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[209.132.180.67]) by mx.google.com with ESMTP id g12si7442199ejd.43.2019.11.26.08.51.16; Tue, 26 Nov 2019 08:51:40 -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=iquYMZ08; 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 S1727925AbfKZQss (ORCPT + 99 others); Tue, 26 Nov 2019 11:48:48 -0500 Received: from mail.kernel.org ([198.145.29.99]:50108 "EHLO mail.kernel.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1727532AbfKZQsr (ORCPT ); Tue, 26 Nov 2019 11:48:47 -0500 Received: from linux-8ccs (x2f7ff09.dyn.telefonica.de [2.247.255.9]) (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 D3E9A20722; Tue, 26 Nov 2019 16:48:44 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=default; t=1574786926; bh=GAmF3TYyk2ddyiuqlE+OzSfxjq6+jpC7YwdpGDEexBw=; h=Date:From:To:Cc:Subject:References:In-Reply-To:From; b=iquYMZ08r+aRZillGk5lb3NuM8yqJDRtbqVNCm1lz6ZgRHr68L/cbW08hq1qycyA3 oO8F1XYt/6KMMn+59RkBmKFwOEK2lMZlhsEKnfFtmhTeskiY4p9Ok8vc8EHUpag1ut RZNCCS7Yno1Z794VxNZP/shqJOsYK+v1MuNTeAjA= Date: Tue, 26 Nov 2019 17:48:40 +0100 From: Jessica Yu To: Masahiro Yamada Cc: Matthias Maennich , Linux Kernel Mailing List , 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: <20191126164840.GA8011@linux-8ccs> References: <20191120145110.8397-1-jeyu@kernel.org> <20191125154217.18640-1-jeyu@kernel.org> <20191126135620.GA38845@google.com> <20191126153153.GA3495@linux-8ccs> 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 [27/11/19 01:12 +0900]: >On Wed, Nov 27, 2019 at 12:32 AM Jessica Yu wrote: >> >> +++ Matthias Maennich [26/11/19 13:56 +0000]: >> >On Tue, Nov 26, 2019 at 05:32:59PM +0900, Masahiro Yamada wrote: >> >>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. >> >>> >> > >> >Thanks for working on this! >> > >> >>>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(). >> > >> >I agree with this, these entries should be consistent. >> > >> >> >> >> >> >> >> >>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) >> >> >> > >> >I would prefer the separate macros though (as initially proposed) as I >> >find them much more readable. The code is already a bit tricky to reason >> >about and I don't think the shorter version is enough of a gain. >> >> Yeah, the macros were more readable IMO. But I could just squash them into one >> __KSTRTAB_ENTRY macro as a compromise for Masahiro maybe? >> >> Is this any better? > >I prefer opposite. > > >__CRC_SYMBOL() is macrofied because it is ifdef'ed by >CONFIG_MODVERSIONS and CONFIG_MOD_REL_CRCS. > >__KSYMTAB_ENTRY() is macrofied because it is ifdef'ed by >CONFIG_HAVE_ARCH_PREL32_RELOCATIONS. > > > >__KSTRTAB_ENTRY() does not depend on any CONFIG option, >so it can be expanded in ___EXPORT_SYMBOL(). > > >You need to check multiple locations >to understand how it works as a whole. >I do not understand why increasing macro-chains is readable. I think it is "readable" in the sense that when someone is reading export.h for the first time, they know exactly what ___EXPORT_SYMBOL() is supposed to do, without requiring them to read too deeply into the asm and swim through the #ifdefs. __CRC_SYMBOL(), depending on modversions, creates an entry in what would eventually be merged to become the __kcrctab section, __KSTRTAB_ENTRY() creates entries in __ksymtab_strings, and __KSYMTAB_ENTRY() creates an entry in (what would eventually become) the __ksymtab{,_gpl} section. It gives a general idea of what it's doing. However, I do understand your reasoning behind not having an extra macro. I'll wait a bit before respinning the patch to see if we can get at a consensus..