From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1752327AbbJFQkE (ORCPT ); Tue, 6 Oct 2015 12:40:04 -0400 Received: from mail-bn1bbn0104.outbound.protection.outlook.com ([157.56.111.104]:22656 "EHLO na01-bn1-obe.outbound.protection.outlook.com" rhost-flags-OK-OK-OK-FAIL) by vger.kernel.org with ESMTP id S1750810AbbJFQkB (ORCPT ); Tue, 6 Oct 2015 12:40:01 -0400 Authentication-Results: spf=none (sender IP is ) smtp.mailfrom=scottwood@freescale.com; Message-ID: <1444149591.5336.279.camel@freescale.com> Subject: Re: [PATCH v2 01/25] powerpc/8xx: Save r3 all the time in DTLB miss handler From: Scott Wood To: Christophe Leroy CC: Benjamin Herrenschmidt , Paul Mackerras , Michael Ellerman , , Date: Tue, 6 Oct 2015 11:39:51 -0500 In-Reply-To: <5613CE1C.40806@c-s.fr> References: <2d35de4435e873f23d37e3b5b5fb34c64421f136.1442939410.git.christophe.leroy@c-s.fr> <20150928220717.GA6161@home.buserror.net> <5613CE1C.40806@c-s.fr> Content-Type: text/plain; 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What is the > > performance impact? Is the performance impact seen with or without > > CONFIG_8xx_CPU6 enabled? Without it, it looks like you're adding an > > mtspr/mfspr combo in order to replace one mfspr. > > > > > The performance impact is not noticeable. Theoritically it adds 1 cycle > on a mean of 65 cycles, that is 1.5%. Even in the worst case where we > spend around 10% of the time in TLB handling exceptions, that represents > only 0.15% of the total CPU time. So that's almost nothing. > Behind the fact to get in simpler, the main reason is because I need a > third register for the following patch in the set, otherwise I would > spend a more time saving and restoring CR several times. If you had said in the changelog that it was because future patches would need the register to be saved, we could have avoided this exchange... Especially with large patchsets, I review the patches one at a time. Don't assume I know what's coming in patch n+1 (and especially not n+m) when I review patch n. -Scott