在Linux内核里，无论是进程还是线程，统一使用 task_struct{} 结构体来表示，也就是统一抽象为任务（task）。task_struct{} 定义在 include/linux/sched.h 文件中，十分复杂，这里简单了解下。
// include/linux/sched.h
// ... 省略
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
/* -1 unrunnable, 0 runnable, >0 stopped: */
volatile long state;
int exit_state;
int exit_code;
int exit_signal;
/*
* This begins the randomizable portion of task_struct. Only
* scheduling-critical items should be added above here.
*/
randomized_struct_fields_start
void *stack;
refcount_t usage;
/* Per task flags (PF_*), defined further below: */
unsigned int flags;
unsigned int ptrace;
#ifdef CONFIG_SMP
int on_cpu;
struct __call_single_node wake_entry;
#ifdef CONFIG_THREAD_INFO_IN_TASK
/* Current CPU: */
unsigned int cpu;
#endif
unsigned int wakee_flips;
unsigned long wakee_flip_decay_ts;
struct task_struct *last_wakee;
// ...省略
struct sched_info sched_info;
struct list_head tasks; // 链表，将所有task_struct串起来
pid_t pid; // process id，指的是线程id
pid_t tgid; // thread group ID，指的是进程的主线程id
struct task_struct *group_leader; // 指向的是进程的主线程
/* Signal handlers: */
struct signal_struct *signal;
struct sighand_struct __rcu *sighand;
sigset_t blocked;
sigset_t real_blocked;
/* Restored if set_restore_sigmask() was used: */
sigset_t saved_sigmask;
struct sigpending pending;
unsigned long sas_ss_sp;
size_t sas_ss_size;
unsigned int sas_ss_flags;
// ... 省略
}查阅相关资料后，对Linux中进程的生命周期总结如下：
从图上可以看出，进程的睡眠状态是最多的，那进程一般在何时进入睡眠状态呢？答案是I/O操作时，因为I/O操作的速度与CPU运行速度相比，相差太大，所以此时进程会释放CPU，进入睡眠状态。
进程状态相关的定义同样在 include/linux/sched.h 文件的开头部分，以 #define TASK_KILLABLE (TASK_WAKEKILL | TASK_UNINTERRUPTIBLE) 为例，TASK_WAKEKILL表示用于在接收到致命信号时唤醒进程，将它与 TASK_UNINTERRUPTIBLE 按位或，就得到了 TASK_KILLABLE。代码注释中提到了 fs/proc/array.c，所以也将其代码贴出，作为补充。
// include/linux/sched.h
// ... 省略
/*
* Task state bitmask. NOTE! These bits are also
* encoded in fs/proc/array.c: get_task_state().
*
* We have two separate sets of flags: task->state
* is about runnability, while task->exit_state are
* about the task exiting. Confusing, but this way
* modifying one set can't modify the other one by
* mistake.
*/
/* Used in tsk->state: */
#define TASK_RUNNING 0x0000
#define TASK_INTERRUPTIBLE 0x0001
#define TASK_UNINTERRUPTIBLE 0x0002
#define __TASK_STOPPED 0x0004
#define __TASK_TRACED 0x0008
/* Used in tsk->exit_state: */
#define EXIT_DEAD 0x0010
#define EXIT_ZOMBIE 0x0020
#define EXIT_TRACE (EXIT_ZOMBIE | EXIT_DEAD)
/* Used in tsk->state again: */
#define TASK_PARKED 0x0040
#define TASK_DEAD 0x0080
#define TASK_WAKEKILL 0x0100
#define TASK_WAKING 0x0200
#define TASK_NOLOAD 0x0400
#define TASK_NEW 0x0800
#define TASK_STATE_MAX 0x1000
/* Convenience macros for the sake of set_current_state: */
#define TASK_KILLABLE (TASK_WAKEKILL | TASK_UNINTERRUPTIBLE)
#define TASK_STOPPED (TASK_WAKEKILL | __TASK_STOPPED)
#define TASK_TRACED (TASK_WAKEKILL | __TASK_TRACED)
#define TASK_IDLE (TASK_UNINTERRUPTIBLE | TASK_NOLOAD)
/* Convenience macros for the sake of wake_up(): */
#define TASK_NORMAL (TASK_INTERRUPTIBLE | TASK_UNINTERRUPTIBLE)
/* get_task_state(): */
#define TASK_REPORT (TASK_RUNNING | TASK_INTERRUPTIBLE | \
TASK_UNINTERRUPTIBLE | __TASK_STOPPED | \
__TASK_TRACED | EXIT_DEAD | EXIT_ZOMBIE | \
TASK_PARKED)
#define task_is_traced(task) ((task->state & __TASK_TRACED) != 0)
#define task_is_stopped(task) ((task->state & __TASK_STOPPED) != 0)
#define task_is_stopped_or_traced(task) ((task->state & (__TASK_STOPPED | __TASK_TRACED)) != 0)
// ... 省略// fs/proc/array.c
// ... 省略
/*
* The task state array is a strange "bitmap" of
* reasons to sleep. Thus "running" is zero, and
* you can test for combinations of others with
* simple bit tests.
