Linux进程pid分配法

2023-12-15 05:29| 来源: 网络整理| 查看: 265

copy from : http://gityuan.com/2017/08/06/linux_process_pid/

一. 概述

Android系统创建进程，最终的实现还是调用linux fork方法，对于linux系统每个进程都有唯一的进程ID(值大于0)，也有pid上限，默认为32768。 pid可重复利用，当进程被杀后会回收该pid，以供后续的进程pid分配。

上一篇文章Linux进程管理详细地介绍了进程fork过程，在copy_process()过程，执行完父进行文件、内存等信息的拷贝，紧接着便是执行alloc_pid()方法去分配pid.

二. 分配法 2.1 copy_process static struct task_struct *copy_process(unsigned long clone_flags, unsigned long stack_start, unsigned long stack_size, int __user *child_tidptr, struct pid *pid, int trace, unsigned long tls) { ... struct task_struct *p; if (pid != &init_struct_pid) { //分配pid[见小节2.2] pid = alloc_pid(p->nsproxy->pid_ns_for_children); } p->pid = pid_nr(pid); //设置pid[见小节2.4] ... } 2.2 alloc_pid

[-> kernel/kernel/pid.c]

struct pid *alloc_pid(struct pid_namespace *ns) { struct pid *pid; //[见小节2.2.1] enum pid_type type; int i, nr; struct pid_namespace *tmp; //[见小节2.2.4] struct upid *upid; int retval = -ENOMEM; //分配pid结构体的内存 pid = kmem_cache_alloc(ns->pid_cachep, GFP_KERNEL); ... tmp = ns; pid->level = ns->level; for (i = ns->level; i >= 0; i--) { nr = alloc_pidmap(tmp); //分配pid【见小节2.3】 ... pid->numbers[i].nr = nr; //nr保存到pid结构体 pid->numbers[i].ns = tmp; tmp = tmp->parent; } ... get_pid_ns(ns); atomic_set(&pid->count, 1); for (type = 0; type < PIDTYPE_MAX; ++type) INIT_HLIST_HEAD(&pid->tasks[type]); //初始化pid的hlist结构体 upid = pid->numbers + ns->level; spin_lock_irq(&pidmap_lock); if (!(ns->nr_hashed & PIDNS_HASH_ADDING)) goto out_unlock; for ( ; upid >= pid->numbers; --upid) { //建立pid_hash的关联关系 hlist_add_head_rcu(&upid->pid_chain, &pid_hash[pid_hashfn(upid->nr, upid->ns)]); upid->ns->nr_hashed++; } spin_unlock_irq(&pidmap_lock); return pid; ... } 2.2.1 pid结构体

[-> kernel/include/linux/pid.h]

struct pid { atomic_t count; unsigned int level; struct hlist_head tasks[PIDTYPE_MAX]; //见enum pid_type struct rcu_head rcu; struct upid numbers[1]; //见结构体upid }; 2.2.2 upid结构体

[-> pid.h]

struct upid { int nr; struct pid_namespace *ns; struct hlist_node pid_chain; }; 2.2.3 pid_type

[-> pid.h]

enum pid_type { PIDTYPE_PID, //进程ID PIDTYPE_PGID, //进程组ID PIDTYPE_SID, //会话组ID PIDTYPE_MAX, __PIDTYPE_TGID //仅用于__task_pid_nr_ns() }; 2.2.4 pid_namespace结构体

[-> kernel/include/linux/pid_namespace.h]

struct pid_namespace { struct kref kref; struct pidmap pidmap[PIDMAP_ENTRIES]; struct rcu_head rcu; int last_pid; unsigned int nr_hashed; struct task_struct *child_reaper; struct kmem_cache *pid_cachep; unsigned int level; struct pid_namespace *parent; ... struct user_namespace *user_ns; struct work_struct proc_work; kgid_t pid_gid; int hide_pid; int reboot; struct ns_common ns; };

PID命名空间，这是为系统提供虚拟化做支撑的功能。

2.3 alloc_pidmap

[-> kernel/kernel/pid.c]

static int alloc_pidmap(struct pid_namespace *pid_ns) { //last_pid为上次分配出去的pid int i, offset, max_scan, pid, last = pid_ns->last_pid; struct pidmap *map; pid = last + 1; if (pid >= pid_max) pid = RESERVED_PIDS; //默认为300 offset = pid & BITS_PER_PAGE_MASK; //最高位值置0，其余位不变 map = &pid_ns->pidmap[pid/BITS_PER_PAGE]; //找到目标pidmap //当offset =0，则扫描一次; //当offset!=0，则扫描两次 max_scan = DIV_ROUND_UP(pid_max, BITS_PER_PAGE) - !offset; for (i = 0; i page)) { void *page = kzalloc(PAGE_SIZE, GFP_KERNEL); spin_lock_irq(&pidmap_lock); if (!map->page) { map->page = page; page = NULL; } spin_unlock_irq(&pidmap_lock); kfree(page); if (unlikely(!map->page)) break; } //当pidmap还有可用pid时 if (likely(atomic_read(&map->nr_free))) { do { //当offset位空闲时返回该pid if (!test_and_set_bit(offset, map->page)) { atomic_dec(&map->nr_free); //可用pid减一 set_last_pid(pid_ns, last, pid); //设置last_pid return pid; } //否则，查询下一个非0的offset值 offset = find_next_offset(map, offset); 根据offset转换成相应的pid pid = mk_pid(pid_ns, map, offset); } while (offset < BITS_PER_PAGE && pid < pid_max); } //当上述pid分配失败，则再次查找offset if (map < &pid_ns->pidmap[(pid_max-1)/BITS_PER_PAGE]) { ++map; offset = 0; } else { map = &pid_ns->pidmap[0]; offset = RESERVED_PIDS; if (unlikely(last == offset)) break; } pid = mk_pid(pid_ns, map, offset); } return -1; }

pid允许分配的最大值为32767，当pid分配轮过一圈之后则允许分配的最小值为300，也就是说前300个pid是不可再分配的。