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
