分类：Linux kernel 笔记

Linux kernel 笔记 （33）——“debugfs“简介

以下摘自维基百科：

debugfs is a simple to use RAM-based file system specially designed for debugging purposes. It exists as a simple way for kernel developers to make information available to user space. Unlike /proc, which is only meant for information about a process, or sysfs, which has strict one-value-per-file rules, debugfs has no rules at all. Developers can put any information they want there.

debugfs是一个用于调试目的，简单的，基于内存的文件系统。kernel的信息可以输出到debugfs中，这样方便user space程序查看和使用。

为了使用debugfs功能，编译kernel时需要把CONFIG_DEBUG_FS置成yes。

典型的挂载debugfs文件系统命令：

mount -t debugfs none /sys/kernel/debug

由于没有具体设备，所以设备的位置使用了none（参考这个帖子）

 

Linux kernel 笔记 （32）——“make“和”make modules“

以下摘自LKD:

After the kernel configuration is set—however you do it—you can build it with a single command:
$ make
Unlike kernels before 2.6, you no longer need to run make dep before building the kernel—the dependency tree is maintained automatically.You also do not need to specify a specific build type, such as bzImage, or build modules separately, as you did in old versions.The default Makefile rule will handle everything.

也就是在2.6版本以后的Linux kernel中，执行make或make all命令即包含了make modules。也可参加这个帖子。

 

Linux kernel 笔记 （31）——“make help”命令

make help命令列出编译kernel时的选项帮助信息：

linux-4cy8:/usr/src/linux/build # make help
make -C /usr/src/linux-3.0.101-63 O=/usr/src/linux-3.0.101-63/build/. help
Cleaning targets:
  clean           - Remove most generated files but keep the config and
                    enough build support to build external modules
  mrproper        - Remove all generated files + config + various backup files
  distclean       - mrproper + remove editor backup and patch files

Configuration targets:
  config          - Update current config utilising a line-oriented program
  nconfig         - Update current config utilising a ncurses menu based program
  menuconfig      - Update current config utilising a menu based program
  xconfig         - Update current config utilising a QT based front-end
  gconfig         - Update current config utilising a GTK based front-end
  oldconfig       - Update current config utilising a provided .config as base
  localmodconfig  - Update current config disabling modules not loaded
  localyesconfig  - Update current config converting local mods to core
  silentoldconfig - Same as oldconfig, but quietly, additionally update deps
  defconfig       - New config with default from ARCH supplied defconfig
  savedefconfig   - Save current config as ./defconfig (minimal config)
  allnoconfig     - New config where all options are answered with no
  allyesconfig    - New config where all options are accepted with yes
  allmodconfig    - New config selecting modules when possible
  alldefconfig    - New config with all symbols set to default
  randconfig      - New config with random answer to all options
  listnewconfig   - List new options
  oldnoconfig     - Same as silentoldconfig but set new symbols to n (unset)

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
  headers_install - Install sanitised kernel headers to INSTALL_HDR_PATH
                    (default: /usr/src/linux-3.0.101-63/build/usr)

Static analysers
  checkstack      - Generate a list of stack hogs
  namespacecheck  - Name space analysis on compiled kernel
  versioncheck    - Sanity check on version.h usage
  includecheck    - Check for duplicate included header files
  export_report   - List the usages of all exported symbols
  headers_check   - Sanity check on exported headers
  headerdep       - Detect inclusion cycles in headers
  coccicheck      - Check with Coccinelle.

Kernel packaging:
  rpm-pkg             - Build both source and binary RPM kernel packages
  binrpm-pkg          - Build only the binary kernel package
  deb-pkg             - Build the kernel as an deb package
  tar-pkg             - Build the kernel as an uncompressed tarball
  targz-pkg           - Build the kernel as a gzip compressed tarball
  tarbz2-pkg          - Build the kernel as a bzip2 compressed tarball
  tarxz-pkg           - Build the kernel as a xz compressed tarball
  perf-tar-src-pkg    - Build perf-3.0.101.tar source tarball
  perf-targz-src-pkg  - Build perf-3.0.101.tar.gz source tarball
  perf-tarbz2-src-pkg - Build perf-3.0.101.tar.bz2 source tarball
  perf-tarxz-src-pkg  - Build perf-3.0.101.tar.xz source tarball

Documentation targets:
 Linux kernel internal documentation in different formats:
  htmldocs        - HTML
  pdfdocs         - PDF
  psdocs          - Postscript
  xmldocs         - XML DocBook
  mandocs         - man pages
  installmandocs  - install man pages generated by mandocs
  cleandocs       - clean all generated DocBook files

