From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org X-Spam-Level: X-Spam-Status: No, score=-8.6 required=3.0 tests=DKIMWL_WL_HIGH,DKIM_SIGNED, DKIM_VALID,DKIM_VALID_AU,INCLUDES_PATCH,MAILING_LIST_MULTI,SIGNED_OFF_BY, SPF_HELO_NONE,SPF_PASS,USER_AGENT_SANE_1 autolearn=ham autolearn_force=no version=3.4.0 Received: from mail.kernel.org (mail.kernel.org [198.145.29.99]) by smtp.lore.kernel.org (Postfix) with ESMTP id 0AE8BC432C0 for ; Tue, 26 Nov 2019 09:55:19 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id BF63F2080F for ; Tue, 26 Nov 2019 09:55:18 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=default; t=1574762118; bh=MzKgqKCa7NaA8Djx5qejgsOtT2+/H2cyYVcKH3ql0Zk=; h=Date:From:To:Cc:Subject:References:In-Reply-To:List-ID:From; b=Nc+TjbMFTWJNrau9W1uM9GiVRL6myaPhvHTTKCh3Nydu5/Zhb7plmY3wfkBjpC11G SjbGl7WfWXlMWxhVknEoktnxqB/2bOwklZgKSQvcJnOpqwW5idGaEhtfz4uQiGVnKb NpoGiLan5ig1LD650H9YL+lGvoOt9McM2q9VWcQ8= Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1727684AbfKZJzS (ORCPT ); Tue, 26 Nov 2019 04:55:18 -0500 Received: from mail.kernel.org ([198.145.29.99]:56140 "EHLO mail.kernel.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1727557AbfKZJzR (ORCPT ); Tue, 26 Nov 2019 04:55:17 -0500 Received: from linux-8ccs (x2f7fc62.dyn.telefonica.de [2.247.252.98]) (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 AE5C42075C; Tue, 26 Nov 2019 09:55:14 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=default; t=1574762116; bh=MzKgqKCa7NaA8Djx5qejgsOtT2+/H2cyYVcKH3ql0Zk=; h=Date:From:To:Cc:Subject:References:In-Reply-To:From; b=GdGD6tkDzZas/Y4hkk9UHE5NsbTPmf86gXupJNtx0Vgt/WN0rWMS2sPrKZn6FTvsU OvbZzZSB6HJaXSD9T+iiGNpButycABEAI1g/M/MQzp0B3sYbMyjRp9eLUVVAPQ21VE bwQAAEdaHAlwa5MOVaClVN2R424QphdxVNm0kRoU= Date: Tue, 26 Nov 2019 10:55:10 +0100 From: Jessica Yu To: Masahiro Yamada Cc: Linux Kernel Mailing List , Matthias Maennich , Rasmus Villemoes , Arnd Bergmann , Greg Kroah-Hartman Subject: Re: [PATCH v2] export.h: reduce __ksymtab_strings string duplication by using "MS" section flags Message-ID: <20191126095510.GA30181@linux-8ccs> References: <20191120145110.8397-1-jeyu@kernel.org> <20191125154217.18640-1-jeyu@kernel.org> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii; format=flowed Content-Disposition: inline In-Reply-To: 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 +++ Masahiro Yamada [26/11/19 17:32 +0900]: >On Tue, Nov 26, 2019 at 12:42 AM Jessica Yu wrote: >> >> 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 >> --- >> >> v2: use %progbits throughout and document the oddity in a comment. >> >> include/asm-generic/export.h | 8 +++++--- >> include/linux/export.h | 27 +++++++++++++++++++++------ >> 2 files changed, 26 insertions(+), 9 deletions(-) >> >> diff --git a/include/asm-generic/export.h b/include/asm-generic/export.h >> index fa577978fbbd..23bc98e97a66 100644 >> --- a/include/asm-generic/export.h >> +++ b/include/asm-generic/export.h >> @@ -26,9 +26,11 @@ >> .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 since usage of the "M" (SHF_MERGE) >> + * section flag requires it. Use '%progbits' instead of '@progbits' since the >> + * former apparently works on all arches according to the binutils source. >> */ >> + >> .macro ___EXPORT_SYMBOL name,val,sec >> #ifdef CONFIG_MODULES >> .globl __ksymtab_\name >> @@ -37,7 +39,7 @@ >> __ksymtab_\name: >> __put \val, __kstrtab_\name >> .previous >> - .section __ksymtab_strings,"a" >> + .section __ksymtab_strings,"aMS",%progbits,1 >> __kstrtab_\name: >> .asciz "\name" >> .previous >> diff --git a/include/linux/export.h b/include/linux/export.h >> index 201262793369..3d835ca34d33 100644 >> --- a/include/linux/export.h >> +++ b/include/linux/export.h >> @@ -81,16 +81,31 @@ struct kernel_symbol { >> >> #else >> >> +/* >> + * note on .section use: we specify progbits since usage of the "M" (SHF_MERGE) >> + * section flag requires it. Use '%progbits' instead of '@progbits' since the >> + * former apparently works on all arches according to the binutils source. >> + */ >> +#define __KSTRTAB_ENTRY(sym) \ >> + asm(" .section \"__ksymtab_strings\",\"aMS\",%progbits,1 \n" \ >> + "__kstrtab_" #sym ": \n" \ >> + " .asciz \"" #sym "\" \n" \ >> + " .previous \n") >> + >> +#define __KSTRTAB_NS_ENTRY(sym, ns) \ >> + asm(" .section \"__ksymtab_strings\",\"aMS\",%progbits,1 \n" \ >> + "__kstrtabns_" #sym ": \n" \ >> + " .asciz " #ns " \n" \ > > >Hmm, it took some time for me to how this code works. > >ns is already a C string, then you added # to it, >then I was confused. > >Personally, I prefer this code: >" .asciz \"" ns "\" \n" > >so it looks in the same way as __KSTRTAB_ENTRY(). > > > >BTW, you duplicated \"aMS\",%progbits,1" and ".previous" > > >I would write it shorter, like this: > > >#define ___EXPORT_SYMBOL(sym, sec, ns) \ > extern typeof(sym) sym; \ > extern const char __kstrtab_##sym[]; \ > extern const char __kstrtabns_##sym[]; \ > __CRC_SYMBOL(sym, sec); \ > asm(" .section \"__ksymtab_strings\",\"aMS\",%progbits,1\n" \ > "__kstrtab_" #sym ": \n" \ > " .asciz \"" #sym "\" \n" \ > "__kstrtabns_" #sym ": \n" \ > " .asciz \"" ns "\" \n" \ > " .previous \n"); \ > __KSYMTAB_ENTRY(sym, sec) > Sure, I can change that. Just thought it'd be easier to read with the separate macros. Thanks for your comments!