*/
static const char * const task_state_array[] = {
/* states in TASK_REPORT: */
"R (running)", /* 0x00 */
"S (sleeping)", /* 0x01 */
"D (disk sleep)", /* 0x02 */
"T (stopped)", /* 0x04 */
"t (tracing stop)", /* 0x08 */
"X (dead)", /* 0x10 */
"Z (zombie)", /* 0x20 */
"P (parked)", /* 0x40 */
/* states beyond TASK_REPORT: */
"I (idle)", /* 0x80 */
};
static inline const char *get_task_state(struct task_struct *tsk)
{
BUILD_BUG_ON(1 + ilog2(TASK_REPORT_MAX) != ARRAY_SIZE(task_state_array));
return task_state_array[task_state_index(tsk)];
}
// ... 省略在单核的CPU上，同一时刻只有一个task会被调度，所以即使看到了 R 状态，也不代表进程就被分配到了CPU时间片。但了解了进程状态之后，我们再通过 top, ps aux 等命令查看进程，分析问题起来效率就更高了:
top - 17:24:07 up 10:20, 1 user, load average: 0.15, 0.08, 0.02
Tasks: 216 total, 1 running, 215 sleeping, 0 stopped, 0 zombie
%Cpu(s): 0.4 us, 0.3 sy, 0.0 ni, 99.2 id, 0.0 wa, 0.0 hi, 0.0 si, 0.0 st
MiB Mem : 5945.2 total, 2655.4 free, 1580.8 used, 1709.1 buff/cache
MiB Swap: 2048.0 total, 2048.0 free, 0.0 used. 4084.6 avail Mem
PID USER PR NI VIRT RES SHR S %CPU %MEM TIME+ COMMAND
1914 demonlee 20 0 5252100 395940 132816 S 1.0 6.5 1:33.09 gnome-shell
824 root 20 0 2495652 89512 48936 S 0.7 1.5 0:05.59 dockerd
1687 demonlee 20 0 1153156 81436 50128 S 0.3 1.3 0:15.90 Xorg
1957 demonlee 20 0 206556 28348 18504 S 0.3 0.5 0:00.26 ibus-x11
2897 demonlee 20 0 874684 61296 44532 S 0.3 1.0 0:06.22 gnome-terminal-
19984 demonlee 20 0 20632 4036 3376 R 0.3 0.1 0:00.02 top
1 root 20 0 169076 12952 8288 S 0.0 0.2 0:04.61 systemd
2 root 20 0 0 0 0 S 0.0 0.0 0:00.01 kthreadd
3 root 0 -20 0 0 0 I 0.0 0.0 0:00.00 rcu_gp
4 root 0 -20 0 0 0 I 0.0 0.0 0:00.00 rcu_par_gp
6 root 0 -20 0 0 0 I 0.0 0.0 0:00.00 kworker/0:0H-kblockd
9 root 0 -20 0 0 0 I 0.0 0.0 0:00.00 mm_percpu_wq
10 root 20 0 0 0 0 S 0.0 0.0 0:00.08 ksoftirqd/0
11 root 20 0 0 0 0 I 0.0 0.0 0:03.60 rcu_sched
12 root rt 0 0 0 0 S 0.0 0.0 0:00.51 migration/0
13 root -51 0 0 0 0 S 0.0 0.0 0:00.00 idle_inject/0
14 root 20 0 0 0 0 S 0.0 0.0 0:00.00 cpuhp/0
15 root 20 0 0 0 0 S 0.0 0.0 0:00.00 cpuhp/1
16 root -51 0 0 0 0 S 0.0 0.0 0:00.00 idle_inject/1
17 root rt 0 0 0 0 S 0.0 0.0 0:00.89 migration/1
18 root 20 0 0 0 0 S 0.0 0.0 0:00.09 ksoftirqd/1
20 root 0 -20 0 0 0 I 0.0 0.0 0:00.00 kworker/1:0H
21 root 20 0 0 0 0 S 0.0 0.0 0:00.00 cpuhp/2
22 root -51 0 0 0 0 S 0.0 0.0 0:00.00 idle_inject/2
23 root rt 0 0 0 0 S 0.0 0.0 0:00.83 migration/2
24 root 20 0 0 0 0 S 0.0 0.0 0:00.07 ksoftirqd/2
