From mboxrd@z Thu Jan 1 00:00:00 1970 From: Mathieu Desnoyers Date: Sat, 22 Jan 2011 17:11:34 +0000 Subject: Re: [patch 1/3] introduce __u64_aligned and U64_ALIGN() for Message-Id: <20110122171134.GA31308@Krystal> List-Id: References: <20110121203630.725922272@efficios.com> <20110121203642.725191236@efficios.com> <20110121.160503.233879710.davem@davemloft.net> In-Reply-To: <20110121.160503.233879710.davem@davemloft.net> MIME-Version: 1.0 Content-Type: text/plain; charset="us-ascii" Content-Transfer-Encoding: 7bit To: David Miller Cc: linux-kernel@vger.kernel.org, rostedt@goodmis.org, fweisbec@gmail.com, mingo@elte.hu, richm@oldelvet.org.uk, ben@decadent.org.uk, sparclinux@vger.kernel.org, stable * David Miller (davem@davemloft.net) wrote: > From: Mathieu Desnoyers > Date: Fri, 21 Jan 2011 15:36:31 -0500 > > > Problem description: > > > > gcc happily align on 32-byte structures defined statically. Ftrace trace events > > and Tracepoints both statically define structures into custom sections (using > > the "section" attribute), to then assign these to symbols with the linker > > scripts to iterate the these sections as an array. > > > > However, gcc uses different alignments for these structures when they are > > defined statically than when they are globally visible and/or in an array. > > Therefore iteration on these arrays sees "holes" of padding. gcc is within its > > rights to increase the alignment of the statically defined structures because, > > normally, there should be no other accesses to them than in the local object. We > > are actually iterating on the generated structures as if they were an array > > without letting gcc knowing anything about it. > > > > This patch introduces __u64_aligned to force gcc to use the u64 type and > > variable alignment, up-aligning or down-aligning the target type if necessary. > > The memory accesses to the target structure are efficient (does not require > > bytewise memory accesses) and the atomic pointer update guarantees required by > > RCU are kept. u64 is considered as the largest type that can generate a trap for > > unaligned accesses (u64 on sparc32 needs to be aligned on 64-bit). > > > > This alignment should be used for both structure definitions and declarations > > (as *both* the type and variable attribute) when using the "section" > > attribute to generate arrays of structures. Given that gcc only uses the type > > attribute "aligned" as a lower-bound for alignment, the structures should not > > contain types which require alignment larger than that of u64. The "aligned" > > variable attribute, on the other hand, forces gcc to use exactly the specified > > alignment. > > > > Also introduce the linker script U64_ALIGN() macro for specification of custom > > section alignment that matches that of __u64_aligned. > > > > Changelog since v2: > > - Drop the "packed" type attribute, because it causes gcc to drop the padding > > between consecutive "int" and "pointer"/"long" fields, which leads to > > unaligned accesses on sparc64. > > > > Signed-off-by: Mathieu Desnoyers > > Acked-by: David S. Miller Thanks David, After these patches get merged -- I would recommend going through the tracing tree -- they'll have to go to -stable too. I'm therefore forwarding this to stable@kernel.org (the same should be done for the two other patches in this series). Mathieu -- Mathieu Desnoyers Operating System Efficiency R&D Consultant EfficiOS Inc. http://www.efficios.com From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1752222Ab1AVRLj (ORCPT ); Sat, 22 Jan 2011 12:11:39 -0500 Received: from mail.openrapids.net ([64.15.138.104]:49079 "EHLO blackscsi.openrapids.net" rhost-flags-OK-OK-OK-FAIL) by vger.kernel.org with ESMTP id S1750967Ab1AVRLh (ORCPT ); Sat, 22 Jan 2011 12:11:37 -0500 Date: Sat, 22 Jan 2011 12:11:34 -0500 From: Mathieu Desnoyers To: David Miller Cc: linux-kernel@vger.kernel.org, rostedt@goodmis.org, fweisbec@gmail.com, mingo@elte.hu, richm@oldelvet.org.uk, ben@decadent.org.uk, sparclinux@vger.kernel.org, stable Subject: Re: [patch 1/3] introduce __u64_aligned and U64_ALIGN() for structure alignment in custom sections (v3) Message-ID: <20110122171134.GA31308@Krystal> References: <20110121203630.725922272@efficios.com> <20110121203642.725191236@efficios.com> <20110121.160503.233879710.davem@davemloft.net> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20110121.160503.233879710.davem@davemloft.net> X-Editor: vi X-Info: http://www.efficios.com X-Operating-System: Linux/2.6.26-2-686 (i686) X-Uptime: 12:08:32 up 59 days, 22:11, 5 users, load average: 0.10, 0.09, 0.04 User-Agent: Mutt/1.5.18 (2008-05-17) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org * David Miller (davem@davemloft.net) wrote: > From: Mathieu Desnoyers > Date: Fri, 21 Jan 2011 15:36:31 -0500 > > > Problem description: > > > > gcc happily align on 32-byte structures defined statically. Ftrace trace events > > and Tracepoints both statically define structures into custom sections (using > > the "section" attribute), to then assign these to symbols with the linker > > scripts to iterate the these sections as an array. > > > > However, gcc uses different alignments for these structures when they are > > defined statically than when they are globally visible and/or in an array. > > Therefore iteration on these arrays sees "holes" of padding. gcc is within its > > rights to increase the alignment of the statically defined structures because, > > normally, there should be no other accesses to them than in the local object. We > > are actually iterating on the generated structures as if they were an array > > without letting gcc knowing anything about it. > > > > This patch introduces __u64_aligned to force gcc to use the u64 type and > > variable alignment, up-aligning or down-aligning the target type if necessary. > > The memory accesses to the target structure are efficient (does not require > > bytewise memory accesses) and the atomic pointer update guarantees required by > > RCU are kept. u64 is considered as the largest type that can generate a trap for > > unaligned accesses (u64 on sparc32 needs to be aligned on 64-bit). > > > > This alignment should be used for both structure definitions and declarations > > (as *both* the type and variable attribute) when using the "section" > > attribute to generate arrays of structures. Given that gcc only uses the type > > attribute "aligned" as a lower-bound for alignment, the structures should not > > contain types which require alignment larger than that of u64. The "aligned" > > variable attribute, on the other hand, forces gcc to use exactly the specified > > alignment. > > > > Also introduce the linker script U64_ALIGN() macro for specification of custom > > section alignment that matches that of __u64_aligned. > > > > Changelog since v2: > > - Drop the "packed" type attribute, because it causes gcc to drop the padding > > between consecutive "int" and "pointer"/"long" fields, which leads to > > unaligned accesses on sparc64. > > > > Signed-off-by: Mathieu Desnoyers > > Acked-by: David S. Miller Thanks David, After these patches get merged -- I would recommend going through the tracing tree -- they'll have to go to -stable too. I'm therefore forwarding this to stable@kernel.org (the same should be done for the two other patches in this series). Mathieu -- Mathieu Desnoyers Operating System Efficiency R&D Consultant EfficiOS Inc. http://www.efficios.com