH1bw8yTf73lb55zJ3SDB6/WIlZoBEcEBZ0XXWoyG3 nH16MwmFYB0qkS52RvBp9q78fvJioPLRacxgp1UpqK88ffmlyBSSvYiHZ5M4fXmTQpHVOzDqx+1Sv Zgopmxfp80F4o4QBRbTEpd2dpOot9ivdUwqV0MqPYCp4o/vNI4JKq+fhrPZS4d8T0O0kh3k3VWmjC hMNfFIkTSHhXajXtdG24BF6NGdpjPpTyTNJNTuvSjL0T5yZ7ieeQloTRIsTKSzic1vxWmeCk4PRgm X48UjPznBkZgys++4vkQ==; Received: from localhost ([::1] helo=bombadil.infradead.org) by bombadil.infradead.org with esmtp (Exim 4.99.1 #2 (Red Hat Linux)) id 1wryAt-00000005uUu-1K7N; Thu, 06 Aug 2026 13:30:19 +0000 Received: from tor.source.kernel.org ([172.105.4.254]) by bombadil.infradead.org with esmtps (Exim 4.99.1 #2 (Red Hat Linux)) id 1wryAr-00000005uUU-39SL for linux-arm-kernel@lists.infradead.org; Thu, 06 Aug 2026 13:30:17 +0000 Received: from smtp.kernel.org (quasi.space.kernel.org [100.103.45.18]) by tor.source.kernel.org (Postfix) with ESMTP id EA42960A5C; Thu, 6 Aug 2026 13:30:16 +0000 (UTC) Received: by smtp.kernel.org (Postfix) with ESMTPSA id 302A71F000E9; Thu, 6 Aug 2026 13:30:11 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=kernel.org; s=k20260515; t=1786023016; bh=rwX9iK0odyquXbuoqMvw+Qag1xkTbpizu9Idecdr2L8=; h=Date:Subject:To:Cc:References:From:In-Reply-To; b=fzwoSCTP5xg+7Rt00qMZIVNKsUtO18dRpDuXs65GcfH8vm+tt5HOxt9f+T5DWSa3y tlRVYu063XH0b7xkS0CIIZXmaHwPWZ4SO+DptqFHPSSd3RSctfAHJewWT1HwyhruMT w3/19YPJmBdlQ7JoiPmvqK9+9oZR/X2ywg3Cj38sth9Og4FXBU9X7/MvCUQ1rliLmP 0MmOSd12aEfPHDT+VAFzZwsaAUr5neJCbfngXyJDTeunMFrUFCzG9t63M37xw6KA3I Aq7MxXhT6qzK/lXAAB0waMKoF5G9XKiYCvhkEWFT8EAANVoD8BIAajEECk6B4Yb/Ig yeulu60HM4WfQ== Message-ID: <9067f9d0-6dfd-4a8b-96a6-f904fb937c8b@kernel.org> Date: Thu, 6 Aug 2026 15:30:10 +0200 MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH v2 13/20] arm64: percpu: Add infrastructure for preemptible this_cpu_*() ops To: David Laight References: <20260804170503.3513916-1-mark.rutland@arm.com> <20260804170503.3513916-14-mark.rutland@arm.com> <7df18d5e-886e-43d1-b53d-52a1d08aba49@kernel.org> <9c3feb9d-3111-4f37-90f7-3ad3c2e094c7@kernel.org> <20260806142527.0c784b42@pumpkin> 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: , Cc: Mark Rutland , vladimir.murzin@arm.com, ryan.roberts@arm.com, peterz@infradead.org, catalin.marinas@arm.com, ruanjinjie@huawei.com, stable@vger.kernel.org, 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 8/6/26 15:25, David Laight wrote: > On Thu, 6 Aug 2026 13:32:52 +0200 > "David Hildenbrand (Arm)" wrote: > >> On 8/6/26 13:21, Mark Rutland wrote: >>> >>> Thanks for the pointer. >>> >>> IIUC in those cases you're using preempt_disable() .. preempt_enable() >>> directly, not this_cpu_*(), right? >> >> It was purely preempt_disable/preempt_enable experiments without any percpu stuff. > > Did you check that preempt_enable() isn't likely to speculatively execute the > schedule() call. > Even if you write: > if (unlikely(a == b)) > function(); > the compiler tends to generate a forwards branch around the function call. I didn't look closer (Michael ran the experiments), in particular, why it added overhead. I was assuming that it's just suboptimal code generation as you and Mark hinted. It was sufficient for us to understand that the overhead we saw can be removed somehow, but as we dropped the approach entirely, we didn't look closer into that. -- Cheers, David