task_struct是程序描述符：

struct task_struct {
#ifdef CONFIG_THREAD_INFO_IN_TASK
/*
 * For reasons of header soup (see current_thread_info()), this
 * must be the first element of task_struct.
 */
struct thread_info thread_info;
#endif
volatile long state;程序狀態/* -1 unrunnable, 0 runnable, >0 stopped */
void *stack;程序核心棧
atomic_t usage;
unsigned int flags;/* per process flags, defined below */
unsigned int ptrace;


#ifdef CONFIG_SMP
struct llist_node wake_entry;
int on_cpu;
#ifdef CONFIG_THREAD_INFO_IN_TASK
unsigned int cpu;/* current CPU */
#endif
unsigned int wakee_flips;
unsigned long wakee_flip_decay_ts;
struct task_struct *last_wakee;


int wake_cpu;
#endif
int on_rq;


int prio, static_prio, normal_prio;
unsigned int rt_priority;
const struct sched_class *sched_class;
struct sched_entity se;
struct sched_rt_entity rt;
#ifdef CONFIG_CGROUP_SCHED
struct task_group *sched_task_group;
#endif
struct sched_dl_entity dl;


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

#ifdef CONFIG_BLK_DEV_IO_TRACE
unsigned int btrace_seq;
#endif

unsigned int policy;
int nr_cpus_allowed;
cpumask_t cpus_allowed;

#ifdef CONFIG_PREEMPT_RCU
int rcu_read_lock_nesting;
union rcu_special rcu_read_unlock_special;
struct list_head rcu_node_entry;
struct rcu_node *rcu_blocked_node;
#endif /* #ifdef CONFIG_PREEMPT_RCU */
#ifdef CONFIG_TASKS_RCU
unsigned long rcu_tasks_nvcsw;
bool rcu_tasks_holdout;
struct list_head rcu_tasks_holdout_list;
int rcu_tasks_idle_cpu;
#endif /* #ifdef CONFIG_TASKS_RCU */

#ifdef CONFIG_SCHED_INFO
struct sched_info sched_info;
#endif

struct list_head tasks;
#ifdef CONFIG_SMP
struct plist_node pushable_tasks;
struct rb_node pushable_dl_tasks;
#endif

struct mm_struct *mm, *active_mm;
/* per-thread vma caching */
u32 vmacache_seqnum;
struct vm_area_struct *vmacache[VMACACHE_SIZE];
#if defined(SPLIT_RSS_COUNTING)
struct task_rss_statrss_stat;
#endif
/* task state */
int exit_state;
int exit_code, exit_signal;
int pdeath_signal;  /*  The signal sent when the parent dies  */
unsigned long jobctl;/* JOBCTL_*, siglock protected */

/* Used for emulating ABI behavior of previous Linux versions */
unsigned int personality;

/* scheduler bits, serialized by scheduler locks */
unsigned sched_reset_on_fork:1;
unsigned sched_contributes_to_load:1;
unsigned sched_migrated:1;
unsigned sched_remote_wakeup:1;
unsigned :0; /* force alignment to the next boundary */

/* unserialized, strictly 'current' */
unsigned in_execve:1; /* bit to tell LSMs we're in execve */
unsigned in_iowait:1;
#if !defined(TIF_RESTORE_SIGMASK)
unsigned restore_sigmask:1;
#endif
#ifdef CONFIG_MEMCG
unsigned memcg_may_oom:1;
#ifndef CONFIG_SLOB
unsigned memcg_kmem_skip_account:1;
#endif
#endif
#ifdef CONFIG_COMPAT_BRK
unsigned brk_randomized:1;
#endif
unsigned long atomic_flags; /* Flags needing atomic access. */

struct restart_block restart_block;

pid_t pid;
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)
 */
struct task_struct __rcu *real_parent; /* real parent process */
struct task_struct __rcu *parent; /* recipient of SIGCHLD, wait4() reports */
/*
 * children/sibling forms the list of my natural children
 */
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 */


/*
 * 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];
struct list_head thread_group;
struct list_head thread_node;


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


cputime_t utime, stime, utimescaled, stimescaled;
cputime_t gtime;
struct prev_cputime prev_cputime;
#ifdef CONFIG_VIRT_CPU_ACCOUNTING_GEN
seqcount_t vtime_seqcount;
unsigned long long vtime_snap;
enum {
/* Task is sleeping or running in a CPU with VTIME inactive */
VTIME_INACTIVE = 0,
/* Task runs in userspace in a CPU with VTIME active */
VTIME_USER,
/* Task runs in kernelspace in a CPU with VTIME active */
VTIME_SYS,
} vtime_snap_whence;
#endif


#ifdef CONFIG_NO_HZ_FULL
atomic_t tick_dep_mask;
#endif
unsigned long nvcsw, nivcsw; /* context switch counts */
u64 start_time;/* monotonic time in nsec */
u64 real_start_time;/* boot based time in nsec */
/* 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 *ptracer_cred; /* Tracer's credentials at attach */
const struct cred __rcu *real_cred; /* objective and real subjective task
 * credentials (COW) */
const struct cred __rcu *cred;/* effective (overridable) subjective task
 * credentials (COW) */
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 */
struct nameidata *nameidata;
#ifdef CONFIG_SYSVIPC
/* ipc stuff */
struct sysv_sem sysvsem;
struct sysv_shm sysvshm;
#endif
#ifdef CONFIG_DETECT_HUNG_TASK
/* hung task detection */
unsigned long last_switch_count;
#endif
/* filesystem information */
struct fs_struct *fs;
/* open file information */
struct files_struct *files;
/* namespaces */
struct nsproxy *nsproxy;
/* signal handlers */
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;
unsigned sas_ss_flags;


struct callback_head *task_works;


struct audit_context *audit_context;
#ifdef CONFIG_AUDITSYSCALL
kuid_t loginuid;
unsigned int sessionid;
#endif
struct seccomp 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;


