进程四要素

• 有一段程序代其执行
• 有进程专用的系统堆栈空间
• 在内核有task_struct数据结构
• 进程有独立的存储空间，拥有专有的用户空间
  如果具备前三点缺少第四条，称为“线程”；如果完全没有用户空间，称为“内核线程”;如果共享用户空间，称为“用户线程”。

进程生命周期

• 运行
• 等待：可以运行，没有得到许可。调度器可以在下一个任务切换时选择该进程。
• 睡眠：等待一个外部事件。
• 转换关系：

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task_struct数据结构

进程描述task_struct数据结构

struct task_struct {
	// 进程状态 
	volatile long state;	/* -1 unrunnable, 0 runnable, >0 stopped */
	void *stack;  // 指向内核栈指针
	atomic_t usage;  // 有几个进程使用此结构
	unsigned int flags;	/* per process flags, defined below */
	unsigned int ptrace;  // ptrace系统调用 实现断点调试 跟踪进程运行 

// 条件编译多处理器用到 
#ifdef CONFIG_SMP
	struct llist_node wake_entry;
	int on_cpu;
	unsigned int wakee_flips;
	unsigned long wakee_flip_decay_ts;
	struct task_struct *last_wakee;

	int wake_cpu;
#endif

	// 运行队列和进程调试相关程序 
	int on_rq;

	int prio, static_prio, normal_prio;  //关于进程调试 
	unsigned int rt_priority; //优先级 

	// 关于进程 
	const struct sched_class *sched_class;
	struct sched_entity se;
	struct sched_rt_entity rt;
	// 结构体链表 
#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

	// 块设备I/O的跟踪工具
#ifdef CONFIG_BLK_DEV_IO_TRACE
	unsigned int btrace_seq;
#endif
	// 进程调试策略相关的字段
	unsigned int policy;
	int nr_cpus_allowed;
	cpumask_t cpus_allowed;

// RCU同步原语 
#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
	//进程链表架构 
	struct list_head tasks;
#ifdef CONFIG_SMP
	struct plist_node pushable_tasks;
	struct rb_node pushable_dl_tasks;
#endif
	// 进程管理地址空间，每个进程有独立的地址空间4G
	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_stat	rss_stat;
#endif

// 进程状态参数 
/* task state */
	int exit_state;
	int exit_code, exit_signal;
	// 接收父进程终止时就会发出信号 
	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 :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;
#ifdef CONFIG_MEMCG
	unsigned memcg_may_oom:1;
#endif
#ifdef CONFIG_MEMCG_KMEM
	unsigned memcg_kmem_skip_account:1;
#endif
#ifdef CONFIG_COMPAT_BRK
	unsigned brk_randomized:1;
#endif

	unsigned long atomic_flags; /* Flags needing atomic access. */

	struct restart_block restart_block;

	//  进程pid, 父进程tgid 
	pid_t pid;
	pid_t tgid;

	// 防止内核堆栈溢出 
#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)
	 */
	// 初始化父进程 
	struct task_struct __rcu *real_parent; /* real parent process */
	// 接收中止进程 
	struct task_struct __rcu *parent; /* recipient of SIGCHLD, wait4() reports */
	/*
	 * children/sibling forms the list of my natural children
	 */
	// 维护子进程链表 
	struct list_head children;	/* list of my children */
	// 兄弟进程链表
	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.
	 */
	// 系统调用 关于断开调试 
	struct list_head ptraced;
	struct list_head ptrace_entry;

	/* PID/PID hash table linkage. */
	// 散列表的关系
	struct pid_link pids[PIDTYPE_MAX];
	struct list_head thread_group;
	struct list_head thread_node;
	// do_fork()函数 
	struct completion *vfork_done;		/* for vfork() */
	int __user *set_child_tid;		/* CLONE_CHILD_SETTID */
	int __user *clear_child_tid;		/* CLONE_CHILD_CLEARTID */
	// 描述CPU时间的内容
	// utime 用户态下的执行时间
	// stime 内核态的执行时间 
	cputime_t utime, stime, utimescaled, stimescaled;
	cputime_t gtime;
	struct prev_cputime prev_cputime;
#ifdef CONFIG_VIRT_CPU_ACCOUNTING_GEN
	seqlock_t vtime_seqlock;
	unsigned long long vtime_snap;
	enum {
		VTIME_SLEEPING = 0,
		VTIME_USER,
		VTIME_SYS,
	} vtime_snap_whence;
#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 *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;

