Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1758915Ab3HOR3u (ORCPT ); Thu, 15 Aug 2013 13:29:50 -0400 Received: from service87.mimecast.com ([91.220.42.44]:41341 "EHLO service87.mimecast.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1757521Ab3HOR3q convert rfc822-to-8bit (ORCPT ); Thu, 15 Aug 2013 13:29:46 -0400 Message-ID: <520D0FFA.1060508@arm.com> Date: Thu, 15 Aug 2013 18:29:30 +0100 From: Jonathan Austin User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:17.0) Gecko/20130308 Thunderbird/17.0.4 MIME-Version: 1.0 To: =?UTF-8?B?QW5kcsOpIEhlbnRzY2hlbA==?= CC: "linux-kernel@vger.kernel.org" , Will Deacon , "linux-arch@vger.kernel.org" , "linux-arm-kernel@lists.infradead.org" , Russell King Subject: Re: arm: Only load TLS values when needed References: <51E42E11.1010903@dawncrow.de> <51E5836B.1010904@arm.com> <51E59E8F.1060501@dawncrow.de> <51E67B98.9040101@arm.com> <51E6F532.1030004@dawncrow.de> <520B8F37.4040609@dawncrow.de> <520BAE58.3060600@arm.com> <520BF4E1.8040708@dawncrow.de> In-Reply-To: <520BF4E1.8040708@dawncrow.de> X-OriginalArrivalTime: 15 Aug 2013 17:29:41.0187 (UTC) FILETIME=[063DF930:01CE99DD] X-MC-Unique: 113081518294400201 Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 8BIT Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 5629 Lines: 154 Hi André (I've put RMK and LAKML back on Cc: they got dropped somewhere along the way) On 14/08/13 22:21, André Hentschel wrote: >>>>> [...] >>>>>>> Now we've only got one instruction between the store and the load >>>>>>> and risk stalling the pipeline... >>>>>>> >>>>>>> Dave M cautiously says "The ancient advice was that one instruction >>>>>>> was enough" but this is very core dependent... I wonder if anyone >>>>>>> has a good idea about whether this is an issue here...? >>>>>> >>>>>> We could use a ldrd at the top, that'd be nearly what we have right >>>>>> now, don't we? >>>>> >>>>> Yea, that'd be good - as far as I can see from an 1136 TRM, the ldrd *may* be two cycles (depending on alignment of the words) but the ldr and ldrne will always be two cycles. Ahhh, the joys of modifying the fast path ;) >> >> Was expecting to see something that reflected this discussion, > > Ah ok, i misunderstood that, sry. > Something like that? > > From: André Hentschel > > This patch intents to reduce loading instructions when the resulting value is not used. > It's a follow up on a4780adeefd042482f624f5e0d577bf9cdcbb760 > You caught me just before running away for a week of holiday, but I've managed to squeeze in a bit of testing - hope this helps - I'll try to be clear about what I have/haven't tested so you can judge for yourself how much weight to give this. What I've tested: * Apply your code to 3.11-rc5 * Boot on Integrator CP (1176 r1p0) and run a full Debian Wheezy init --> It works! I also applied the following diff and rebuilt to check that we were really testing the V6 NOT V6K path here: diff --git a/arch/arm/include/asm/tls.h b/arch/arm/include/asm/tls.h index 31743f7..71dfe82 100644 --- a/arch/arm/include/asm/tls.h +++ b/arch/arm/include/asm/tls.h @@ -43,6 +43,7 @@ #define switch_tls switch_tls_v6 #elif defined(CONFIG_CPU_32v6K) #define tls_emu 0 +#error #define has_tls_reg 1 #define switch_tls switch_tls_v6k #else What I've *not* tested: * As this CPU does not have the tls register, I haven't run any tests that try to read/write it, but we wouldn't expect that to work, right? * Haven't tried a V7 platform, as I understand you've already done that. * Haven't verified it still builds for v4 (which was the problem for ldrd in the past and now we've put it back, so that's important) * Performance impact I think it'd be good if you can verify the bottom two points - especially the ldrd one: perhaps Russell can tell you which platform it broke on before? So, that's a Tested-by: Jonathan Austin If you want it :) Hope that helps, Jonny > Signed-off-by: André Hentschel > > --- > This patch is against 28fbc8b6a29c849a3f03a6b05010d4b584055665 > > diff --git a/arch/arm/include/asm/tls.h b/arch/arm/include/asm/tls.h > index 83259b8..31743f7 100644 > --- a/arch/arm/include/asm/tls.h > +++ b/arch/arm/include/asm/tls.h > @@ -3,29 +3,31 @@ > > #ifdef __ASSEMBLY__ > #include > - .macro switch_tls_none, base, tp, tpuser, tmp1, tmp2 > + .macro switch_tls_none, prev, next, tp, tpuser, tmp1, tmp2 > .endm > > - .macro switch_tls_v6k, base, tp, tpuser, tmp1, tmp2 > + .macro switch_tls_v6k, prev, next, tp, tpuser, tmp1, tmp2 > + ldrd \tp, \tpuser, [\next, #TI_TP_VALUE] @ get the next TLS and user r/w register > mrc p15, 0, \tmp2, c13, c0, 2 @ get the user r/w register > mcr p15, 0, \tp, c13, c0, 3 @ set TLS register > mcr p15, 0, \tpuser, c13, c0, 2 @ and the user r/w register > - str \tmp2, [\base, #TI_TP_VALUE + 4] @ save it > + str \tmp2, [\prev, #TI_TP_VALUE + 4] @ save it > .endm > > - .macro switch_tls_v6, base, tp, tpuser, tmp1, tmp2 > + .macro switch_tls_v6, prev, next, tp, tpuser, tmp1, tmp2 > + ldrd \tp, \tpuser, [\next, #TI_TP_VALUE] > ldr \tmp1, =elf_hwcap > ldr \tmp1, [\tmp1, #0] > mov \tmp2, #0xffff0fff > tst \tmp1, #HWCAP_TLS @ hardware TLS available? > streq \tp, [\tmp2, #-15] @ set TLS value at 0xffff0ff0 > - mrcne p15, 0, \tmp2, c13, c0, 2 @ get the user r/w register > + mrcne p15, 0, \tmp2, c13, c0, 2 @ get the previous user r/w register > mcrne p15, 0, \tp, c13, c0, 3 @ yes, set TLS register > mcrne p15, 0, \tpuser, c13, c0, 2 @ set user r/w register > - strne \tmp2, [\base, #TI_TP_VALUE + 4] @ save it > + strne \tmp2, [\prev, #TI_TP_VALUE + 4] @ save it > .endm > > - .macro switch_tls_software, base, tp, tpuser, tmp1, tmp2 > + .macro switch_tls_software, prev, next, tp, tpuser, tmp1, tmp2 > mov \tmp1, #0xffff0fff > str \tp, [\tmp1, #-15] @ set TLS value at 0xffff0ff0 > .endm > diff --git a/arch/arm/kernel/entry-armv.S b/arch/arm/kernel/entry-armv.S > index d40d0ef..11112de 100644 > --- a/arch/arm/kernel/entry-armv.S > +++ b/arch/arm/kernel/entry-armv.S > @@ -689,12 +689,10 @@ ENTRY(__switch_to) > THUMB( stmia ip!, {r4 - sl, fp} ) @ Store most regs on stack > THUMB( str sp, [ip], #4 ) > THUMB( str lr, [ip], #4 ) > - ldr r4, [r2, #TI_TP_VALUE] > - ldr r5, [r2, #TI_TP_VALUE + 4] > #ifdef CONFIG_CPU_USE_DOMAINS > ldr r6, [r2, #TI_CPU_DOMAIN] > #endif > - switch_tls r1, r4, r5, r3, r7 > + switch_tls r1, r2, r4, r5, r3, r7 > #if defined(CONFIG_CC_STACKPROTECTOR) && !defined(CONFIG_SMP) > ldr r7, [r2, #TI_TASK] > ldr r8, =__stack_chk_guard > > > -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/