Linux 操作系统内核

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进程

进程有很多解释的版本，比如说是程序的执行过程，其实每个人的理解不一样，简单的来说，进程就是cpu分配资源的基本单元。

ps：线程是计算机调度的基本单元（面试经常问的：进程和线程区别）

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一、进程描述符

进程描述符主要是说明进程的相关信息，可以看linux 内核源码2.6.3以上的版本，
struct task_struct根据描述(include/linux/sched.h)

struct task_struct {
	volatile long state;	/* -1 unrunnable, 0 runnable, >0 stopped */
	struct thread_info *thread_info;
	atomic_t usage;
	unsigned long flags;	/* per process flags, defined below */
	unsigned long ptrace;

	int lock_depth;		/* Lock depth */

	int prio, static_prio;
	struct list_head run_list;
	prio_array_t *array;

	unsigned long sleep_avg;
	long interactive_credit;
	unsigned long long timestamp;
	int activated;

	unsigned long policy;
	cpumask_t cpus_allowed;
	unsigned int time_slice, first_time_slice;

	struct list_head tasks;
	struct list_head ptrace_children;
	struct list_head ptrace_list;

	struct mm_struct *mm, *active_mm;

/* task state */
	struct linux_binfmt *binfmt;
	int exit_code, exit_signal;
	int pdeath_signal;  /*  The signal sent when the parent dies  */
	/* ??? */
	unsigned long personality;
	int did_exec:1;
	pid_t pid;
	pid_t __pgrp;		/* Accessed via process_group() */
	pid_t tty_old_pgrp;
	pid_t session;
	pid_t tgid;
	/* boolean value for session group leader */
	int leader;
	/* 
	 * pointers to (original) parent process, youngest child, younger sibling,
	 * older sibling, respectively.  (p->father can be replaced with 
	 * p->parent->pid)
	 */
	struct task_struct *real_parent; /* real parent process (when being debugged) */
	struct task_struct *parent;	/* parent process */
	struct list_head children;	/* list of my children */
	struct list_head sibling;	/* linkage in my parent's children list */
	struct task_struct *group_leader;	/* threadgroup leader */

	/* PID/PID hash table linkage. */
	struct pid_link pids[PIDTYPE_MAX];

	wait_queue_head_t wait_chldexit;	/* for wait4() */
	struct completion *vfork_done;		/* for vfork() */
	int __user *set_child_tid;		/* CLONE_CHILD_SETTID */
	int __user *clear_child_tid;		/* CLONE_CHILD_CLEARTID */

	unsigned long rt_priority;
	unsigned long it_real_value, it_prof_value, it_virt_value;
	unsigned long it_real_incr, it_prof_incr, it_virt_incr;
	struct timer_list real_timer;
	struct list_head posix_timers; /* POSIX.1b Interval Timers */
	unsigned long utime, stime, cutime, cstime;
	unsigned long nvcsw, nivcsw, cnvcsw, cnivcsw; /* context switch counts */
	u64 start_time;
/* mm fault and swap info: this can arguably be seen as either mm-specific or thread-specific */
	unsigned long min_flt, maj_flt, nswap, cmin_flt, cmaj_flt, cnswap;
/* process credentials */
	uid_t uid,euid,suid,fsuid;
	gid_t gid,egid,sgid,fsgid;
	int ngroups;
	gid_t	groups[NGROUPS];
	kernel_cap_t   cap_effective, cap_inheritable, cap_permitted;
	int keep_capabilities:1;
	struct user_struct *user;
/* limits */
	struct rlimit rlim[RLIM_NLIMITS];
	unsigned short used_math;
	char comm[16];
/* file system info */
	int link_count, total_link_count;
	struct tty_struct *tty; /* NULL if no tty */
/* ipc stuff */
	struct sysv_sem sysvsem;
/* CPU-specific state of this task */
	struct thread_struct thread;
/* filesystem information */
	struct fs_struct *fs;
/* open file information */
	struct files_struct *files;
/* namespace */
	struct namespace *namespace;
/* signal handlers */
	struct signal_struct *signal;
	struct sighand_struct *sighand;

	sigset_t blocked, real_blocked;
	struct sigpending pending;

	unsigned long sas_ss_sp;
	size_t sas_ss_size;
	int (*notifier)(void *priv);
	void *notifier_data;
	sigset_t *notifier_mask;
	
	void *security;

/* Thread group tracking */
   	u32 parent_exec_id;
   	u32 self_exec_id;
/* Protection of (de-)allocation: mm, files, fs, tty */
	spinlock_t alloc_lock;
/* Protection of proc_dentry: nesting proc_lock, dcache_lock, write_lock_irq(&tasklist_lock); */
	spinlock_t proc_lock;
/* context-switch lock */
	spinlock_t switch_lock;

/* journalling filesystem info */
	void *journal_info;

/* VM state */
	struct reclaim_state *reclaim_state;

	struct dentry *proc_dentry;
	struct backing_dev_info *backing_dev_info;

	struct io_context *io_context;

	unsigned long ptrace_message;
	siginfo_t *last_siginfo; /* For ptrace use.  */
};

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内核中是使用双向链表将 task_struct 串起来，

但是对于每个链表都要初始化，插入删除的话，非常浪费时间，linux内核封装了list_head, 使用list_head构造双向链表

struct list_head {
	struct list_head *next, *prev;
};
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同时添加了处理函数：

list_add(n, p): 把n指向的元素指向p指向的元素之后
list_add_tail(n, p)：把n指向的元素指向p指向的元素之前
list_del(p）：删除p所指向的元素
list_empty(p）:检查p链表是否为空
…

然后通过esp寄存器快速获取到当前运行进程的task_struct结构

static inline struct task_struct * get_current(void) { 
          struct task_struct *current; __asm__(
        "andl %%esp,%0; ":"=r" (current) : "0" (~8191UL)
); 
return current;
}
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二、进程状态

1.进程描述符中有字段state表示了进程的状态

volatile long state;	/* -1 unrunnable, 0 runnable, >0 stopped */
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2.有7种状态：

TASK_RUNNING : 可执行状态
TASK_INTETRRUPTIBLE : 可中断的等待状态
TASK_UNINTERRUPTIBLE : 不可中断的等待状态
TASK_STOPPED : 暂停状态
TASK_TRACED : 跟踪状态
EXIT_ZOMBLE : 僵死状态
EXIT_DEAD : 僵死撤销状态
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总结

本文大概讲了进程描述符和进程状态，后面会说进程的关系和转换。
参考书籍：
《linux内核设计和实现》
《深入理解linux内核》