Messages in this thread | | | Date | Thu, 12 Dec 2019 14:29:40 +0000 | From | Matthias Maennich <> | Subject | Re: [PATCH v4] export.h: reduce __ksymtab_strings string duplication by using "MS" section flags |
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On Thu, Dec 12, 2019 at 03:16:13PM +0100, 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 <linux@rasmusvillemoes.dk> >Reviewed-by: Masahiro Yamada <masahiroy@kernel.org> >Signed-off-by: Jessica Yu <jeyu@kernel.org> >--- >v4: > - fix the comment above ___EXPORT_SYMBOL to be more specific about what > entries are being placed in their respective sections. > > 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 afddc5442e92..365345f9a9e3 100644 >--- a/include/asm-generic/export.h >+++ b/include/asm-generic/export.h >@@ -27,9 +27,11 @@ > .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 > .section ___ksymtab\sec+\name,"a" >@@ -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 627841448293..c166d35e3d76 100644 >--- a/include/linux/export.h >+++ b/include/linux/export.h >@@ -82,16 +82,29 @@ struct kernel_symbol { > > #else > >-/* For every exported symbol, place a struct in the __ksymtab section */ >+/* >+ * For every exported symbol, do the following: >+ * >+ * - If applicable, place a CRC entry in the __kcrctab section. >+ * - Put the name of the symbol and namespace (empty string "" for none) in >+ * __ksymtab_strings. >+ * - Place a struct kernel_symbol entry in the __ksymtab section. >+ * >+ * 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"); \
nit: You might want to align the newline characters up to the asm line.
Thanks for working on this!
Reviewed-by: Matthias Maennich <maennich@google.com>
Cheers, Matthias
> __KSYMTAB_ENTRY(sym, sec) > > #endif >-- >2.16.4 >
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