Re: [PATCH v2] export.h: reduce __ksymtab_strings string duplication by using "MS" section flags

From: Masahiro Yamada
Date: Tue Nov 26 2019 - 11:13:17 EST

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On Wed, Nov 27, 2019 at 12:32 AM Jessica Yu <jeyu@xxxxxxxxxx> wrote:
>
> +++ Matthias Maennich [26/11/19 13:56 +0000]:
> >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@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.
> >>>
> >
> >Thanks for working on this!
> >
> >>>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().
> >
> >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.
>
> Yeah, the macros were more readable IMO. But I could just squash them into one
> __KSTRTAB_ENTRY macro as a compromise for Masahiro maybe?
>
> Is this any better?

I prefer opposite.

__CRC_SYMBOL() is macrofied because it is ifdef'ed by
CONFIG_MODVERSIONS and CONFIG_MOD_REL_CRCS.

__KSYMTAB_ENTRY() is macrofied because it is ifdef'ed by
CONFIG_HAVE_ARCH_PREL32_RELOCATIONS.

__KSTRTAB_ENTRY() does not depend on any CONFIG option,
so it can be expanded in ___EXPORT_SYMBOL().

You need to check multiple locations
to understand how it works as a whole.
I do not understand why increasing macro-chains is readable.

Masahiro

>
> diff --git a/include/linux/export.h b/include/linux/export.h
> index 201262793369..f4a8fc798a1b 100644
> --- a/include/linux/export.h
> +++ b/include/linux/export.h
> @@ -81,16 +81,30 @@ 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.
> + *
> + * This basically corresponds to:
> + * const char __kstrtab_##sym[] __attribute__((section("__ksymtab_strings")) = #sym;
> + * const char __kstrtabns_##sym[] __attribute__((section("__ksymtab_strings")) = ns;
> + */
> +#define __KSTRTAB_ENTRY(sym, ns) \
> + asm(" .section \"__ksymtab_strings\",\"aMS\",%progbits,1 \n" \
> + "__kstrtab_" #sym ": \n" \
> + " .asciz \"" #sym "\" \n" \
> + "__kstrtabns_" #sym ": \n" \
> + " .asciz \"" ns "\" \n" \
> + " .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, ns); \
> __KSYMTAB_ENTRY(sym, sec)
>
> #endif
>
> >>
> >>
> >>
> >>
> >>
> >>
> >>
> >>>+ " .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@xxxxxxxxxx>
> >
> >Cheers,
> >Matthias
> >
> >>
> >>
> >>--
> >>Best Regards
> >>Masahiro Yamada

--
Best Regards
Masahiro Yamada

Next message: Marc Gonzalez: "[PATCH v1] clk: Convert managed get functions to devm_add_action API"
Previous message: Jules Irenge: "Re: [PATCH] cpu: microcode: replace 0 with NULL"
In reply to: Matthias Maennich: "Re: [PATCH v2] export.h: reduce __ksymtab_strings string duplication by using "MS" section flags"
Next in thread: Jessica Yu: "Re: [PATCH v2] export.h: reduce __ksymtab_strings string duplication by using "MS" section flags"
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