Architecture specific targets (x86):
* bzImage      - Compressed kernel image (arch/x86/boot/bzImage)
  install      - Install kernel using
                  (your) ~/bin/installkernel or
                  (distribution) /sbin/installkernel or
                  install to $(INSTALL_PATH) and run lilo
  fdimage      - Create 1.4MB boot floppy image (arch/x86/boot/fdimage)
  fdimage144   - Create 1.4MB boot floppy image (arch/x86/boot/fdimage)
  fdimage288   - Create 2.8MB boot floppy image (arch/x86/boot/fdimage)
  isoimage     - Create a boot CD-ROM image (arch/x86/boot/image.iso)
                  bzdisk/fdimage*/isoimage also accept:
                  FDARGS="..."  arguments for the booted kernel
                  FDINITRD=file initrd for the booted kernel

  i386_defconfig           - Build for i386
  x86_64_defconfig         - Build for x86_64

  make V=0|1 [targets] 0 => quiet build (default), 1 => verbose build
  make V=2   [targets] 2 => give reason for rebuild of target
  make O=dir [targets] Locate all output files in "dir", including .config
  make C=1   [targets] Check all c source with $CHECK (sparse by default)
  make C=2   [targets] Force check of all c source with $CHECK
  make W=n   [targets] Enable extra gcc checks, n=1,2,3 where
                1: warnings which may be relevant and do not occur too often
                2: warnings which occur quite often but may still be relevant
                3: more obscure warnings, can most likely be ignored
                Multiple levels can be combined with W=12 or W=123
  make RECORDMCOUNT_WARN=1 [targets] Warn about ignored mcount sections

Execute "make" or "make all" to build all targets marked with [*]
For further info see the ./README file

很有用！！

 

Linux kernel 笔记 （30）——“.ko”和“.o”文件区别

原帖参见这里。

假设你编写的模块文件是mod.c，编译会生成mod.o文件。kernel编译系统还会自动生成一个mod_kmod.c文件（包含需要的kernel数据结构），编译这个文件生成另一个object文件。把mod_kmod.c所新生成的object文件和之前的mod.o文件链接生成最后的mod.ko文件。也就是可以被kernel加载运行的模块文件。

 

Linux kernel 笔记 （27）——“make clean”和“make mrproper”的区别

参考Recompiling the kernel：

Sometimes, you’ll change things so much that make can’t figure out how to recompile the files correctly. make clean will remove all the object and kernel object files (ending in .o and .ko) and a few other things. make mrproper will do everything make clean does, plus remove your config file, the dependency files, and everything else that make config creates. Be sure to save your config file in another file before running make mrproper. Afterwards, copy the config file back to .config and start over, beginning at make menuconfig. A make mrproper will often fix strange kernel crashes that make no sense and strange compilation errors that make no sense.

make clean删除object和kernel object文件，还有其它一些文件。make mrproper是make clean的超集，它还会删除config文件，依赖文件，以及其它make config生成的文件。

 

Linux kernel 笔记 （26）——利用“make -j”提高编译`kernel`速度

首先看一下-j选项在make命令中的含义：

-j [jobs], --jobs[=jobs]
        Specifies the number of jobs (commands) to run simultaneously.   If  there  is  more  than  one  -j
        option,  the  last  one is effective.  If the -j option is given without an argument, make will not
        limit the number of jobs that can run simultaneously.

也就是-j指定并行工作的job数量，例如make -j4。如果-j选项后面没有参数，则不会限制job数。

再参考《Linux kernel development》:

By default, make spawns only a single job because Makefiles all too often have incorrect dependency information.With incorrect dependencies, multiple jobs can step on each other’s toes, resulting in errors in the build process.The kernel’s Makefiles have correct dependency information, so spawning multiple jobs does not result in failures.To build the kernel with multiple make jobs, use

$ make -jn

Here, n is the number of jobs to spawn. Usual practice is to spawn one or two jobs per processor. For example, on a 16-core machine, you might do

$ make -j32 > /dev/null

可以看到指定job数是系统core数量2倍是一种推荐的做法。

 

Linux kernel 笔记 （24）——Kdump是如何工作的

先介绍两个术语：
a）Standard（production）kernel：正常使用的kernel；
b）Crash（capture）kernel：用来收集crash dump的kernel。

Kdump有两个重要的组件：Kdump和Kexec：
a）Kexec：
是一种fastboot mechanism。Kexec允许不通过BIOS，而是从运行的kernel中启动另一个kernel，这样做速度很快,可以节省大量时间。
b）Kdump：
是一种新的，可靠的crash dumping mechanism。Crash dump是从新启动的kernel中去捕获，而不是从已经crashed kernel中。当系统crash后，Kdump使用Kexec启动第二个kernel（Crash kernel），而第一个kernel（Standard kernel）会保留一部分内存供第二个kernel使用。由于Kexec没有通过BIOS启动第二个kernel，因此第一个kernel的内存得到保护，也就是最终的kernel crash dump。

参考资料：
Linux Kernel Crash Book。