On Wed, 24 Aug 2016 19:05:30 +1000 Nicholas Piggin <npiggin@xxxxxxxxx> wrote: > On Wed, 24 Aug 2016 09:41:39 +0200 > Arnd Bergmann <arnd@xxxxxxxx> wrote: > > > On Wednesday, August 24, 2016 2:00:44 PM CEST Nicholas Piggin wrote: > > > On Tue, 23 Aug 2016 14:01:29 +0200 > > > Arnd Bergmann <arnd@xxxxxxxx> wrote: > > > > > > > On Friday, August 12, 2016 6:19:17 PM CEST Nicholas Piggin wrote: > > > > > Erratum 657417 is worked around by the linker by inserting additional > > > > > branch trampolines to avoid problematic branch target locations. This > > > > > results in much higher linking time and presumably slower and larger > > > > > generated code. The workaround also seems to only be required when > > > > > linking thumb2 code, but the linker applies it for non-thumb2 code as > > > > > well. > > > > > > > > > > The workaround today is left to the linker to apply, which is overly > > > > > conservative. > > > > > > > > > > https://sourceware.org/ml/binutils/2009-05/msg00297.html > > > > > > > > > > This patch adds an option which defaults to "y" in cases where we > > > > > could possibly be running Cortex A8 and using Thumb2 instructions. > > > > > In reality the workaround might not be required at all for the kernel > > > > > if virtual instruction memory is linear in physical memory. However it > > > > > is more conservative to keep the workaround, and it may be the case > > > > > that the TLB lookup would be required in order to catch branches to > > > > > unmapped or no-execute pages. > > > > > > > > > > In an allyesconfig build, this workaround causes a large load on > > > > > the linker's branch stub hash and slows down the final link by a > > > > > factor of 5. > > > > > > > > > > Signed-off-by: Nicholas Piggin <npiggin@xxxxxxxxx> > > > > > > > > > > > > > Thanks a lot for finding this issue. I can confirm that your patch > > > > helps noticeably in all configurations, reducing time for a relink > > > > from 18 minutes to 9 minutes on my machine in the best case, but > > > > the factor 10 slowdown of the final link with your thin archives > > > > and gc-sections patches remains. > > > > > > > > I suspect there is still something else going on besides the 657417 > > > > slowing things down, but it's also possible that I'm doing something > > > > wrong here. > > > > > > Okay, I was only testing thin archives. gc-sections I didn't look at. > > > With thin archives, one final arm allyesconfig link with this patch is > > > not showing a regression. gc-sections must be causing something else > > > ARM specific, because powerpc seems to link fast with gc-sections. > > > > Ok, I see. For completeness, here are my results with thin archives and > > without gc-sections on ARM: > > This is about what I saw. > > > > > || no THUMB2, thin archives, no gc-sections, before: 144 seconds > > > > 09:29:51 LINK vmlinux > > 09:29:51 AR built-in.o > > 09:29:52 LD vmlinux.o > > 09:30:12 MODPOST vmlinux.o > > 09:30:14 GEN .version > > 09:30:14 CHK include/generated/compile.h > > UPD include/generated/compile.h > > 09:30:14 CC init/version.o > > 09:30:15 AR init/built-in.o > > 09:30:43 KSYM .tmp_kallsyms1.o > > 09:31:28 KSYM .tmp_kallsyms2.o > > 09:31:40 LD vmlinux > > 09:32:13 SORTEX vmlinux > > 09:32:13 SYSMAP System.map > > 09:32:15 OBJCOPY arch/arm/boot/Image > > > > || no THUMB2, thin archives, no gc-sections, after: 70 seconds > > > > 09:33:54 LINK vmlinux > > 09:33:54 AR built-in.o > > 09:33:55 LD vmlinux.o > > 09:34:13 MODPOST vmlinux.o > > 09:34:15 GEN .version > > 09:34:16 CHK include/generated/compile.h > > UPD include/generated/compile.h > > 09:34:16 CC init/version.o > > 09:34:16 AR init/built-in.o > > 09:34:24 KSYM .tmp_kallsyms1.o > > 09:34:43 KSYM .tmp_kallsyms2.o > > 09:34:55 LD vmlinux > > 09:35:03 SORTEX vmlinux > > 09:35:03 SYSMAP System.map > > 09:35:04 OBJCOPY arch/arm/boot/Image > > > > The final 'LD' step is much faster here as you also found, and now > > the time for the complete link is mainly the initial 'LD vmlinux.o' > > step, which did not get faster with your patch. > > The info here isn't very good because KSYM is printed after the link > but before the kallsyms generation. We can kind of see what's happening > if we take the time between the KSYM and the LD vmlinux as the time for > kallsyms: > before after > KSYM ~12s ~12s > LD vmlinux.o 20s 18s > LD .tmp_kallsyms1.o 28s 8s > LD .tmp_kallsyms2.o 33s 7s > LD vmlinux 33s 8s > > Probably the cortex a8 workaround does not get applied to vmlinux.o > link (due to being incremental), so we don't see any speedup with the > patch. It takes longer overall I guess because it keeps a lot of > symbols in the output file (due to incremental). > > > > > Can you send your latest ARM patch to enable this and I'll have a look > > > at it? > > > > See below. I have not updated the patch description yet, but included > > the changes that Nico suggested. The test above used the same patch > > but left out the 'select LD_DEAD_CODE_DATA_ELIMINATION' line. > > Thanks, I'll take a look. Okay, I can't reproduce your bad linking times even with gc-sections. It's possible I'm doing something wrong, but with my patches + your patch and standard arm allyesconfig: 20:33:56 AR built-in.o 20:33:57 LD vmlinux.o MODPOST vmlinux.o 20:34:12 GEN .version CHK include/generated/compile.h UPD include/generated/compile.h CC init/version.o AR init/built-in.o 20:34:24 KSYM .tmp_kallsyms1.o 20:34:45 KSYM .tmp_kallsyms2.o 20:34:54 LD vmlinux 20:35:07 SORTEX vmlinux 20:35:07 SYSMAP System.map I have about 71 seconds for the final link phase. Command is: make ARCH=arm CROSS_COMPILE=arm-linux-gnueabi- -j3 vmlinux $ arm-linux-gnueabi-ld -v GNU ld (GNU Binutils for Debian) 2.27 Confirming function/data sections: $ objdump -h built-in.o | grep ^[[:space:]]*[0-9][0-9]* | wc -l 1012848 $ size vmlinux text data bss dec hex filename 74205771 36746855 19072744 130025370 7c0079a vmlinux $ file vmlinux vmlinux: ELF 32-bit LSB executable, ARM, EABI5 version 1 (SYSV), statically linked, BuildID[sha1]=37d207288f81f83291dfb45294a94c555d9fbdb8, not stripped Final link command is: arm-linux-gnueabi-ld -EL --no-fix-cortex-a8 -p --no-undefined -X --pic-veneer --build-id --gc-sections -X -o vmlinux -T ./arch/arm/kernel/vmlinux.lds --whole-archive built-in.o .tmp_kallsyms2.o Which takes 12s I'm building THUMB2 now, but if you have any hints for me... Thanks, Nick -- To unsubscribe from this list: send the line "unsubscribe linux-kbuild" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
