Messages in this thread | | | Date | Sat, 03 Jan 2015 01:12:43 +0100 | From | Daniel Borkmann <> | Subject | Re: [PATCH akpm/next] lib: crc32: conditionally constify crc32 lookup table |
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On 01/03/2015 12:35 AM, Andrew Morton wrote: > On Wed, 31 Dec 2014 20:03:28 +0100 Daniel Borkmann <dborkman@redhat.com> wrote: > >> Commit 8f243af42ade ("sections: fix const sections for crc32 table") > > The 8f243af42ade changelog is rather poor :(. With the help of this > changelog I can now see what 8f243af42ade was doing. I must have been > asleep at the time. > >> removed the compile-time generated crc32 tables from the RO sections, >> because it conflicts with the definition of __cacheline_aligned >> which puts all such aligned data into .data..cacheline_aligned section >> optimized for wasting less space, and causes const align issues with >> some GCC versions (see #52181, for example). > > (searches several bugzilla databases) > > "https://gcc.gnu.org/bugzilla/show_bug.cgi?id=52181" would be more > reader-friendly.
Okay, will add it like that.
>> We can fix that in two steps: 1) by using the ____cacheline_aligned >> version, which only aligns the data but doesn't move it into specific >> sections, 2) test GCC and in problematic cases fall back to the current >> code, otherwise use const and proper alignment for the lookup tables. >> >> After patch tables are in RO: >> >> $ nm -v lib/crc32.o | grep -1 -E "crc32c?table" >> 0000000000000000 t arch_local_irq_enable >> 0000000000000000 r crc32ctable_le >> 0000000000000000 t crc32_exit >> -- >> 0000000000000960 t test_buf >> 0000000000002000 r crc32table_be >> 0000000000004000 r crc32table_le >> 000000001d1056e5 A __crc_crc32_be >> >> Signed-off-by: Daniel Borkmann <dborkman@redhat.com> >> Cc: Joe Mario <jmario@redhat.com> >> --- >> Makefile | 5 +++++ >> lib/Makefile | 3 +++ >> lib/gen_crc32table.c | 21 +++++++++++++++------ >> scripts/gcc-const-align.sh | 21 +++++++++++++++++++++ >> 4 files changed, 44 insertions(+), 6 deletions(-) > > Seems a lot of fuss. Why are these tables cacheline aligned anyway? > To avoid one cache miss (most of the time, presumably) in a 16k table. > Pretty marginal benefit, I suspect.
I guess, it actually came in with the slice-by-8 algorithm (e.g. used in SCTP checksumming if no offloading is available) that was added back then, that is, commit 324eb0f17d9dc ("crc32: add slice-by-8 algorithm to existing code").
>> --- a/lib/Makefile >> +++ b/lib/Makefile >> @@ -171,6 +171,9 @@ obj-$(CONFIG_FONT_SUPPORT) += fonts/ >> hostprogs-y := gen_crc32table >> clean-files := crc32table.h >> >> +# We need to transfer this flag to the host compiler if present >> +HOSTCFLAGS_gen_crc32table.o := $(findstring -DCC_HAVE_CONST_ALIGN,$(KBUILD_CFLAGS)) >> + >> $(obj)/crc32.o: $(obj)/crc32table.h >> >> quiet_cmd_crc32 = GEN $@ >> diff --git a/lib/gen_crc32table.c b/lib/gen_crc32table.c >> index 71fcfcd..2f06893 100644 >> --- a/lib/gen_crc32table.c >> +++ b/lib/gen_crc32table.c >> @@ -21,6 +21,14 @@ >> # define BE_TABLE_SIZE (1 << CRC_BE_BITS) >> #endif >> >> +#ifdef CC_HAVE_CONST_ALIGN >> +# define TABLE_CONST_ATTR "const" >> +# define TABLE_ALIGNMENT "____cacheline_aligned" >> +#else >> +# define TABLE_CONST_ATTR "" >> +# define TABLE_ALIGNMENT "__cacheline_aligned" >> +#endif > > Pity out poor readers, trying to work out what all this does and why it > is here. It is totally unobvious that this is working around some gcc > bug. Can we please have a nice comment which explains everything?
Will add a comment, sure.
Thanks, Andrew!
I'll send out v2 with your feedback tomorrow.
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