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 vger.kernel.org (vger.kernel.org [23.128.96.18]) by smtp.lore.kernel.org (Postfix) with ESMTP id 324C4C433EF for ; Thu, 14 Jul 2022 09:10:35 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S232087AbiGNJKd (ORCPT ); Thu, 14 Jul 2022 05:10:33 -0400 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:60386 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S229719AbiGNJKb (ORCPT ); Thu, 14 Jul 2022 05:10:31 -0400 Received: from metis.ext.pengutronix.de (metis.ext.pengutronix.de [IPv6:2001:67c:670:201:290:27ff:fe1d:cc33]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id 241321EEC4 for ; Thu, 14 Jul 2022 02:10:30 -0700 (PDT) Received: from ptx.hi.pengutronix.de ([2001:67c:670:100:1d::c0]) by metis.ext.pengutronix.de with esmtps (TLS1.3:ECDHE_RSA_AES_256_GCM_SHA384:256) (Exim 4.92) (envelope-from ) id 1oBurc-00017G-F1; Thu, 14 Jul 2022 11:10:28 +0200 Received: from sha by ptx.hi.pengutronix.de with local (Exim 4.92) (envelope-from ) id 1oBura-0007zV-O5; Thu, 14 Jul 2022 11:10:26 +0200 Date: Thu, 14 Jul 2022 11:10:26 +0200 From: Sascha Hauer To: Steven Rostedt Cc: linux-kernel@vger.kernel.org, linux-arm-kernel@lists.infradead.org, Ingo Molnar , kernel@pengutronix.de Subject: Re: Performance impact of CONFIG_FUNCTION_TRACER Message-ID: <20220714091026.GM2387@pengutronix.de> References: <20220705105416.GE5208@pengutronix.de> <20220705103901.41a70cf0@rorschach.local.home> <20220705215948.GK5208@pengutronix.de> <20220705182746.4ce53681@rorschach.local.home> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20220705182746.4ce53681@rorschach.local.home> X-Sent-From: Pengutronix Hildesheim X-URL: http://www.pengutronix.de/ X-IRC: #ptxdist @freenode X-Accept-Language: de,en X-Accept-Content-Type: text/plain X-Uptime: 11:06:53 up 105 days, 21:36, 81 users, load average: 0.15, 0.38, 0.40 User-Agent: Mutt/1.10.1 (2018-07-13) X-SA-Exim-Connect-IP: 2001:67c:670:100:1d::c0 X-SA-Exim-Mail-From: sha@pengutronix.de X-SA-Exim-Scanned: No (on metis.ext.pengutronix.de); SAEximRunCond expanded to false X-PTX-Original-Recipient: linux-kernel@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Tue, Jul 05, 2022 at 06:27:46PM -0400, Steven Rostedt wrote: > On Tue, 5 Jul 2022 23:59:48 +0200 > Sascha Hauer wrote: > > > > > > > As I believe due to using a link register for function calls, ARM > > > requires adding two 4 byte nops to every function where as x86 only > > > adds a single 5 byte nop. > > > > > > Although nops are very fast (they should not be processed in the CPU's > > > pipe line, but I don't know if that's true for every arch). It also > > > affects instruction cache misses, as adding 8 bytes around the code > > > will cause more cache misses than when they do not exist. > > > > Just digged around a bit and saw that on ARM it's not even a real nop. > > The compiler emits: > > > > push {lr} > > bl 8010e7c0 <__gnu_mcount_nc> > > > > Which is then turned into a nop by replacing the second instruction with > > > > add sp, sp, #4 > > > > to bring the stack pointer back to its original value. This indeed must > > be processed by the CPU pipeline. I wonder if that could be optimized by > > replacing both instructions with a nop. I have no idea though if that's > > feasible at all or if the overhead would even get smaller by that. > > The problem is that there's no easy way to do that, because a task > could have been preempted after doing the 'push {lr}' and before the > 'bl'. Thus, you create a race by changing either one to a nop first. > > I wonder if it would have been better to change the first one to a jump > passed the second :-/ I gave this a try, but the performance was not better compared to the stack push/pop operations we have now. I also tried to replace both instructions with nops (mov r0, r0), still no better performance. I guess we have to live with it then. Sascha -- Pengutronix e.K. | | Steuerwalder Str. 21 | http://www.pengutronix.de/ | 31137 Hildesheim, Germany | Phone: +49-5121-206917-0 | Amtsgericht Hildesheim, HRA 2686 | Fax: +49-5121-206917-5555 |