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2 \! V* i# w6 d/ G3 @3 `linux 内核庞大而复杂。内核代码阅读的时候，有没有遇到因为宏定义或者inline层次太深而不知道到底代码是什么样子。代码预处理可以解决这个难题。" o, S9 P$ x$ _+ L
& }9 Q, h1 q q4 B2 A7 {平台：linux 3.4.5 ARM，PC linux上类似，更简单些。9 q) [. {9 A8 `* T4 w
$ G) a7 S4 W4 _! ~- L# b加V=1重新编译内核+ U; L# D6 D+ E1 t: B
+ I( }& v8 U8 bmake内核增加V=1选项，会详细打印编译过程，-B是要求重新编译内核所有模块。
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- cd linux-3.4.5 && make ARCH=arm defconfig && make ARCH=arm CROSS_COMPILE=arm-buildroot-linux-uclibcgnueabi- EXTRAVERSION=- -B V=1 uImage0 w- f; b$ F! P O: h! [3 v
/ Y) \8 F0 _# W- j编译内核并保存编译log到文件，搜索你要预编译的文件，如mm/slab.c，会找到如下编译命令：; U3 a- j+ v& u" g' o. f, d9 x
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- arm-buildroot-linux-uclibcgnueabi-gcc -Wp,-MD,mm/.slab.o.d -nostdinc -isystem /home/test/build/gcc-4.9.8/build_arm/staging_dir/usr/bin/../lib/gcc/arm-buildroot-linux-uclibcgnueabi/4.9.8/include -I/home/test/linux/kernels/linux-3.4.5/arch/arm/include -Iarch/arm/include/generated -Iinclude -include /home/test/linux/kernels/linux-3.4.5/include/linux/kconfig.h -D__KERNEL__ -mlittle-endian -Iarch/arm/mach-zx297510/include -Wall -Wundef -Wstrict-prototypes -Wno-trigraphs -fno-strict-aliasing -fno-common -Werror-implicit-function-declaration -Wno-format-security -fno-delete-null-pointer-checks -O2 -marm -fno-dwaRF2-cfi-asm -mabi=aapcs-linux -mno-thumb-interwork -funwind-tables -D__LINUX_ARM_ARCH__=7 -march=armv7-a -msoft-float -Uarm -Wframe-larger-than=1024 -fno-stack-protector -Wno-unused-but-set-variable -fomit-frame-pointer -g -fno-inline-functions-called-once -Wdeclaration-after-statement -Wno-pointer-sign -fno-strict-overflow -fconserve-stack -DCC_HAVE_ASM_GOTO -D"KBUILD_STR(s)=#s" -D"KBUILD_BASENAME=KBUILD_STR(slab)" -D"KBUILD_MODNAME=KBUILD_STR(slab)" -c -o mm/.tmp_slab.o mm/slab.c1 a0 H+ @2 G8 K; B1 @$ z
& Q! f- X/ g6 f; e编译预处理指定文件
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9 K4 F0 r; \. A- Z/ e把编译命令修改成预处理命令：-c -o mm/.tmp_slab.o修改成-E -o mm/slab.E mm/slab.c，在内核目录linux-3.4.5直接执行。如果是交叉编译链，可能需要把arm-buildroot-linux-uclibcgnueabi-gcc所在路径加入到环境变量PATH里。) K7 Q' D. A6 T: \
2 R4 Z7 ]8 x3 `. C7 B* V- arm-buildroot-linux-uclibcgnueabi-gcc -Wp,-MD,mm/.slab.o.d -nostdinc -isystem /home/test/build/gcc-4.9.8/build_arm/staging_dir/usr/bin/../lib/gcc/arm-buildroot-linux-uclibcgnueabi/4.9.8/include -I/home/test/linux/kernels/linux-3.4.5/arch/arm/include -Iarch/arm/include/generated -Iinclude -include /home/test/linux/kernels/linux-3.4.5/include/linux/kconfig.h -D__KERNEL__ -mlittle-endian -Iarch/arm/mach-zx297510/include -Wall -Wundef -Wstrict-prototypes -Wno-trigraphs -fno-strict-aliasing -fno-common -Werror-implicit-function-declaration -Wno-format-security -fno-delete-null-pointer-checks -O2 -marm -fno-dwarf2-cfi-asm -mabi=aapcs-linux -mno-thumb-interwork -funwind-tables -D__LINUX_ARM_ARCH__=7 -march=armv7-a -msoft-float -Uarm -Wframe-larger-than=1024 -fno-stack-protector -Wno-unused-but-set-variable -fomit-frame-pointer -g -fno-inline-functions-called-once -Wdeclaration-after-statement -Wno-pointer-sign -fno-strict-overflow -fconserve-stack -DCC_HAVE_ASM_GOTO -D"KBUILD_STR(s)=#s" -D"KBUILD_BASENAME=KBUILD_STR(slab)" -D"KBUILD_MODNAME=KBUILD_STR(slab)" -E -o mm/slab.E mm/slab.c" {% ]) ^. L# J# A' F0 M9 ~ w
5 B- R, J" A/ S# [ U执行完命令，在内核的mm目录就能看到slab.c的预处理后文件slab.E文件了。看一下kmalloc函数代码，是不是清晰很多了。
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slab_def.h里的原始kmalloc
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- static __always_inline void *kmalloc(size_t size, gfp_t flags)
- {
- struct kmem_cache *cachep;
- void *ret;
- if (__builtin_constant_p(size)) {
- int i = 0;
- if (!size)
- return ZERO_SIZE_PTR;
- #define CACHE(x) \
- if (size <= x) \
- goto found; \
- else \
- i++;
- #include <linux/kmalloc_sizes.h>
- #undef CACHE
- return NULL;
- found:
- #ifdef CONFIG_ZONE_DMA
- if (flags & GFP_DMA)
- cachep = malloc_sizes.cs_dmacachep;
- else
- #endif
- cachep = malloc_sizes.cs_cachep;
- ret = kmem_cache_alloc_trace(size, cachep, flags);
- return ret;
- }
- return __kmalloc(size, flags);
- }
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预处理后的kmalloc,流程是不是清晰多了。3 x- p8 C& }$ L. @2 h1 a- n$ L
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- static inline __attribute__((always_inline)) __attribute__((always_inline)) void *kmalloc(size_t size, gfp_t flags)
- {
- struct kmem_cache *cachep;
- void *ret;
- if (__builtin_constant_p(size)) {
- int i = 0;
- if (!size)
- return ((void *)16);
- # 1 "include/linux/kmalloc_sizes.h" 1
- if (size <= 32) goto found; else i++;
- if (size <= 64) goto found; else i++;
- if (size <= 128) goto found; else i++;
- if (size <= 192) goto found; else i++;
- if (size <= 256) goto found; else i++;
- if (size <= 512) goto found; else i++;
- if (size <= 1024) goto found; else i++;
- if (size <= 2048) goto found; else i++;
- if (size <= 4096) goto found; else i++;
- if (size <= 8192) goto found; else i++;
- if (size <= 16384) goto found; else i++;
- if (size <= 32768) goto found; else i++;
- if (size <= 65536) goto found; else i++;
- if (size <= 131072) goto found; else i++;
- if (size <= 262144) goto found; else i++;
- if (size <= 524288) goto found; else i++;
- if (size <= 1048576) goto found; else i++;
- if (size <= 2097152) goto found; else i++;
- if (size <= 4194304) goto found; else i++;
- # 145 "include/linux/slab_def.h" 2
- return ((void *)0);
- found:
- cachep = malloc_sizes.cs_cachep;
- ret = kmem_cache_alloc_trace(size, cachep, flags);
- return ret;
- }
- return __kmalloc(size, flags);
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发表于 2020-2-6 09:47
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