Messages in this thread | | | Date | Tue, 26 Nov 2019 13:56:20 +0000 | From | Matthias Maennich <> | 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 05:32:59PM +0900, Masahiro Yamada wrote: >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. >>
Thanks for working on this!
>> 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().
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.
> > > > > > > >> + " .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; \
You could keep simplified versions of these statements as comment for the above macros to increase readability.
>> - 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 >>
With the above addressed, please feel free to add
Reviewed-by: Matthias Maennich <maennich@google.com>
Cheers, Matthias
> > >-- >Best Regards >Masahiro Yamada
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