窥探 kernel --- task_struct

本系列文章由张同浩编写，转载请注明出处：http://blog.csdn.net/muge0913/article/details/7477083

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程序和进程：

程序是存放在磁盘上的一系列代码和数据的可执行镜像，是一个静态的实体；

进程是一个执行的程序，它是动态的实体，它除了包含指令段，数据段等静态数据外（数据是可以是动态变化的），还包括当前的状态信息，如临时数据堆栈信息，当前处理器的寄存器信息等动态信息。这些动态信息通常称为进程上下文。

从内核角度来看，进程是操作系统分配内存，cpu时间片等资源的最小单位。其中它用到的数据和信息大部分都是在动态变化的。在linux内核中进程上下文通常用task_struct来描述，进程切换负责保存当前进程的上下文，恢复合适进程的上下文到cpu和寄存器中。

进程和线程：

 随着计算机产业的发展，计算机的应用范围越来越广，计算机要解决的范围从处理器密集型的科学计算向IO密集型的用户交互式程序。为了解决日益复杂的问题。人们提出了分而治之（divide and comquer）的思想，也就是提出了进程。随着计算机的发展和对此技术的研究，人们发现，进程间的切换带来了相当大的系统开销（overload），人们又提出了线程的概念。线程是对进程的进一步抽象。一个进程有两部分组成：线程集合和资源集合。线程是进程中的一个动态对象，一组动态的指令流。进程中的所有线程将共享进程的中的资源，但每个线程又有独立的程序计数器，堆栈和寄存器。

linux中线程、进程都是用struct task_struct来描述。进程描述符task_struct用来刻画进程的状态属性，是内核操作和维护进程状态的唯一手段,其定义在linux 2.6.xx/include/linux/sched.h中。这个结构体相当的大

 truct task_struct {
     /*这个是进程的运行时状态，-1代表不可运行，0代表可运行，>0代表已停止*/
     volatile long state;    /* -1 unrunnable, 0 runnable, >0 stopped */
     void *stack;
     atomic_t usage;
     unsigned int flags; /* per process flags, defined below */
     unsigned int ptrace;

     int lock_depth;     /* BKL lock depth */

 #ifdef CONFIG_SMP
 #ifdef __ARCH_WANT_UNLOCKED_CTXSW
     int oncpu;
 #endif
 #endif

     int prio, static_prio, normal_prio;
     /*表示此进程的运行优先级*/
     unsigned int rt_priority;
     const struct sched_class *sched_class;
     struct sched_entity se;
     struct sched_rt_entity rt;

 #ifdef CONFIG_PREEMPT_NOTIFIERS
     /* list of struct preempt_notifier: */
     struct hlist_head preempt_notifiers;
 #endif

     /*
      * fpu_counter contains the number of consecutive context switches
      * that the FPU is used. If this is over a threshold, the lazy fpu
      * saving becomes unlazy to save the trap. This is an unsigned char
      * so that after 256 times the counter wraps and the behavior turns
      * lazy again; this to deal with bursty apps that only use FPU for
      * a short time
      */
     unsigned char fpu_counter;
 #ifdef CONFIG_BLK_DEV_IO_TRACE
     unsigned int btrace_seq;
 #endif

     unsigned int policy;
     cpumask_t cpus_allowed;

 #ifdef CONFIG_PREEMPT_RCU
     int rcu_read_lock_nesting;
     char rcu_read_unlock_special;
     struct list_head rcu_node_entry;
 #endif /* #ifdef CONFIG_PREEMPT_RCU */
 #ifdef CONFIG_TREE_PREEMPT_RCU
     struct rcu_node *rcu_blocked_node;
 #endif /* #ifdef CONFIG_TREE_PREEMPT_RCU */

 #if defined(CONFIG_SCHEDSTATS) || defined(CONFIG_TASK_DELAY_ACCT)
     struct sched_info sched_info;
 #endif

     struct list_head tasks;
     struct plist_node pushable_tasks;
     /*该结构体记录了进程内存使用的相关情况*/
     struct mm_struct *mm, *active_mm;
 #if defined(SPLIT_RSS_COUNTING)
     struct task_rss_stat    rss_stat;
 #endif
 /* task state */
     /*进程退出时的状态*/
     int exit_state;
     int exit_code, exit_signal;
     int pdeath_signal;  /*  The signal sent when the parent dies  */
     /* ??? */
     unsigned int personality;
     unsigned did_exec:1;
     unsigned in_execve:1;   /* Tell the LSMs that the process is doing an
                  * execve */
     unsigned in_iowait:1;


     /* Revert to default priority/policy when forking */
     unsigned sched_reset_on_fork:1;

     /*进程号*/
     pid_t pid;
     /*组进程号*/
     pid_t tgid;

 #ifdef CONFIG_CC_STACKPROTECTOR
     /* Canary value for the -fstack-protector gcc feature */
     unsigned long stack_canary;
 #endif

