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. Aside from that, I notice that for the purpose of speeding up "allyesconfig", we don't actually need to make this user configurable, it's sufficient to disable the workaround when CONFIG_THUMB2_KERNEL is disabled, which is what allyesconfig and all the defconfig files (but not randconfig) use. I also found that using THUMB2_KERNEL itself causes a 50% slowdown. I have patches on my "randconfig" test tree that have the side-effect of enabling THUMB2_KERNEL for allyesconfig, which is one reason I have been getting worse results than others. I could also try to revive an older patch I started, to annotate the specific CPU core on each ARMv7 platform. I think I have all the information we need for that, and there are other advantages in doing it: we could be more selective with all the ARMv7 errata, and automatically determine whether some optional CPU features (LPAE, virtualization, integer divide) are available on all of the selected CPU cores. Arnd --- Full link timing results follow || THUMB2, thin archive + gc-sections, before: 18 minutes 09:56:47 LINK vmlinux 09:56:47 AR built-in.o 09:56:49 LD vmlinux.o 10:04:27 MODPOST vmlinux.o 10:04:29 GEN .version 10:04:29 CHK include/generated/compile.h UPD include/generated/compile.h 10:04:29 CC init/version.o 10:04:29 AR init/built-in.o 10:07:39 KSYM .tmp_kallsyms1.o 10:11:05 KSYM .tmp_kallsyms2.o 10:11:16 LD vmlinux 10:14:30 SORTEX vmlinux 10:14:30 SYSMAP System.map || THUMB2, thin archive + gc-sections, after: 9 minutes 10:16:01 CHK include/generated/uapi/linux/version.h 10:16:02 LINK vmlinux 10:16:02 AR built-in.o 10:16:03 LD vmlinux.o 10:23:43 MODPOST vmlinux.o 10:23:46 GEN .version 10:23:46 CHK include/generated/compile.h UPD include/generated/compile.h 10:23:46 CC init/version.o 10:23:47 AR init/built-in.o 10:24:04 KSYM .tmp_kallsyms1.o 10:24:32 KSYM .tmp_kallsyms2.o 10:24:45 LD vmlinux 10:25:00 SORTEX vmlinux 10:25:00 SYSMAP System.map || THUMB2, no thin archive + gc-sections, before: 93 seconds 10:44:35 CHK include/generated/uapi/linux/version.h 10:44:35 LINK vmlinux 10:44:35 LD vmlinux.o 10:44:39 MODPOST vmlinux.o 10:44:41 GEN .version 10:44:41 CHK include/generated/compile.h UPD include/generated/compile.h 10:44:41 CC init/version.o 10:44:41 LD init/built-in.o 10:45:02 KSYM .tmp_kallsyms1.o 10:45:35 KSYM .tmp_kallsyms2.o 10:45:47 LD vmlinux 10:46:06 SORTEX vmlinux 10:46:06 SYSMAP System.map 10:46:08 OBJCOPY arch/arm/boot/Image || THUMB2, no thin archive + gc-sections, after: 52 seconds 10:41:46 LINK vmlinux 10:41:46 LD vmlinux.o 10:41:49 MODPOST vmlinux.o 10:41:52 GEN .version 10:41:52 CHK include/generated/compile.h UPD include/generated/compile.h 10:41:52 CC init/version.o 10:41:52 LD init/built-in.o 10:41:58 KSYM .tmp_kallsyms1.o 10:42:17 KSYM .tmp_kallsyms2.o 10:42:31 LD vmlinux 10:42:36 SORTEX vmlinux 10:42:36 SYSMAP System.map 10:42:38 OBJCOPY arch/arm/boot/Image || THUMB2_KERNEL disabled, no thin archives + gc-sections, before: 59 seconds 11:25:05 LINK vmlinux 11:25:05 LD vmlinux.o 11:25:07 MODPOST vmlinux.o 11:25:10 GEN .version 11:25:10 CHK include/generated/compile.h UPD include/generated/compile.h 11:25:10 CC init/version.o 11:25:10 LD init/built-in.o 11:25:19 KSYM .tmp_kallsyms1.o 11:25:41 KSYM .tmp_kallsyms2.o 11:25:53 LD vmlinux 11:26:03 SORTEX vmlinux 11:26:03 SYSMAP System.map Building modules, stage 2. || THUMB2_KERNEL disabled, no thin archives + gc-sections, after: 46 seconds 11:27:36 LINK vmlinux 11:27:36 LD vmlinux.o 11:27:39 MODPOST vmlinux.o 11:27:41 GEN .version 11:27:41 CHK include/generated/compile.h UPD include/generated/compile.h 11:27:41 CC init/version.o 11:27:41 LD init/built-in.o 11:27:46 KSYM .tmp_kallsyms1.o 11:28:04 KSYM .tmp_kallsyms2.o 11:28:15 LD vmlinux 11:28:20 SORTEX vmlinux 11:28:20 SYSMAP System.map 11:28:22 OBJCOPY arch/arm/boot/Image || THUMB2_KERNEL disabled, thin archives+gc-sections, before: 12 minutes 13:18:39 LINK vmlinux 13:18:39 AR built-in.o 13:18:40 LD vmlinux.o 13:24:44 MODPOST vmlinux.o 13:24:46 GEN .version 13:24:46 CHK include/generated/compile.h UPD include/generated/compile.h 13:24:46 CC init/version.o 13:24:46 AR init/built-in.o 13:26:34 KSYM .tmp_kallsyms1.o 13:28:32 KSYM .tmp_kallsyms2.o 13:28:43 LD vmlinux 13:30:31 SORTEX vmlinux 13:30:31 SYSMAP System.map 13:30:33 OBJCOPY arch/arm/boot/Image || THUMB2_KERNEL disabled, thin archives+gc-sections, after: 7 minutes 12:43:15 LINK vmlinux 12:43:15 AR built-in.o 12:43:16 LD vmlinux.o 12:49:19 MODPOST vmlinux.o 12:49:21 GEN .version 12:49:21 CHK include/generated/compile.h UPD include/generated/compile.h 12:49:22 CC init/version.o 12:49:22 AR init/built-in.o 12:49:33 KSYM .tmp_kallsyms1.o 12:49:56 KSYM .tmp_kallsyms2.o 12:50:07 LD vmlinux 12:50:19 SORTEX vmlinux 12:50:19 SYSMAP System.map 12:50:21 OBJCOPY arch/arm/boot/Image Building modules, stage 2. -- 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