	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

/* 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 */ 
	// 虚拟内存状态参数 内存回收 
	struct reclaim_state *reclaim_state;
	// 存放块设备I/O流量信息 
	struct backing_dev_info *backing_dev_info;
	// I/O调度器所用的信息
	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
	// 内存检测工具
#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; //管道

	struct page_frag task_frag;
// 延迟计数 
#ifdef	CONFIG_TASK_DELAY_ACCT
	struct task_delay_info *delays;
#endif
#ifdef CONFIG_FAULT_INJECTION
	int make_it_fail;
#endif

进程优先级

• 实时进程 非实时进程(普通进程)
  实时进程优先级（0-99）比普通进程的优先级高（100-139）高。当系统中有实时进程运行时，普通进程几乎无法分到时间片（只能分到5%的CPU时间）。

进程系统调用

进程复制

1. fork是重量级调用，因为它建立了父进程的一个完整副本，然后作为子进程执行。 为减少与该调用相关的工作量，Linux使用了写时复制（copy-on-write）技术。
2. vfork类似于fork，但并不创建父进程数据的副本。相反，父子进程之间共享数据。 这节省了大量CPU时间（如果一个进程操纵共享数据，则另一个会自动注意到）。
3. clone产生线程，可以对父子进程之间的共享、复制进行精确控制

写时复制（Copy On Write）

内核使用了写时复制（Copy-On-Write，COW）技术，以防止在fork执行时将父进程的所有数据 复制到子进程。在调用fork时，内核通常对父进程的每个内存页，都为子进程创建一个相同的副本。

问题：主进程修改页z的数据，此时会发生父子进程在内存分离。

只有在不得不复制数据内容时采取复制数据内容，这就是写时复制的核心思想，可以看到因为修改页z导致子进程不得不去复制原来页z来保证父子进程互不干扰。
内核只为新生成的子进程创建虚拟空间结构，它们来复制父进程的虚拟结构，但是不为这些结构分配物理内存，它们共享父进程的物理空间，父进程有更改相应段的行为发生时，再为子进程相应段分配物理空间。

内核线程

内核线程是直接由内核本身启动的进程。内核线程实际上是将内核函数委托给独立的 进程，与系统中其他进程“并行”执行（实际上，也并不等于内核自身的执行）。内核线程经常 称之为（内核）守护进程。它们用于执行下列任务。

• 周期性地将修改的内存页与页来源块设备同步（例如，使用mmap的文件映射）。
• 如果内存页很少使用，则写入交换区。
• 管理延时动作（deferred action）。
• 实现文件系统的事务日志。

进程退出

进程必须用exit系统调用终止。这使得内核有机会将该进程使用的资源释放回系统。见kernel/exit.c------>do_exit。简而言之， 该函数的实现就是将各个引用计数器减1，如果引用计数器归0而没有进程再使用对应的结构，那么将相应的内存区域返还给内存管理模块

void do_exit(long code)
{
	struct task_struct *tsk = current;
	int group_dead;
	TASKS_RCU(int tasks_rcu_i);

	profile_task_exit(tsk);

	WARN_ON(blk_needs_flush_plug(tsk));

	if (unlikely(in_interrupt()))
		panic("Aiee, killing interrupt handler!");
	if (unlikely(!tsk->pid))
		panic("Attempted to kill the idle task!");

	/*
	 * If do_exit is called because this processes oopsed, it's possible
	 * that get_fs() was left as KERNEL_DS, so reset it to USER_DS before
	 * continuing. Amongst other possible reasons, this is to prevent
	 * mm_release()->clear_child_tid() from writing to a user-controlled
	 * kernel address.
	 */
	set_fs(USER_DS);

	ptrace_event(PTRACE_EVENT_EXIT, code);

	validate_creds_for_do_exit(tsk);

	/*
	 * We're taking recursive faults here in do_exit. Safest is to just
	 * leave this task alone and wait for reboot.
	 */
	if (unlikely(tsk->flags & PF_EXITING)) {
		pr_alert("Fixing recursive fault but reboot is needed!\n");
		/*
		 * We can do this unlocked here. The futex code uses
		 * this flag just to verify whether the pi state
		 * cleanup has been done or not. In the worst case it
		 * loops once more. We pretend that the cleanup was
		 * done as there is no way to return. Either the
		 * OWNER_DIED bit is set by now or we push the blocked
		 * task into the wait for ever nirwana as well.
		 */
		tsk->flags |= PF_EXITPIDONE;
		set_current_state(TASK_UNINTERRUPTIBLE);
		schedule();
	}

	exit_signals(tsk);  /* sets PF_EXITING */
	/*
	 * tsk->flags are checked in the futex code to protect against
	 * an exiting task cleaning up the robust pi futexes.
	 */
	smp_mb();
	raw_spin_unlock_wait(&tsk->pi_lock);

	if (unlikely(in_atomic())) {
		pr_info("note: %s[%d] exited with preempt_count %d\n",
			current->comm, task_pid_nr(current),
			preempt_count());
		preempt_count_set(PREEMPT_ENABLED);
	}

