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[209.132.180.67]) by mx.google.com with ESMTP id t17si7636708eji.394.2019.11.20.07.52.59; Wed, 20 Nov 2019 07:53:23 -0800 (PST) Received-SPF: pass (google.com: best guess record for domain of linux-kernel-owner@vger.kernel.org designates 209.132.180.67 as permitted sender) client-ip=209.132.180.67; Authentication-Results: mx.google.com; dkim=pass header.i=@kernel.org header.s=default header.b=Cw3bB3o7; spf=pass (google.com: best guess record for domain of linux-kernel-owner@vger.kernel.org designates 209.132.180.67 as permitted sender) smtp.mailfrom=linux-kernel-owner@vger.kernel.org; dmarc=pass (p=NONE sp=NONE dis=NONE) header.from=kernel.org Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1728417AbfKTPCY (ORCPT + 99 others); Wed, 20 Nov 2019 10:02:24 -0500 Received: from mail.kernel.org ([198.145.29.99]:51518 "EHLO mail.kernel.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1728096AbfKTPCY (ORCPT ); Wed, 20 Nov 2019 10:02:24 -0500 Received: from linux-8ccs (x2f7fef5.dyn.telefonica.de [2.247.254.245]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by mail.kernel.org (Postfix) with ESMTPSA id 39A7020731; Wed, 20 Nov 2019 15:02:19 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=default; t=1574262142; bh=yxx+qE1WrejWUHpUk0uPaXwqaaabpk1t3tYxoMbmI4E=; h=Date:From:To:Cc:Subject:References:In-Reply-To:From; b=Cw3bB3o7CtrF9VC80gZEKMIo5f9IDXKl8uk5lRrwC/Z29SxQ0gD8cvrNgX1evegkr rlNHZnDiG+l41WRpx5OzC/fLZg6WBpDP9S3qS6xldCsAdGvHam0w9Pr0PlCfBpJjTa U1pP0DZeBmrL/zg+MZjeULMd82pwIXEdBHokePYQ= Date: Wed, 20 Nov 2019 16:02:16 +0100 From: Jessica Yu To: linux-kernel@vger.kernel.org Cc: Matthias Maennich , Masahiro Yamada , Rasmus Villemoes , Arnd Bergmann , Greg Kroah-Hartman Subject: Re: [PATCH] export.h: reduce __ksymtab_strings string duplication by using "MS" section flags Message-ID: <20191120150215.GA756@linux-8ccs> References: <20191120145110.8397-1-jeyu@kernel.org> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii; format=flowed Content-Disposition: inline In-Reply-To: <20191120145110.8397-1-jeyu@kernel.org> X-OS: Linux linux-8ccs 5.4.0-rc5-lp150.12.61-default+ x86_64 User-Agent: Mutt/1.10.1 (2018-07-13) Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org +++ Jessica Yu [20/11/19 15:51 +0100]: >Commit c3a6cf19e695 ("export: avoid code duplication in >include/linux/export.h") refactors export.h quite nicely, but introduces >a slight increase in memory usage due to using the empty string "" >instead of NULL to indicate that an exported symbol has no namespace. As >mentioned in that commit, this meant an increase of 1 byte per exported >symbol without a namespace. For example, if a kernel configuration has >about 10k exported symbols, this would mean that the size of >__ksymtab_strings would increase by roughly 10kB. > >We can alleviate this situation by utilizing the SHF_MERGE and >SHF_STRING section flags. SHF_MERGE|SHF_STRING indicate to the linker >that the data in the section are null-terminated strings that can be >merged to eliminate duplication. More specifically, from the binutils >documentation - "for sections with both M and S, a string which is a >suffix of a larger string is considered a duplicate. Thus "def" will be >merged with "abcdef"; A reference to the first "def" will be changed to >a reference to "abcdef"+3". Thus, all the empty strings would be merged >as well as any strings that can be merged according to the cited method >above. For example, "memset" and "__memset" would be merged to just >"__memset" in __ksymtab_strings. > >As of v5.4-rc5, the following statistics were gathered with x86 >defconfig with approximately 10.7k exported symbols. > >Size of __ksymtab_strings in vmlinux: >------------------------------------- >v5.4-rc5: 213834 bytes >v5.4-rc5 with commit c3a6cf19e695: 224455 bytes >v5.4-rc5 with this patch: 205759 bytes > >So, we already see memory savings of ~8kB compared to vanilla -rc5 and >savings of nearly 18.7kB compared to -rc5 with commit c3a6cf19e695 on top. > >Unfortunately, as of this writing, strings will not get deduplicated for >kernel modules, as ld does not do the deduplication for >SHF_MERGE|SHF_STRINGS sections for relocatable files (ld -r), which >kernel modules are. A patch for ld is currently being worked on to >hopefully allow for string deduplication in relocatable files in the >future. > >Suggested-by: Rasmus Villemoes >Signed-off-by: Jessica Yu >--- > include/asm-generic/export.h | 13 ++++++++++--- > include/linux/export.h | 28 ++++++++++++++++++++++------ > 2 files changed, 32 insertions(+), 9 deletions(-) > >diff --git a/include/asm-generic/export.h b/include/asm-generic/export.h >index fa577978fbbd..d0704f2602f4 100644 >--- a/include/asm-generic/export.h >+++ b/include/asm-generic/export.h >@@ -26,9 +26,16 @@ > .endm > > /* >- * note on .section use: @progbits vs %progbits nastiness doesn't matter, >- * since we immediately emit into those sections anyway. >+ * note on .section use: we specify @progbits vs %progbits since usage of >+ * "M" (SHF_MERGE) section flag requires it. > */ >+ >+#ifdef CONFIG_ARM >+#define ARCH_PROGBITS %progbits >+#else >+#define ARCH_PROGBITS @progbits >+#endif >+ > .macro ___EXPORT_SYMBOL name,val,sec > #ifdef CONFIG_MODULES > .globl __ksymtab_\name >@@ -37,7 +44,7 @@ > __ksymtab_\name: > __put \val, __kstrtab_\name > .previous >- .section __ksymtab_strings,"a" >+ .section __ksymtab_strings,"aMS",ARCH_PROGBITS,1 > __kstrtab_\name: > .asciz "\name" > .previous >diff --git a/include/linux/export.h b/include/linux/export.h >index 201262793369..ab325a8e6bee 100644 >--- a/include/linux/export.h >+++ b/include/linux/export.h >@@ -81,16 +81,32 @@ struct kernel_symbol { > > #else > >+#ifdef CONFIG_ARM >+#define ARCH_PROGBITS "%progbits" >+#else >+#define ARCH_PROGBITS "@progbits" >+#endif Just a comment, I don't like the duplication of this block, but I wasn't sure where was the best place to have this define since asm-generic/export.h and linux/export.h don't share includes, and includes are generally kept to a minimum. If anyone has a better idea, please let me know.