窥探 kernel，just for fun --- task_struct

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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

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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。