Messages in this thread | | | From | Masahiro Yamada <> | Date | Tue, 26 Nov 2019 17:32:59 +0900 | Subject | Re: [PATCH v2] export.h: reduce __ksymtab_strings string duplication by using "MS" section flags |
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On Tue, Nov 26, 2019 at 12:42 AM 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> > --- > > 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().
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)
> + " .previous \n") > + > /* For every exported symbol, place a struct in the __ksymtab section */ > #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; \ > + __KSTRTAB_ENTRY(sym); \ > + __KSTRTAB_NS_ENTRY(sym, ns); \ > __KSYMTAB_ENTRY(sym, sec) > > #endif > -- > 2.16.4 >
-- Best Regards Masahiro Yamada
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