Hi Casey,
2018-08-23 8:21 GMT+09:00 Casey Fitzpatrick <kcghost@xxxxxxxxx>:
> I have observed that attempting to rebuild the kernel with no changes
> to source files:
> 1. Takes more time than I should hope is necessary; I would hope it
> would only check timestamps and determine the target does not need
> updating rather than do work.
> 2. Switches between two states of determining what work is needed to do.
> 3. Unnecessarily updates timestamp on bzImage (and maybe others)
> despite not actually modifying the file.
>
> I am working with v4.14.66 building from an Ubuntu 17.10 machine.
> The following summarized output demonstrates the issue (I believe the
> same happens with the default target).
> Notice that despite no changes made to sources, a rebuild takes 3
> seconds on my machine, does some work and typically updates the
> timestamp of the target.
> Also notice that the work is oddly inconsistent, the third time I ran
> the same build with no changes it did different work. And in that case
> it does not update the timestamp of the target, which is preferable.
>
> make distclean
> make x86_64_defconfig
> time make -j16 bzImage
> ...(lots of output, actually building all the sources)
> OBJCOPY arch/x86/boot/setup.bin
> BUILD arch/x86/boot/bzImage
> Setup is 15548 bytes (padded to 15872 bytes).
> System is 7789 kB
> CRC 5b2b5059
> Kernel: arch/x86/boot/bzImage is ready (#1)
>
> real 4m15.147s
> user 27m21.182s
> sys 2m34.607s
> stat --printf 'mtime: %y\n' arch/x86/boot/bzImage
> mtime: 2018-08-22 18:57:10.000000000 -0400
> time make -j16 bzImage
> CHK include/config/kernel.release
> CHK include/generated/uapi/linux/version.h
> DESCEND objtool
> CHK scripts/mod/devicetable-offsets.h
> CHK include/generated/utsrelease.h
> CHK include/generated/timeconst.h
> CHK include/generated/bounds.h
> CHK include/generated/asm-offsets.h
> CALL scripts/checksyscalls.sh
> CHK include/generated/compile.h
> DATAREL arch/x86/boot/compressed/vmlinux
> BUILD arch/x86/boot/bzImage
> Setup is 15548 bytes (padded to 15872 bytes).
> System is 7789 kB
> CRC 5b2b5059
> Kernel: arch/x86/boot/bzImage is ready (#1)
>
> real 0m3.273s
> user 0m11.260s
> sys 0m4.727s
> stat --printf 'mtime: %y\n' arch/x86/boot/bzImage
> mtime: 2018-08-22 19:00:26.587233951 -0400
> time make -j16 bzImage
> CHK include/config/kernel.release
> CHK include/generated/uapi/linux/version.h
> DESCEND objtool
> CHK include/generated/utsrelease.h
> CHK scripts/mod/devicetable-offsets.h
> CHK include/generated/timeconst.h
> CHK include/generated/bounds.h
> CHK include/generated/asm-offsets.h
> CALL scripts/checksyscalls.sh
> CHK include/generated/compile.h
> LD arch/x86/boot/compressed/vmlinux
> ZOFFSET arch/x86/boot/zoffset.h
> OBJCOPY arch/x86/boot/vmlinux.bin
> AS arch/x86/boot/header.o
> LD arch/x86/boot/setup.elf
> OBJCOPY arch/x86/boot/setup.bin
> BUILD arch/x86/boot/bzImage
> Setup is 15548 bytes (padded to 15872 bytes).
> System is 7789 kB
> CRC 5b2b5059
> Kernel: arch/x86/boot/bzImage is ready (#1)
>
> real 0m3.136s
> user 0m11.411s
> sys 0m4.775s
> stat --printf 'mtime: %y\n' arch/x86/boot/bzImage
> mtime: 2018-08-22 19:01:11.583377706 -0400
> time make -j16 bzImage
> CHK include/config/kernel.release
> CHK include/generated/uapi/linux/version.h
> DESCEND objtool
> CHK include/generated/utsrelease.h
> CHK scripts/mod/devicetable-offsets.h
> CHK include/generated/timeconst.h
> CHK include/generated/bounds.h
> CHK include/generated/asm-offsets.h
> CALL scripts/checksyscalls.sh
> CHK include/generated/compile.h
> DATAREL arch/x86/boot/compressed/vmlinux
> Kernel: arch/x86/boot/bzImage is ready (#1)
>
> real 0m3.118s
> user 0m11.476s
> sys 0m4.505s
> stat --printf 'mtime: %y\n' arch/x86/boot/bzImage
> mtime: 2018-08-22 19:01:11.583377706 -0400
>
> Sincerely,
> Casey Fitzpatrick
Could you check if the following commit fixes the problem?
commit 92a4728608a8fd228c572bc8ff50dd98aa0ddf2a
Author: Kees Cook <keescook@xxxxxxxxxxxx>
Date: Tue Jul 24 16:08:27 2018 -0700
x86/boot: Fix if_changed build flip/flop bug
I am not sure whether it was back-ported to 4.14 kernel or not...
--
Best Regards
Masahiro Yamada
