百度关键词搜索排名查询,长沙百度网站优化,西安网站建设管理,php网站开发套模板文章目录 背景1. static-key的使用方法1.1. static-key定义1.2 初始化1.3 条件判断1.4 修改判断条件 2、示例代码参考链接 背景
内核中有很多判断条件在正常情况下的结果都是固定的#xff0c;除非极其罕见的场景才会改变#xff0c;通常单个的这种判断的代价很低可以忽略除非极其罕见的场景才会改变通常单个的这种判断的代价很低可以忽略但是如果这种判断数量巨大且被频繁执行那就会带来性能损失了。内核的static-key机制就是为了优化这种场景,其优化的结果是对于大多数情况对应的判断被优化为一个NOP指令在非常有场景的时候就变成jump XXX一类的指令使得对应的代码段得到执行。
1. static-key的使用方法
1.1. static-key定义
static_key 结构体的定义如下
#ifdef CONFIG_JUMP_LABELstruct static_key {atomic_t enabled;
/** Note:* To make anonymous unions work with old compilers, the static* initialization of them requires brackets. This creates a dependency* on the order of the struct with the initializers. If any fields* are added, STATIC_KEY_INIT_TRUE and STATIC_KEY_INIT_FALSE may need* to be modified.** bit 0 1 if key is initially true* 0 if initially false* bit 1 1 if points to struct static_key_mod* 0 if points to struct jump_entry*/union {unsigned long type;struct jump_entry *entries;struct static_key_mod *next;};
};#else
struct static_key {atomic_t enabled;
};
#endif /* CONFIG_JUMP_LABEL */如果没有定义CONFIG_JUMP_LABEL则static_key 退化成atomic变量。
1.2 初始化
#define DEFINE_STATIC_KEY_TRUE(name) \struct static_key_true name STATIC_KEY_TRUE_INIT
#define DEFINE_STATIC_KEY_FALSE(name) \struct static_key_false name STATIC_KEY_FALSE_INIT#define STATIC_KEY_TRUE_INIT (struct static_key_true) { .key STATIC_KEY_INIT_TRUE, }
#define STATIC_KEY_FALSE_INIT (struct static_key_false){ .key STATIC_KEY_INIT_FALSE, }#define STATIC_KEY_INIT_TRUE \{ .enabled { 1 }, \.entries (void *)JUMP_TYPE_TRUE }
#define STATIC_KEY_INIT_FALSE \{ .enabled { 0 }, \.entries (void *)JUMP_TYPE_FALSE }false和true的主要区别就是enabled 是否为1.
1.3 条件判断
#ifdef CONFIG_JUMP_LABEL/** Combine the right initial value (type) with the right branch order* to generate the desired result.*** type\branch| likely (1) | unlikely (0)* ----------------------------------------------------* | |* true (1) | ... | ...* | NOP | JMP L* | br-stmts | 1: ...* | L: ... |* | |* | | L: br-stmts* | | jmp 1b* | |* ----------------------------------------------------* | |* false (0) | ... | ...* | JMP L | NOP* | br-stmts | 1: ...* | L: ... |* | |* | | L: br-stmts* | | jmp 1b* | |* ----------------------------------------------------** The initial value is encoded in the LSB of static_key::entries,* type: 0 false, 1 true.** The branch type is encoded in the LSB of jump_entry::key,* branch: 0 unlikely, 1 likely.** This gives the following logic table:** enabled type branch instuction* ----------------------------------------* 0 0 0 | NOP* 0 0 1 | JMP* 0 1 0 | NOP* 0 1 1 | JMP** 1 0 0 | JMP* 1 0 1 | NOP* 1 1 0 | JMP* 1 1 1 | NOP** Which gives the following functions:** dynamic: instruction enabled ^ branch* static: instruction type ^ branch** See jump_label_type() / jump_label_init_type().*/#define static_branch_likely(x) \
({ \bool branch; \if (__builtin_types_compatible_p(typeof(*x), struct static_key_true)) \branch !arch_static_branch((x)-key, true); \else if (__builtin_types_compatible_p(typeof(*x), struct static_key_false)) \branch !arch_static_branch_jump((x)-key, true); \else \branch ____wrong_branch_error(); \likely(branch); \
})#define static_branch_unlikely(x) \