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


struct wake_q_node wake_q;


#ifdef CONFIG_RT_MUTEXES
/* PI waiters blocked on a rt_mutex held by this task */
struct rb_root pi_waiters;
struct rb_node *pi_waiters_leftmost;
/* 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
#ifdef CONFIG_UBSAN
unsigned int in_ubsan;
#endif


/* journalling filesystem info */
void *journal_info;


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


#ifdef CONFIG_BLOCK
/* stack plugging */
struct blk_plug *plug;
#endif


/* 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 */
seqcount_t mems_allowed_seq;/* Seqence no to catch updates */
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_DEBUG_PREEMPT
unsigned long preempt_disable_ip;
#endif
#ifdef CONFIG_NUMA
struct mempolicy *mempolicy;/* Protected by alloc_lock */
short il_next;
short pref_node_fork;
#endif
#ifdef CONFIG_NUMA_BALANCING
int numa_scan_seq;
unsigned int numa_scan_period;
unsigned int numa_scan_period_max;
int numa_preferred_nid;
unsigned long numa_migrate_retry;
u64 node_stamp;/* migration stamp  */
u64 last_task_numa_placement;
u64 last_sum_exec_runtime;
struct callback_head numa_work;


struct list_head numa_entry;
struct numa_group *numa_group;


/*
 * numa_faults is an array split into four regions:
 * faults_memory, faults_cpu, faults_memory_buffer, faults_cpu_buffer
 * in this precise order.
 *
 * faults_memory: Exponential decaying average of faults on a per-node
 * basis. Scheduling placement decisions are made based on these
 * counts. The values remain static for the duration of a PTE scan.
 * faults_cpu: Track the nodes the process was running on when a NUMA
 * hinting fault was incurred.
 * faults_memory_buffer and faults_cpu_buffer: Record faults per node
 * during the current scan window. When the scan completes, the counts
 * in faults_memory and faults_cpu decay and these values are copied.
 */
unsigned long *numa_faults;
unsigned long total_numa_faults;


/*
 * numa_faults_locality tracks if faults recorded during the last
 * scan window were remote/local or failed to migrate. The task scan
 * period is adapted based on the locality of the faults with different
 * weights depending on whether they were shared or private faults
 */
unsigned long numa_faults_locality[3];


unsigned long numa_pages_migrated;
#endif /* CONFIG_NUMA_BALANCING */


#ifdef CONFIG_ARCH_WANT_BATCHED_UNMAP_TLB_FLUSH
struct tlbflush_unmap_batch tlb_ubc;
#endif


struct rcu_head rcu;


/*
 * cache last used pipe for splice
 */
struct pipe_inode_info *splice_pipe;


struct page_frag task_frag;


#ifdefCONFIG_TASK_DELAY_ACCT
struct task_delay_info *delays;
#endif
#ifdef CONFIG_FAULT_INJECTION
int make_it_fail;
#endif
/*
 * when (nr_dirtied >= nr_dirtied_pause), it's time to call
 * balance_dirty_pages() for some dirty throttling pause
 */
int nr_dirtied;
int nr_dirtied_pause;
unsigned long dirty_paused_when; /* start of a write-and-pause period */


#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.
 */
u64 timer_slack_ns;
u64 default_timer_slack_ns;


#ifdef CONFIG_KASAN
unsigned int kasan_depth;
#endif
#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 and counter of trace recursion */
unsigned long trace_recursion;
#endif /* CONFIG_TRACING */
#ifdef CONFIG_KCOV
/* Coverage collection mode enabled for this task (0 if disabled). */
enum kcov_mode kcov_mode;
/* Size of the kcov_area. */
unsignedkcov_size;
/* Buffer for coverage collection. */
void*kcov_area;
/* kcov desciptor wired with this task or NULL. */
struct kcov*kcov;
#endif
#ifdef CONFIG_MEMCG
struct mem_cgroup *memcg_in_oom;
gfp_t memcg_oom_gfp_mask;
int memcg_oom_order;


/* number of pages to reclaim on returning to userland */
unsigned int memcg_nr_pages_over_high;
#endif
#ifdef CONFIG_UPROBES
struct uprobe_task *utask;
#endif
#if defined(CONFIG_BCACHE) || defined(CONFIG_BCACHE_MODULE)
unsigned intsequential_io;
unsigned intsequential_io_avg;
#endif
#ifdef CONFIG_DEBUG_ATOMIC_SLEEP
unsigned longtask_state_change;
#endif
int pagefault_disabled;
#ifdef CONFIG_MMU
struct task_struct *oom_reaper_list;
#endif
#ifdef CONFIG_VMAP_STACK
struct vm_struct *stack_vm_area;
#endif
#ifdef CONFIG_THREAD_INFO_IN_TASK
/* A live task holds one reference. */
atomic_t stack_refcount;
#endif
/* CPU-specific state of this task */
struct thread_struct thread;
/*
 * WARNING: on x86, 'thread_struct' contains a variable-sized
 * structure.  It *MUST* be at the end of 'task_struct'.
 *
 * Do not put anything below here!
 */
};

程序描述符當然是在程序建立時候被建立，也就是在fork時候。