26 root 0 -20 0 0 0 I 0.0 0.0 0:00.00 kworker/2:0H-kblockd
27 root 20 0 0 0 0 S 0.0 0.0 0:00.00 cpuhp/3
28 root -51 0 0 0 0 S 0.0 0.0 0:00.00 idle_inject/3
29 root rt 0 0 0 0 S 0.0 0.0 0:00.77 migration/3
30 root 20 0 0 0 0 S 0.0 0.0 0:00.18 ksoftirqd/3
32 root 0 -20 0 0 0 I 0.0 0.0 0:00.00 kworker/3:0H-kblockd
33 root 20 0 0 0 0 S 0.0 0.0 0:00.00 kdevtmpfs
34 root 0 -20 0 0 0 I 0.0 0.0 0:00.00 netns
35 root 20 0 0 0 0 S 0.0 0.0 0:00.00 rcu_tasks_kthre
36 root 20 0 0 0 0 S 0.0 0.0 0:00.00 rcu_tasks_rude_
37 root 20 0 0 0 0 S 0.0 0.0 0:00.00 rcu_tasks_trace
38 root 20 0 0 0 0 S 0.0 0.0 0:00.00 kauditd
39 root 20 0 0 0 0 S 0.0 0.0 0:00.03 khungtaskd
demonlee@demonlee-ubuntu:~$最后，再补充一个知识点：使用ps命令查看进程时，会发现状态上面有其他符号，比如 S+、Z+ 等，如下所示，
demonlee@demonlee-ubuntu:~$
demonlee 1704 0.0 0.6 557904 37256 ? Sl 05:01 0:00 /usr/libexec/goa-daemon
demonlee 1707 0.0 0.1 172652 6936 tty2 Ssl+ 05:01 0:00 /usr/lib/gdm3/gdm-x-session --run-script env GNOME_SHELL_SESSION_MODE=ubuntu /usr/bin/g
demonlee 1714 0.0 0.1 323388 9068 ? Sl 05:01 0:00 /usr/libexec/goa-identity-service
demonlee 1720 0.1 1.3 1151880 80576 tty2 Sl+ 05:01 1:05 /usr/lib/xorg/Xorg vt2 -displayfd 3 -auth /run/user/1000/gdm/Xauthority -background non
demonlee 1723 0.0 0.1 325356 9016 ? Ssl 05:01 0:02 /usr/libexec/gvfs-afc-volume-monitor
demonlee 1728 0.0 0.1 244336 6532 ? Ssl 05:01 0:00 /usr/libexec/gvfs-mtp-volume-monitor
demonlee 1759 0.0 0.2 197052 14276 tty2 Sl+ 05:01 0:00 /usr/libexec/gnome-session-binary --systemd --systemd --session=ubuntu
...这个 + 是啥意思呢，其实 man ps 中就有描述，只是我们从来都没认真看说明文档：
PROCESS STATE CODES
Here are the different values that the s, stat and state output specifiers (header "STAT" or "S") will display to describe the state of a
process:
D uninterruptible sleep (usually IO)
I Idle kernel thread
R running or runnable (on run queue)
S interruptible sleep (waiting for an event to complete)
T stopped by job control signal
t stopped by debugger during the tracing
W paging (not valid since the 2.6.xx kernel)
X dead (should never be seen)
Z defunct ("zombie") process, terminated but not reaped by its parent
For BSD formats and when the stat keyword is used, additional characters may be displayed:
< high-priority (not nice to other users)
N low-priority (nice to other users)
L has pages locked into memory (for real-time and custom IO)
s is a session leader
l is multi-threaded (using CLONE_THREAD, like NPTL pthreads do)
+ is in the foreground process group