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[209.132.180.67]) by mx.google.com with ESMTP id g28si19302909edg.327.2019.11.20.06.58.06; Wed, 20 Nov 2019 06:58:31 -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=Wn8bN9YE; 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 S1731836AbfKTOzW (ORCPT + 99 others); Wed, 20 Nov 2019 09:55:22 -0500 Received: from mail.kernel.org ([198.145.29.99]:48904 "EHLO mail.kernel.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1731775AbfKTOzW (ORCPT ); Wed, 20 Nov 2019 09:55:22 -0500 Received: from linux-8ccs.suse.de (x2f7fef5.dyn.telefonica.de [2.247.254.245]) (using TLSv1.2 with cipher ECDHE-RSA-AES128-GCM-SHA256 (128/128 bits)) (No client certificate requested) by mail.kernel.org (Postfix) with ESMTPSA id 8A467206EC; Wed, 20 Nov 2019 14:55:18 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=default; t=1574261721; bh=K2L2QTaADtfrEnvPFmBvRj5rP4HoJezLv58+2MpX48I=; h=From:To:Cc:Subject:Date:From; b=Wn8bN9YERfEwrf6yFkGA6+Y3SU5YpROZ2qGwOjXY/0hFxUYVSsNb382gZuzcvcRFq +BMR2QCLMjkqKUfR1ISCsJb3xKG4k9P+uMvkETqJFadZ8FSLazgiYcc6iuY+lAeKJi gKYayiNDBWVp87XlC/6oLTPpEehn1xi+MX5CKp0Y= From: Jessica Yu To: linux-kernel@vger.kernel.org Cc: Matthias Maennich , Masahiro Yamada , Rasmus Villemoes , Arnd Bergmann , Greg Kroah-Hartman , Jessica Yu Subject: [PATCH] export.h: reduce __ksymtab_strings string duplication by using "MS" section flags Date: Wed, 20 Nov 2019 15:51:10 +0100 Message-Id: <20191120145110.8397-1-jeyu@kernel.org> X-Mailer: git-send-email 2.16.4 Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org 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 + +#define __KSTRTAB_ENTRY(sym) \ + asm(" .section \"__ksymtab_strings\",\"aMS\","ARCH_PROGBITS",1\n" \ + "__kstrtab_" #sym ": \n" \ + " .asciz \"" #sym "\" \n" \ + " .previous \n") + +#define __KSTRTAB_NS_ENTRY(sym, ns) \ + asm(" .section \"__ksymtab_strings\",\"aMS\","ARCH_PROGBITS",1\n" \ + "__kstrtabns_" #sym ": \n" \ + " .asciz " #ns " \n" \ + " .previous \n") + /* For every exported symbol, place a struct in the __ksymtab section */ #define ___EXPORT_SYMBOL(sym, sec, ns) \ extern typeof(sym) sym; \ + extern const char __kstrtab_##sym[]; \ + extern const char __kstrtabns_##sym[]; \ __CRC_SYMBOL(sym, sec); \ - static const char __kstrtab_##sym[] \ - __attribute__((section("__ksymtab_strings"), used, aligned(1))) \ - = #sym; \ - static const char __kstrtabns_##sym[] \ - __attribute__((section("__ksymtab_strings"), used, aligned(1))) \ - = ns; \ + __KSTRTAB_ENTRY(sym); \ + __KSTRTAB_NS_ENTRY(sym, ns); \ __KSYMTAB_ENTRY(sym, sec) #endif -- 2.16.4