     /*
      * pointers to (original) parent process, youngest child, younger sibling,
      * older sibling, respectively.  (p->father can be replaced with
      * p->real_parent->pid)
      */
      /*创建该进程的父进程*/
     struct task_struct *real_parent; /* real parent process */
     /*parent是该进程现在的父进程，有可能是”继父“*/
     struct task_struct *parent; /* recipient of SIGCHLD, wait4() reports */
     /*
      * children/sibling forms the list of my natural children
      */
      /*这里children指的是该进程孩子的链表，可以得到所有孩子的进程描述符*/
     struct list_head children;  /* list of my children */
     /*sibling该进程兄弟的链表，也就是其父亲的所有孩子的链表*/
     struct list_head sibling;   /* linkage in my parent's children list */
     /*这个是主线程的进程描述符，linux并没有单独实现线程的相关结构体，只是用一个进程来代替线程，然后对其做一些特殊的处理*/
     struct task_struct *group_leader;   /* threadgroup leader */

     /*
      * ptraced is the list of tasks this task is using ptrace on.
      * This includes both natural children and PTRACE_ATTACH targets.
      * p->ptrace_entry is p's link on the p->parent->ptraced list.
      */
     struct list_head ptraced;
     struct list_head ptrace_entry;

     /* PID/PID hash table linkage. */
     struct pid_link pids[PIDTYPE_MAX];
     /*该进程所有线程的链表*/
     struct list_head thread_group;

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

     /*这个是该进程使用cpu时间的信息，utime是在用户态下执行的时间，stime是在内核态下执行的时间*/
     cputime_t utime, stime, utimescaled, stimescaled;
     cputime_t gtime;
 #ifndef CONFIG_VIRT_CPU_ACCOUNTING
     cputime_t prev_utime, prev_stime;
 #endif
     unsigned long nvcsw, nivcsw; /* context switch counts */
     /*启动的时间，只是时间基准不一样*/
     struct timespec start_time;         /* monotonic time */
     struct timespec real_start_time;    /* boot based time */
 /* mm fault and swap info: this can arguably be seen as either mm-specific or thread-specific */
     unsigned long min_flt, maj_flt;

     struct task_cputime cputime_expires;
     struct list_head cpu_timers[3];

 /* process credentials */
     const struct cred __rcu *real_cred; /* objective and real subjective task
                      * credentials (COW) */
     const struct cred __rcu *cred;  /* effective (overridable) subjective task
                      * credentials (COW) */
     struct cred *replacement_session_keyring; /* for KEYCTL_SESSION_TO_PARENT */

     /*保存该进程名字的字符数组*/
     char comm[TASK_COMM_LEN]; /* executable name excluding path
                      - access with [gs]et_task_comm (which lock
                        it with task_lock())
                      - initialized normally by setup_new_exec */
 /* file system info */
 /* 文件系统信息计数*/
     int link_count, total_link_count;
 #ifdef CONFIG_SYSVIPC
 /* ipc stuff */
     struct sysv_sem sysvsem;
 #endif
 #ifdef CONFIG_DETECT_HUNG_TASK
 /* hung task detection */
     unsigned long last_switch_count;
 #endif
 /* CPU-specific state of this task */
 /*该进程在特定CPU下的状态*/
     struct thread_struct thread;
 /* filesystem information */
 /* 文件系统相关信息结构体*/
     struct fs_struct *fs;
 /* open file information */
 /* 打开的文件相关信息结构体,对驱动开发者来说此结构会常见到*/
     struct files_struct *files;
 /* namespaces */
     struct nsproxy *nsproxy;
 /* signal handlers */
 /* 信号相关信息的句柄*/
     struct signal_struct *signal;
     struct sighand_struct *sighand;

     sigset_t blocked, real_blocked;
     sigset_t saved_sigmask; /* restored if set_restore_sigmask() was used */
     struct sigpending pending;

     unsigned long sas_ss_sp;
     size_t sas_ss_size;
     int (*notifier)(void *priv);
     void *notifier_data;
     sigset_t *notifier_mask;
     struct audit_context *audit_context;
 #ifdef CONFIG_AUDITSYSCALL
     uid_t loginuid;
     unsigned int sessionid;
 #endif
     seccomp_t seccomp;

 /* Thread group tracking */
     u32 parent_exec_id;
     u32 self_exec_id;
 /* Protection of (de-)allocation: mm, files, fs, tty, keyrings, mems_allowed,
  * mempolicy */
     spinlock_t alloc_lock;

 #ifdef CONFIG_GENERIC_HARDIRQS
     /* IRQ handler threads */
     struct irqaction *irqaction;
 #endif

     /* Protection of the PI data structures: */
     raw_spinlock_t pi_lock;

 #ifdef CONFIG_RT_MUTEXES
     /* PI waiters blocked on a rt_mutex held by this task */
     struct plist_head pi_waiters;
     /* Deadlock detection and priority inheritance handling */
     struct rt_mutex_waiter *pi_blocked_on;
 #endif

