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When debugging kernel, I often rebuild the kernel with just make. Even if I just modify one-liner, I have to pass through all the below commands:

make[1]: Nothing to be done for `all'.
  CHK     include/generated/uapi/linux/version.h
make[1]: Nothing to be done for `relocs'.
  CHK     include/generated/utsrelease.h
  CALL    scripts/checksyscalls.sh
  CHK     include/generated/compile.h
  PASYMS  arch/x86/realmode/rm/pasyms.h
  LDS     arch/x86/realmode/rm/realmode.lds
  LD      arch/x86/realmode/rm/realmode.elf
  RELOCS  arch/x86/realmode/rm/realmode.relocs
  OBJCOPY arch/x86/realmode/rm/realmode.bin
  AS      arch/x86/realmode/rmpiggy.o
  CHK     kernel/config_data.h
  LD      arch/x86/realmode/built-in.o
  VDSOSYM arch/x86/vdso/vdso-syms.lds
  VDSOSYM arch/x86/vdso/vdso32-int80-syms.lds
  VDSOSYM arch/x86/vdso/vdso32-syscall-syms.lds
  VDSOSYM arch/x86/vdso/vdso32-sysenter-syms.lds
  VDSOSYM arch/x86/vdso/vdso32-syms.lds
  LD      arch/x86/vdso/built-in.o
  LD      arch/x86/built-in.o
  CC      drivers/gpu/drm/i915/i915_debugfs.o
  LD      drivers/gpu/drm/i915/i915.o
  LD      drivers/gpu/drm/i915/built-in.o
  LD      drivers/gpu/drm/built-in.o
  LD      drivers/gpu/built-in.o
  LD      drivers/built-in.o
  CHK     include/generated/uapi/linux/version.h
make[2]: Nothing to be done for `all'.
make[2]: Nothing to be done for `relocs'.
  LINK    vmlinux
  LD      vmlinux.o
  MODPOST vmlinux.o
  GEN     .version
  CHK     include/generated/compile.h
  UPD     include/generated/compile.h
  CC      init/version.o
  LD      init/built-in.o
  KSYM    .tmp_kallsyms1.o
  KSYM    .tmp_kallsyms2.o
  LD      vmlinux
  SORTEX  vmlinux
  SYSMAP  System.map
  Building modules, stage 2.
  VOFFSET arch/x86/boot/voffset.h
  CC      arch/x86/boot/version.o
  OBJCOPY arch/x86/boot/compressed/vmlinux.bin
  GZIP    arch/x86/boot/compressed/vmlinux.bin.gz
  MKPIGGY arch/x86/boot/compressed/piggy.S
  AS      arch/x86/boot/compressed/piggy.o
  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 16688 bytes (padded to 16896 bytes).
System is 4107 kB
CRC 9346a092
Kernel: arch/x86/boot/bzImage is ready  (#29)
  MODPOST 664 modules

For passing all over the above commands, it almost take 1~2 minutes, which I still feel boring. Is there any kind of tricks to reduce the kernel re-build time?

jaeyong
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2 Answers2

1

The -j flag will tell make how many jobs to do in parallel. Eg. make -j4 will do four jobs in parallel.

Regarding what should you set -j to; I don't believe there's a straightforward answer.

There's a good thread about it here: GNU make: should the number of jobs equal the number of CPU cores in a system?

Community
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Andy J
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  • Is this -j flag really help a lot for recompiling a small part of codes? Most of the steps I mentioned are interdependent each other so I don't think -j option could help a lot. Nevertheless, I already using -j option. – jaeyong Nov 10 '14 at 07:40
  • GNU `make` will do everything it can in parallel when `-j` is used. But if A depends on B, which depends on C, none of them can be done in parallel, and `make` is forced to do them in serial. – Andy J Nov 10 '14 at 09:25
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Check out some of the options from make help in the kernel source directory.

$make help
Other generic targets:
all - Build all targets marked with [*]

  • vmlinux - Build the bare kernel
  • modules - Build all modules modules_install - Install all modules to INSTALL_MOD_PATH (default: /) firmware_install- Install all firmware to INSTALL_FW_PATH (default: $(INSTALL_MOD_PATH)/lib/firmware) dir/ - Build all files in dir and below dir/file.[oisS] - Build specified target only dir/file.lst - Build specified mixed source/assembly target only (requires a recent binutils and recent build (System.map)) dir/file.ko - Build module including final link
    modules_prepare - Set up for building external modules tags/TAGS
  • Generate tags file for editors cscope - Generate cscope index gtags - Generate GNU GLOBAL index kernelrelease
  • Output the release version string kernelversion - Output the version stored in Makefile image_name - Output the image name
    headers_install - Install sanitised kernel headers to INSTALL_HDR_PATH

It depends on the changes you are doing. If the changes are specific to a kernel module. then you module should be ready to load and test.

$make M=/path/to/module/dir/

If the changes are in the config and require re-generating the build scripts

$make prepare $make scripts $make M=/path/to/module/dir/

In some cases if its required to regenerate just the vmlinux

$make vmlinux

askb
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