From mboxrd@z Thu Jan 1 00:00:00 1970 From: Robin Murphy Subject: Re: [PATCH] iommu/arm-smmu: add a shortcut when the @dev_node is NULL Date: Wed, 20 Jan 2016 14:46:34 +0000 Message-ID: <569F9DCA.3030808@arm.com> References: <1452564905-2662-1-git-send-email-shijie.huang@arm.com> <20160120120225.GD18805@8bytes.org> <20160120133401.GA3487@localhost.localdomain> Mime-Version: 1.0 Content-Type: text/plain; charset="us-ascii"; Format="flowed" Content-Transfer-Encoding: 7bit Return-path: In-Reply-To: <20160120133401.GA3487-bi+AKbBUZKY6gyzm1THtWbp2dZbC/Bob@public.gmane.org> List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Sender: iommu-bounces-cunTk1MwBs9QetFLy7KEm3xJsTq8ys+cHZ5vskTnxNA@public.gmane.org Errors-To: iommu-bounces-cunTk1MwBs9QetFLy7KEm3xJsTq8ys+cHZ5vskTnxNA@public.gmane.org To: Huang Shijie Cc: will.deacon-5wv7dgnIgG8@public.gmane.org, iommu-cunTk1MwBs9QetFLy7KEm3xJsTq8ys+cHZ5vskTnxNA@public.gmane.org, linux-arm-kernel-IAPFreCvJWM7uuMidbF8XUB+6BGkLq7r@public.gmane.org, linux-kernel-u79uwXL29TY76Z2rM5mHXA@public.gmane.org List-Id: iommu@lists.linux-foundation.org On 20/01/16 13:34, Huang Shijie wrote: > On Wed, Jan 20, 2016 at 01:02:25PM +0100, Joerg Roedel wrote: >> On Tue, Jan 12, 2016 at 10:15:05AM +0800, Huang Shijie wrote: >>> This patch adds a shortcut for the code when the @device_node is NULL. >>> In my juno-r1 board, the boot time can be faster by 0.004014s. >> >> How have you made sure this number is reliable and not just noise in the >> boot process? > In the boot process, there are 5 or more modules whose @dev_node are > NULL. Without the patch, the kernel will waste some cycles to do the > meaningless calculations for all these modules. With a quick counting hack, booting 4.4 on my r1 indeed shows 5 calls where dev_node is null. Plus 68 calls in which we waste cycles doing meaningless calculations when dev_node is non-null. The fundamental issue at hand is that the "platform bus" is a rubbish abstraction. > In theory, it is not noise. > If you have interest, I can send you the kernel boot logs. :) > > 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). Robin. > > Thanks > Huang Shijie > > _______________________________________________ > iommu mailing list > iommu-cunTk1MwBs9QetFLy7KEm3xJsTq8ys+cHZ5vskTnxNA@public.gmane.org > https://lists.linuxfoundation.org/mailman/listinfo/iommu >