Messages in this thread Patch in this message | | | Date | Tue, 26 Nov 2019 17:48:40 +0100 | From | Jessica Yu <> | Subject | Re: [PATCH v2] export.h: reduce __ksymtab_strings string duplication by using "MS" section flags |
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+++ Masahiro Yamada [27/11/19 01:12 +0900]: >On Wed, Nov 27, 2019 at 12:32 AM Jessica Yu <jeyu@kernel.org> 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@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. >> >> 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.
I think it is "readable" in the sense that when someone is reading export.h for the first time, they know exactly what ___EXPORT_SYMBOL() is supposed to do, without requiring them to read too deeply into the asm and swim through the #ifdefs. __CRC_SYMBOL(), depending on modversions, creates an entry in what would eventually be merged to become the __kcrctab section, __KSTRTAB_ENTRY() creates entries in __ksymtab_strings, and __KSYMTAB_ENTRY() creates an entry in (what would eventually become) the __ksymtab{,_gpl} section. It gives a general idea of what it's doing.
However, I do understand your reasoning behind not having an extra macro. I'll wait a bit before respinning the patch to see if we can get at a consensus..
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