From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S934346AbcATQAQ (ORCPT ); Wed, 20 Jan 2016 11:00:16 -0500 Received: from smarthost01b.mail.zen.net.uk ([212.23.1.3]:51738 "EHLO smarthost01b.mail.zen.net.uk" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S933838AbcATQAN (ORCPT ); Wed, 20 Jan 2016 11:00:13 -0500 Message-ID: <1453305601.3417.12.camel@linaro.org> Subject: Re: [PATCH] iommu/arm-smmu: add a shortcut when the @dev_node is NULL From: "Jon Medhurst (Tixy)" To: Robin Murphy Cc: Huang Shijie , Joerg Roedel , will.deacon@arm.com, iommu@lists.linux-foundation.org, linux-arm-kernel@lists.infradead.org, linux-kernel@vger.kernel.org Date: Wed, 20 Jan 2016 16:00:01 +0000 In-Reply-To: <569F9DCA.3030808@arm.com> References: <1452564905-2662-1-git-send-email-shijie.huang@arm.com> <20160120120225.GD18805@8bytes.org> <20160120133401.GA3487@localhost.localdomain> <569F9DCA.3030808@arm.com> Content-Type: text/plain; charset="UTF-8" X-Mailer: Evolution 3.12.9-1+b1 Mime-Version: 1.0 Content-Transfer-Encoding: 7bit X-Originating-smarthost01b-IP: [82.69.122.217] Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Wed, 2016-01-20 at 14:46 +0000, Robin Murphy wrote: > > Of course, the 0.004014s maybe not accurate enough, it is just an > > approximate number. > > A mean and standard deviation of at least, say, 5 runs each with and > without the patch would be considerably more meaningful (even if > still > far from statistically significant). It wouldn't surprise me if replacing the proposed change with an 'asm volatile("nop")' or two also give a boot time delta of several milliseconds (due to change in cache line alignment of functions). I don't believe you can reliably measure such minor changes. It doesn't mean that the proposed change isn't a good addition though, it obviously results in less code getting executed for the cost of one or two instructions for a compare and branch. -- Tixy