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=-7.1 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 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 EE5D2C43603 for ; Thu, 12 Dec 2019 06:24:08 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id B4ACE2173E for ; Thu, 12 Dec 2019 06:24:08 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=default; t=1576131848; bh=iFk0l3fNk+DznF5EHYeeRyW1NKE7uSXwlKEpYfm9jdU=; h=References:In-Reply-To:From:Date:Subject:To:Cc:List-ID:From; b=VIEWyPZTVJ7bhqlzpL6EbOG5H3PVQtkbDOAvvcrNJ4PNAZd6UzFdkBQaTOISQv1U7 yI3Q8178hng/TL/+oXs927xfcTaLtEcdFFLIn7/zUP+pGVdn1gow3nfBIE8E2iUEjb RtzPTGCshDNJkwCCA/wgjT4zlSUTHW147lwehO9c= Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1727982AbfLLGXe (ORCPT ); Thu, 12 Dec 2019 01:23:34 -0500 Received: from conssluserg-04.nifty.com ([210.131.2.83]:35320 "EHLO conssluserg-04.nifty.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1727954AbfLLGXd (ORCPT ); Thu, 12 Dec 2019 01:23:33 -0500 Received: from mail-vk1-f179.google.com (mail-vk1-f179.google.com [209.85.221.179]) (authenticated) by conssluserg-04.nifty.com with ESMTP id xBC6NSf0023112 for ; Thu, 12 Dec 2019 15:23:29 +0900 DKIM-Filter: OpenDKIM Filter v2.10.3 conssluserg-04.nifty.com xBC6NSf0023112 DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=nifty.com; s=dec2015msa; t=1576131809; bh=i7ocOMSc/MW6eFTX2hPMVcZEp0K52RbWh5zz/YcJAPs=; h=References:In-Reply-To:From:Date:Subject:To:Cc:From; b=RCvQa6T4un0P7modVhShMH6LZJ/CgaCV3mrsFBlaq86vjUfPhDTY/QhMpkIwp2/U1 h3vRYpNFhng5TYc0TQJyvj/BFRbWoBYiHXjMgCxsa/Q4TfMfD4sNwn39c0JxdG09gI Y0XlXry3GSHJ0dynVSkbhIM61AcIq+F9DMzc3m9cNEkKX8B2g2unYlnVC7hRlA/Lz3 TMIUBH0LW3Q/cPcrO2F9gXFuqxFbjFsbHzU2Frg5AiOcOQY9RpOrc/1Gw+PTUvL6dv qmPGAHqJ5f/kGKbASBefcF3K7vrCt1ii5KQhDmjXmWp1MWI/dOwqPrBBlM+1K9Y2vP GvbbPvYPRZ7Sg== X-Nifty-SrcIP: [209.85.221.179] Received: by mail-vk1-f179.google.com with SMTP id u6so79837vkn.13 for ; Wed, 11 Dec 2019 22:23:28 -0800 (PST) X-Gm-Message-State: APjAAAXGe96yFKzs9LynNo9oOQZQwVHa8EtNFvfEqZgOsbXxGECpBaP8 Ve6uQCM1RIXbYzkxsxGrj56CBOlMLhEmDyquII0= X-Google-Smtp-Source: APXvYqwEm/gQyJa2MpP2RIiSjW99Fc7GxuRNeWtbPxHbHRKlC4nJ8ewJTlxrhsE89XU5uf1j8vv0e+FaBmkUoZSUrBQ= X-Received: by 2002:a1f:6344:: with SMTP id x65mr7376302vkb.26.1576131807703; Wed, 11 Dec 2019 22:23:27 -0800 (PST) MIME-Version: 1.0 References: <20191125154217.18640-1-jeyu@kernel.org> <20191206124102.12334-1-jeyu@kernel.org> In-Reply-To: <20191206124102.12334-1-jeyu@kernel.org> From: Masahiro Yamada Date: Thu, 12 Dec 2019 15:22:50 +0900 X-Gmail-Original-Message-ID: Message-ID: Subject: Re: [PATCH v3] export.h: reduce __ksymtab_strings string duplication by using "MS" section flags To: Jessica Yu Cc: Linux Kernel Mailing List , Matthias Maennich , Rasmus Villemoes , Arnd Bergmann , Greg Kroah-Hartman Content-Type: text/plain; charset="UTF-8" Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Fri, Dec 6, 2019 at 9:41 PM 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 > --- > v3: > - remove __KSTRTAB_ENTRY macros in favor of just putting the asm directly > in ___EXPORT_SYMBOL > - Document more clearly what the ___EXPORT_SYMBOL macro does > > include/asm-generic/export.h | 8 +++++--- > include/linux/export.h | 27 ++++++++++++++++++++------- > 2 files changed, 25 insertions(+), 10 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..18dcdcd118e7 100644 > --- a/include/linux/export.h > +++ b/include/linux/export.h > @@ -81,16 +81,29 @@ struct kernel_symbol { > > #else > > -/* For every exported symbol, place a struct in the __ksymtab section */ > +/* > + * For every exported symbol, do the following: Just a nit. You mention "an entry" twice to talk about different classes of structures. It might be better to be explicit about "which entry". > + * > + * - If applicable, place an entry in the __kcrctab section. "place a CRC entry in the __kcrctab section" might be clearer ? > + * - Put the name of the symbol and namespace (empty string "" for none) in > + * __ksymtab_strings. > + * - Place an entry in the __ksymtab section. "Place a struct kernel_symbol entry in __ksymtab section" ? might be clearer ? Other than that: Reviewed-by: Masahiro Yamada > + * > + * 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 ___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; \ > + 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) > > #endif > -- > 2.16.4 > -- Best Regards Masahiro Yamada