Linux per-cpu机制

Linux操作系统，特别是针对SMP或者NUMA架构的多CPU系统的时候，描述每个CPU的私有数据的时候，Linux操作系统提供了per_cpu机制。 

1.1     定义

per_cpu机制就是让每个CPU都有自己的私有数据段，便于保护与访问。

相关宏定义在include/linux/percpu-defs.h文件中:

/*     

 * Normal declaration and definition macros.

 */

#define DECLARE_PER_CPU_SECTION(type, name, sec)                        \

        extern __PCPU_ATTRS(sec) __typeof__(type) name

 

#define DEFINE_PER_CPU_SECTION(type, name, sec)                         \

        __PCPU_ATTRS(sec) PER_CPU_DEF_ATTRIBUTES                        \

        __typeof__(type) name

#endif

 

/*

 * Variant on the per-CPU variable declaration/definition theme used for

 * ordinary per-CPU variables.

 */

#define DECLARE_PER_CPU(type, name)                                     \

        DECLARE_PER_CPU_SECTION(type, name, "")

 

#define DEFINE_PER_CPU(type, name)                                      \

        DEFINE_PER_CPU_SECTION(type, name, "")

include/linux/percpu-defs.h文件

#define __PCPU_ATTRS(sec)                                               \

        __percpu __attribute__((section(PER_CPU_BASE_SECTION sec)))     \

        PER_CPU_ATTRIBUTES

这个__PCPU_ATTRS是每个CPU变量声明和定义的最基本实现。总之就是创建CPU的一个私有变量。其中#define PER_CPU_BASE_SECTION ".data..percpu"定义其存放的数据段，定义在arch/ia64/include/asm/percpu.h文件中。

1.2     示例

例如定义描述每个IA64CPU信息的数据结构变量ia64_cpu_info, 在文件

arch/ia64/include/asm/processor.h，DEFINE_PER_CPU，定义这种私有数据，放在特定的数据段中。

DECLARE_PER_CPU(struct cpuinfo_ia64, ia64_cpu_info);

在文件arch/ia64/include/asm/processor.h中，定义了结构体cpuinfo_ia64。其中定义了CPU类型，硬件BUG标志， CPU状态等。

struct cpuinfo_ia64 {

        unsigned int softirq_pending;  

        unsigned long itm_delta;        /* # of clock cycles between clock ticks */

        unsigned long itm_next;         /* interval timer mask value to use for next clock tick */

        unsigned long nsec_per_cyc;     /* (1000000000<<IA64_NSEC_PER_CYC_SHIFT)/itc_freq */

        unsigned long unimpl_va_mask;   /* mask of unimplemented virtual address bits (from PAL) */

        unsigned long unimpl_pa_mask;   /* mask of unimplemented physical address bits (from PAL) */

        unsigned long itc_freq;         /* frequency of ITC counter */

        unsigned long proc_freq;        /* frequency of processor */

        unsigned long cyc_per_usec;     /* itc_freq/1000000 */

        unsigned long ptce_base;

        unsigned int ptce_count[2];

        unsigned int ptce_stride[2];

        struct task_struct *ksoftirqd;  /* kernel softirq daemon for this CPU */

 

#ifdef CONFIG_SMP

        unsigned long loops_per_jiffy;

        int cpu;

        unsigned int socket_id; /* physical processor socket id */

        unsigned short core_id; /* core id */

        unsigned short thread_id; /* thread id */

        unsigned short num_log; /* Total number of logical processors on

                                 * this socket that were successfully booted */

        unsigned char cores_per_socket; /* Cores per processor socket */

        unsigned char threads_per_core; /* Threads per core */

#endif 

       

        /* CPUID-derived information: */

        unsigned long ppn;

        unsigned long features;

        unsigned char number;

        unsigned char revision;

        unsigned char model;

        unsigned char family;

        unsigned char archrev;

        char vendor[16];

        char *model_name;

 

#ifdef CONFIG_NUMA

        struct ia64_node_data *node_data;

#endif

};

1.3     初始化

　　在start_kernel函数中调用执行函数setup_per_cpu_areas( )，其会将.data.percpu中的数据拷贝到每个CPU的数据段中，每个CPU一份。 其中CPU n 对应的专有数据区的首地址为__per_cpu_offset[n]。 

1.4     读取变量

在不同的内核版本中，略有差异，这里取的是最新的2018/3/19日的主线版本中代码。include/linux/percpu-defs.h

#define get_cpu_var(var)                                                \

(*({                                                                    \

        preempt_disable();                                              \

        this_cpu_ptr(&var);                                             \

}))