Messages in this thread Patch in this message | | | Date | Thu, 12 Dec 2019 11:36:07 +0100 | From | Jessica Yu <> | Subject | Re: [PATCH v3] export.h: reduce __ksymtab_strings string duplication by using "MS" section flags |
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+++ Masahiro Yamada [12/12/19 15:22 +0900]: >On Fri, Dec 6, 2019 at 9:41 PM Jessica Yu <jeyu@kernel.org> 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 <linux@rasmusvillemoes.dk> >> Signed-off-by: Jessica Yu <jeyu@kernel.org> >> --- >> 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 <masahiroy@kernel.org>
Yeah, that is much clearer, will resend. Thanks!
Jessica
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