({ \bool branch; \if (__builtin_types_compatible_p(typeof(*x), struct static_key_true)) \branch arch_static_branch_jump((x)-key, false); \else if (__builtin_types_compatible_p(typeof(*x), struct static_key_false)) \branch arch_static_branch((x)-key, false); \else \branch ____wrong_branch_error(); \unlikely(branch); \
})#else /* !CONFIG_JUMP_LABEL */#define static_branch_likely(x) likely(static_key_enabled((x)-key))
#define static_branch_unlikely(x) unlikely(static_key_enabled((x)-key))#endif /* CONFIG_JUMP_LABEL */可见同样依赖HAVE_JUMP_LABEL。如果没有定义的话直接退化成likely和unlikely static_branch_unlikely 和 static_branch_likely 只是填充指令的方式不同可以参考上面的代码注释 当static_key为false时都会进入else逻辑语句中。
if (static_branch_unlikely((static_key)))do likely work;
elsedo unlikely work1.4 修改判断条件
使用static_branch_enable 和 static_branch_disable可以改变static_key 状态
#define static_branch_enable(x) static_key_enable((x)-key)
#define static_branch_disable(x) static_key_disable((x)-key)底层是调用static_key_slow_dec, static_key_slow_dec来改变key-enabled计数。
static inline void static_key_enable(struct static_key *key)
{int count static_key_count(key);WARN_ON_ONCE(count 0 || count 1);if (!count)static_key_slow_inc(key);
}
static inline void static_key_disable(struct static_key *key)
{int count static_key_count(key);WARN_ON_ONCE(count 0 || count 1);if (count)static_key_slow_dec(key);
}static inline void static_key_slow_inc(struct static_key *key)
{STATIC_KEY_CHECK_USE(key);atomic_inc(key-enabled);
}static inline void static_key_slow_dec(struct static_key *key)
{STATIC_KEY_CHECK_USE(key);atomic_dec(key-enabled);
}2、示例代码
下面我们用一段代码来分析static-key对程序分支跳转硬编码的影响。
#include linux/module.h
#include linux/kernel.h
#include linux/init.h
#include linux/static_key.hDEFINE_STATIC_KEY_FALSE(key);void func(int a){if (static_branch_unlikely(key)) { printk(my_module: Feature is enabled\n);} else {printk(my_module: Feature is disabled\n);}
}static int __init my_module_init(void) {pr_info(my_module: Module loaded\n);int a 1;func(a);static_branch_enable(key);func(a);return 0;
}static void __exit my_module_exit(void) {pr_info(my_module: Module unloaded\n);
}module_init(my_module_init);
module_exit(my_module_exit);MODULE_LICENSE(GPL);
MODULE_AUTHOR(Your Name);
MODULE_DESCRIPTION(Sample Kernel Module with Static Key);func汇编代码如下
0000000000000000 func:0: a9bf7bfd stp x29, x30, [sp, #-16]!4: 910003fd mov x29, sp8: d503201f nopc: 90000000 adrp x0, 0 func10: 91000000 add x0, x0, #0x014: 94000000 bl 0 printk18: a8c17bfd ldp x29, x30, [sp], #161c: d65f03c0 ret20: 90000000 adrp x0, 0 func24: 91000000 add x0, x0, #0x028: 94000000 bl 0 printk2c: 17fffffb b 18 func0x18func中不适用static-key时汇编代码如下
void func(int a){if (a) { printk(my_module: Feature is enabled\n);} else {printk(my_module: Feature is disabled\n);}
}0000000000000000 func:0: a9bf7bfd stp x29, x30, [sp, #-16]!4: 910003fd mov x29, sp8: 340000a0 cbz w0, 1c func0x1cc: 90000000 adrp x0, 0 func10: 91000000 add x0, x0, #0x014: 94000000 bl 0 printk18: 14000004 b 28 func0x281c: 90000000 adrp x0, 0 func20: 91000000 add x0, x0, #0x024: 94000000 bl 0 printk28: a8c17bfd ldp x29, x30, [sp], #162c: d65f03c0 ret对比可以发现在0x8地址处使用static-key编码在编译时将cbz指令替换为了nop指令减少了程序运行时对比次数。
参考链接
Linux内核中的static-key机制Linux内核jump label与static key的原理与示例static-keys.html | 静态键Linux Jump Label/static-key机制详解
