From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1753232AbbJFUaM (ORCPT ); Tue, 6 Oct 2015 16:30:12 -0400 Received: from 2.236.17.93.rev.sfr.net ([93.17.236.2]:63656 "EHLO mailhub1.si.c-s.fr" rhost-flags-OK-OK-OK-FAIL) by vger.kernel.org with ESMTP id S1753145AbbJFUaK (ORCPT ); Tue, 6 Oct 2015 16:30:10 -0400 From: christophe leroy Subject: Re: [PATCH v2 01/25] powerpc/8xx: Save r3 all the time in DTLB miss handler To: Scott Wood References: <2d35de4435e873f23d37e3b5b5fb34c64421f136.1442939410.git.christophe.leroy@c-s.fr> <20150928220717.GA6161@home.buserror.net> <5613CE1C.40806@c-s.fr> <1444149995.5336.282.camel@freescale.com> Cc: Benjamin Herrenschmidt , Paul Mackerras , Michael Ellerman , linux-kernel@vger.kernel.org, linuxppc-dev@lists.ozlabs.org Message-ID: <56142F4D.5090405@c-s.fr> Date: Tue, 6 Oct 2015 22:30:05 +0200 User-Agent: Mozilla/5.0 (Windows NT 6.1; WOW64; rv:38.0) Gecko/20100101 Thunderbird/38.3.0 MIME-Version: 1.0 In-Reply-To: <1444149995.5336.282.camel@freescale.com> Content-Type: text/plain; charset=utf-8; format=flowed Content-Transfer-Encoding: 8bit X-Antivirus: avast! (VPS 151006-1, 06/10/2015), Outbound message X-Antivirus-Status: Clean Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Le 06/10/2015 18:46, Scott Wood a écrit : > On Tue, 2015-10-06 at 15:35 +0200, Christophe Leroy wrote: >> Le 29/09/2015 00:07, Scott Wood a écrit : >>> On Tue, Sep 22, 2015 at 06:50:29PM +0200, Christophe Leroy wrote: >>>> We are spending between 40 and 160 cycles with a mean of 65 cycles in >>>> the TLB handling routines (measured with mftbl) so make it more >>>> simple althought it adds one instruction. >>>> >>>> Signed-off-by: Christophe Leroy >>> Does this just make it simpler or does it make it faster? 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. > FWIW, the added instruction is an SPR access and I doubt that's only one > cycle. > > According to the mpc885 reference manual (table 9-1), Instruction Execution Timing for "Move to: mtspr, mtcrf, mtmsr, mcrxr except mtspr to LR and CTR and to SPRs external to the core" is "serialize + 1 cycle". Taking into account we preeceeding instructions are also 'mtspr', we are already serialized, so it is only one cycle I believe. Am I interpreting it wrong ? Christophe --- L'absence de virus dans ce courrier électronique a été vérifiée par le logiciel antivirus Avast. https://www.avast.com/antivirus