2.1 进程优先级
1. 进程分类
1）硬实时进程: 有严格时间限制
2）软实时进程，
3）普通进程：大多数进程
2.2 进程生命周期
进程的生命周期可归结为以下三个状态：
- 运行：进程正在执行；
- 睡眠：进程正在睡眠，不能执行，它在等待一个外部事件；
- 等待：进程可以运行，但是需要等到下一任务切换时执行。
更细致的，从task_struct中state中得到具体状态：
1）运行时状态：
TASK_RUNNING：进程处于可运行状态，但并不意味着已实际分配了CPU，而是进程可以无需等待外部条件执行；
TASK_INTERRUPTIBLE：进程处于睡眠状态，可由外部信号唤醒；
TASK_UNINTERRUPTIBLE：睡眠状态，但不能有外部信号唤醒，例如IO等待，这种状态主要是为了保持强一致性，例如读取磁盘时若外部信号能中断，那么磁盘读取则会处理不完整状态，影响正常使用。
__TASK_STOPPED：进程特意停止运行，如由调试器暂停；
__TASK_TRACED：ptrace跟踪用，在调试时区分常规进程；
2）退出时状态：exit_state
EXIT_ZOMBIE: 进程处于僵尸状态。当进程被另一个进程或用户杀死的同时，父进程在该子进程终止时，未调用wait函数，则会出现僵尸进程。
EXIT_DEAD：指父进程已发出wait调用，但进程还未完全从系统中移除之前的状态
2.3 进程表示
进程的表示主要通过task_struct结构：
struct task_struct {
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;
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 */
#ifdef CONFIG_RCU_BOOST
struct rt_mutex *rcu_boost_mutex;
#endif /* #ifdef CONFIG_RCU_BOOST */
#if defined(CONFIG_SCHEDSTATS) || defined(CONFIG_TASK_DELAY_ACCT)
struct sched_info sched_info;
#endif
struct list_head tasks;
#ifdef CONFIG_SMP
struct plist_node pushable_tasks;
#endif
struct mm_struct *mm, *active_mm;
#ifdef CONFIG_COMPAT_BRK
unsigned brk_randomized:1;
#endif
#if defined(SPLIT_RSS_COUNTING)
struct task_rss_stat rss_stat;
#endif
/* task state */
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;
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 *real_parent; /* real parent process */
struct task_struct *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 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;
#ifndef CONFIG_VIRT_CPU_ACCOUNTING
cputime_t prev_utime, prev_stime;
#endif
unsigned long nvcsw, nivcsw; /* context switch counts */
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 */
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 */
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 */
struct thread_struct thread;
/* 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;
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;
#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 */
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;
short pref_node_fork;
#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.
*/
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 nr_pages; /* uncharged usage */
unsigned long memsw_nr_pages; /* uncharged mem+swap usage */
} memcg_batch;
#endif
#ifdef CONFIG_HAVE_HW_BREAKPOINT
atomic_t ptrace_bp_refcnt;
#endif
};
进程管理中需要注意的一些重要成员：
1、state: 见上文；
2、资源管理rlim数组，见signal_struct中:
struct rlimit rlim[RLIM_NLIMITS];
rlimit定义如下：
struct rlmit {
unsigned long rlim_cur; // 进程当前的资源限制，叫软限制
unsigned long rlim_max; // 进程最大容许值，叫硬限制
}
rlim数组的索引标识类型，资源限制可通过查看limits文件得知：
cat /proc/pid/limits
2.3.1 进程类型
1、新进程是通过fork、exec系统调用产生的；
2、clone也可以产生新进程，但是clone主要用于实现线程，它和fork的主要不同点在于父子进程共享的资源不同，本质上是没有任务区别的。
2.3.2 命名空间
作用
不同于KVM或VMWare，Linux的命名空间只使用一个内核在一台物理计算机上运作，只需要很少的资源，便可虚拟化出多台计算机。namespace主要做资源的隔离，正如目前很热门的Docker技术也是使用类似的原理。
创建方式
创建方式有三种：
1、clone() – 实现线程的系统调用，用来创建一个新的进程，并可以通过设计参数达到各类资源的隔离。
2、unshare() – 使某进程脱离某个namespace
3、setns() – 把某进程加入到某个namespace
实现方式
namespace的实现主要由nsproxy结构：
struct nsproxy {
atomic_t count; // 引用计数，
struct uts_namespace *uts_ns; // UTS命名空间，包括内核名称版本等信息
struct ipc_namespace *ipc_ns; // 进程间通信相关信息
struct mnt_namespace *mnt_ns; // 文件系统挂载信息
struct user_namespace *user_ns; // 用于保存限制每个用户资源使用的信息
struct pid_namespace *pid_ns; // 进程ID相关信息
struct net *net_ns; // 网络相关命名空间信息
};
具体可参考：DOCKER基础技术：LINUX NAMESPACE（上）
2.3.2 进程ID号
pid.png
上图很好的阐述了进程ID的结构信息
首先从task_struct开始：
struct task_struct {
...