 #ifdef CONFIG_DEBUG_MUTEXES
     /* mutex deadlock detection */
     struct mutex_waiter *blocked_on;
 #endif
 #ifdef CONFIG_TRACE_IRQFLAGS
     unsigned int irq_events;
     unsigned long hardirq_enable_ip;
     unsigned long hardirq_disable_ip;
     unsigned int hardirq_enable_event;
     unsigned int hardirq_disable_event;
     int hardirqs_enabled;
     int hardirq_context;
     unsigned long softirq_disable_ip;
     unsigned long softirq_enable_ip;
     unsigned int softirq_disable_event;
     unsigned int softirq_enable_event;
     int softirqs_enabled;
     int softirq_context;
 #endif
 #ifdef CONFIG_LOCKDEP
 # define MAX_LOCK_DEPTH 48UL
     u64 curr_chain_key;
     int lockdep_depth;
     unsigned int lockdep_recursion;
     struct held_lock held_locks[MAX_LOCK_DEPTH];
     gfp_t lockdep_reclaim_gfp;
 #endif

 /* journalling filesystem info */
     void *journal_info;

 /* stacked block device info */
     struct bio_list *bio_list;

 /* VM state */
     struct reclaim_state *reclaim_state;

     struct backing_dev_info *backing_dev_info;

     struct io_context *io_context;

     unsigned long ptrace_message;
     siginfo_t *last_siginfo; /* For ptrace use.  */
     struct task_io_accounting ioac;
 #if defined(CONFIG_TASK_XACCT)
     u64 acct_rss_mem1;  /* accumulated rss usage */
     u64 acct_vm_mem1;   /* accumulated virtual memory usage */
     cputime_t acct_timexpd; /* stime + utime since last update */
 #endif
 #ifdef CONFIG_CPUSETS
     nodemask_t mems_allowed;    /* Protected by alloc_lock */
     int mems_allowed_change_disable;
     int cpuset_mem_spread_rotor;
     int cpuset_slab_spread_rotor;
 #endif
 #ifdef CONFIG_CGROUPS
     /* Control Group info protected by css_set_lock */
     struct css_set __rcu *cgroups;
     /* cg_list protected by css_set_lock and tsk->alloc_lock */
     struct list_head cg_list;
 #endif
 #ifdef CONFIG_FUTEX
     struct robust_list_head __user *robust_list;
 #ifdef CONFIG_COMPAT
     struct compat_robust_list_head __user *compat_robust_list;
 #endif
     struct list_head pi_state_list;
     struct futex_pi_state *pi_state_cache;
 #endif
 #ifdef CONFIG_PERF_EVENTS
     struct perf_event_context *perf_event_ctxp[perf_nr_task_contexts];
     struct mutex perf_event_mutex;
     struct list_head perf_event_list;
 #endif
 #ifdef CONFIG_NUMA
     struct mempolicy *mempolicy;    /* Protected by alloc_lock */
     short il_next;
 #endif
     atomic_t fs_excl;   /* holding fs exclusive resources */
     struct rcu_head rcu;

     /*
      * cache last used pipe for splice
      */
     struct pipe_inode_info *splice_pipe;
 #ifdef  CONFIG_TASK_DELAY_ACCT
     struct task_delay_info *delays;
 #endif
 #ifdef CONFIG_FAULT_INJECTION
     int make_it_fail;
 #endif
     struct prop_local_single dirties;
 #ifdef CONFIG_LATENCYTOP
     int latency_record_count;
     struct latency_record latency_record[LT_SAVECOUNT];
 #endif
     /*
      * time slack values; these are used to round up poll() and
      * select() etc timeout values. These are in nanoseconds.
      */
      /*这些是松弛时间值，用来规定select()和poll()的超时时间，单位是纳秒nanoseconds  */
     unsigned long timer_slack_ns;
     unsigned long default_timer_slack_ns;

     struct list_head    *scm_work_list;
 #ifdef CONFIG_FUNCTION_GRAPH_TRACER
     /* Index of current stored address in ret_stack */
     int curr_ret_stack;
     /* Stack of return addresses for return function tracing */
     struct ftrace_ret_stack *ret_stack;
     /* time stamp for last schedule */
     unsigned long long ftrace_timestamp;
     /*
      * Number of functions that haven't been traced
      * because of depth overrun.
      */
     atomic_t trace_overrun;
     /* Pause for the tracing */
     atomic_t tracing_graph_pause;
 #endif
 #ifdef CONFIG_TRACING
     /* state flags for use by tracers */
     unsigned long trace;
     /* bitmask of trace recursion */
     unsigned long trace_recursion;
 #endif /* CONFIG_TRACING */
 #ifdef CONFIG_CGROUP_MEM_RES_CTLR /* memcg uses this to do batch job */
     struct memcg_batch_info {
         int do_batch;   /* incremented when batch uncharge started */
         struct mem_cgroup *memcg; /* target memcg of uncharge */
         unsigned long bytes;        /* uncharged usage */
         unsigned long memsw_bytes; /* uncharged mem+swap usage */
     } memcg_batch;
 #endif
 };

上面只是一些简单注释，后面会重点介绍某些重要的结构体，和它们的相关操作和用途。