Messages in this thread Patch in this message | | | Date | Mon, 16 Dec 2019 10:41:42 +0100 | From | Jessica Yu <> | Subject | Re: [PATCH v4] export.h: reduce __ksymtab_strings string duplication by using "MS" section flags |
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+++ Matthias Maennich [12/12/19 14:29 +0000]: >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>
I've fixed up the newlines and applied to modules-next. Thanks!
Jessica
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