	/* sync mm's RSS info before statistics gathering */
	if (tsk->mm)
		sync_mm_rss(tsk->mm);
	acct_update_integrals(tsk);
	group_dead = atomic_dec_and_test(&tsk->signal->live);
	if (group_dead) {
		hrtimer_cancel(&tsk->signal->real_timer);
		exit_itimers(tsk->signal);
		if (tsk->mm)
			setmax_mm_hiwater_rss(&tsk->signal->maxrss, tsk->mm);
	}
	acct_collect(code, group_dead);
	if (group_dead)
		tty_audit_exit();
	audit_free(tsk);

	tsk->exit_code = code;
	taskstats_exit(tsk, group_dead);

	exit_mm(tsk);

	if (group_dead)
		acct_process();
	trace_sched_process_exit(tsk);

	exit_sem(tsk);
	exit_shm(tsk);
	exit_files(tsk);
	exit_fs(tsk);
	if (group_dead)
		disassociate_ctty(1);
	exit_task_namespaces(tsk);
	exit_task_work(tsk);
	exit_thread();

	/*
	 * Flush inherited counters to the parent - before the parent
	 * gets woken up by child-exit notifications.
	 *
	 * because of cgroup mode, must be called before cgroup_exit()
	 */
	perf_event_exit_task(tsk);

	cgroup_exit(tsk);

	/*
	 * FIXME: do that only when needed, using sched_exit tracepoint
	 */
	flush_ptrace_hw_breakpoint(tsk);

	TASKS_RCU(preempt_disable());
	TASKS_RCU(tasks_rcu_i = __srcu_read_lock(&tasks_rcu_exit_srcu));
	TASKS_RCU(preempt_enable());
	exit_notify(tsk, group_dead);
	proc_exit_connector(tsk);
#ifdef CONFIG_NUMA
	task_lock(tsk);
	mpol_put(tsk->mempolicy);
	tsk->mempolicy = NULL;
	task_unlock(tsk);
#endif
#ifdef CONFIG_FUTEX
	if (unlikely(current->pi_state_cache))
		kfree(current->pi_state_cache);
#endif
	/*
	 * Make sure we are holding no locks:
	 */
	debug_check_no_locks_held();
	/*
	 * We can do this unlocked here. The futex code uses this flag
	 * just to verify whether the pi state cleanup has been done
	 * or not. In the worst case it loops once more.
	 */
	tsk->flags |= PF_EXITPIDONE;

	if (tsk->io_context)
		exit_io_context(tsk);

	if (tsk->splice_pipe)
		free_pipe_info(tsk->splice_pipe);

	if (tsk->task_frag.page)
		put_page(tsk->task_frag.page);

	validate_creds_for_do_exit(tsk);

	check_stack_usage();
	preempt_disable();
	if (tsk->nr_dirtied)
		__this_cpu_add(dirty_throttle_leaks, tsk->nr_dirtied);
	exit_rcu();
	TASKS_RCU(__srcu_read_unlock(&tasks_rcu_exit_srcu, tasks_rcu_i));

	/*
	 * The setting of TASK_RUNNING by try_to_wake_up() may be delayed
	 * when the following two conditions become true.
	 *   - There is race condition of mmap_sem (It is acquired by
	 *     exit_mm()), and
	 *   - SMI occurs before setting TASK_RUNINNG.
	 *     (or hypervisor of virtual machine switches to other guest)
	 *  As a result, we may become TASK_RUNNING after becoming TASK_DEAD
	 *
	 * To avoid it, we have to wait for releasing tsk->pi_lock which
	 * is held by try_to_wake_up()
	 */
	smp_mb();
	raw_spin_unlock_wait(&tsk->pi_lock);

	/* causes final put_task_struct in finish_task_switch(). */
	tsk->state = TASK_DEAD;
	tsk->flags |= PF_NOFREEZE;	/* tell freezer to ignore us */
	schedule();
	BUG();
	/* Avoid "noreturn function does return".  */
	for (;;)
		cpu_relax();	/* For when BUG is null */
}

总结

本文介绍了进程四要素、进程生命周期，列举了task_struct数据结构的主要内容，系统调用写时复制思想，守护进程，进程退出原理等内容。