Re: [PATCH v2] export.h: reduce __ksymtab_strings string duplication by using "MS" section flags
From: Masahiro Yamada
Date: Tue Nov 26 2019 - 03:33:41 EST
On Tue, Nov 26, 2019 at 12:42 AM Jessica Yu <jeyu@xxxxxxxxxx> 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@xxxxxxxxxxxxxxxxxx>
> Signed-off-by: Jessica Yu <jeyu@xxxxxxxxxx>
> ---
>
> 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