struct pid_link pids[PIDTYPE_MAX];
...
}
其中pids数组是一个将task_struct关联到pid的散列表。
struct pid_link {
struct hlist_node node; 用作散列表元素
struct pid *pid;
}
struct upid {
/* Try to keep pid_chain in the same cacheline as nr for find_vpid */
int nr; // ID数值
struct pid_namespace *ns; // 关联到pid_namespace的指针
struct hlist_node pid_chain;
};
struct pid
{
atomic_t count; // 引用计数
unsigned int level; // 层级，子命名空间的level为父命名空间level+1
/* lists of tasks that use this pid */
struct hlist_head tasks[PIDTYPE_MAX]; // 一个HASH数组，每一项都是一个链表头。分别是PID链表头，进程组ID表头，会话ID表头；
struct rcu_head rcu;
struct upid numbers[1]; // 是一个UPID数组，记录对应层级的命名空间中的UPID，所以可以想到，该PID处于第几层，那么这个数组应该有几项（当然都是从0开始）。
};
enum pid_type
{
PIDTYPE_PID, // 进程PID
PIDTYPE_PGID, // 进程组PID
PIDTYPE_SID, // 会话PID
PIDTYPE_MAX
};
这里没加上TGID的原因是线程组也是一种PID：线程组长PID，再单独定义一个id没有必要。
upid中关联到pid_namespace，再看看pid_namespace定义：
struct pid_namespace {
struct kref kref;
struct pidmap pidmap[PIDMAP_ENTRIES];
int last_pid;
struct task_struct *child_reaper;
struct kmem_cache *pid_cachep;
unsigned int level;
struct pid_namespace *parent;
#ifdef CONFIG_PROC_FS
struct vfsmount *proc_mnt;
#endif
#ifdef CONFIG_BSD_PROCESS_ACCT
struct bsd_acct_struct *bacct;
#endif
};
在这里只关注child_reaper指针、level和parent指针。
其中child_reaper作用类似于fork函数中父进程调用wait系列函数，用于托管进程：就是当父进程先于子进程结束的时候，就把子进程的父进程更新为child_reaper。
level即为命名空间的层级关系；
parent：父pid命名空间指针。
参考文章：Pid NameSpace浅分析
生成唯一PID
唯一pid的生成其实是通过一个大的bitmap生成，bitmap有高效、节省空间的作用，本质即是寻找bitmap中第一个为0的比特用于分配新pid。该bitmap可见pid_namespace:
struct pid_namespace {
...
struct pidmap pidmap[PIDMAP_ENTRIES];
...
}
#define PIDMAP_ENTRIES ((PID_MAX_LIMIT + 8*PAGE_SIZE - 1)/PAGE_SIZE/8);
#define PID_MAX_LIMIT (CONFIG_BASE_SMALL ? PAGE_SIZE * 8 : \
(sizeof(long) > 4 ? 4 * 1024 * 1024 : PID_MAX_DEFAULT));
alloc_pidmap函数用于分配一个PID，而free_pidmap用于地方一个PID，具体见kernel/pid.c
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