转自：http://gityuan.com/2017/08/06/linux_process_pid/
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.
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]
...
}
[-> 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;
...
}
[-> 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
};
[-> pid.h]
struct upid
{
int nr;
struct pid_namespace *ns;
struct hlist_node pid_chain;
};
[-> 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()
};
[-> 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。
[-> 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 <= max_scan; ++i) {
if (unlikely(!map->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。
相关常量如下：
#define PAGE_SHIFT 12
#define PAGE_SIZE (1UL << PAGE_SHIFT) // 2^12
#define BITS_PER_PAGE (PAGE_SIZE * 8) // 2^15
#define BITS_PER_PAGE_MASK (BITS_PER_PAGE-1) //2^15-1
#define PAGE_MASK (~(PAGE_SIZE-1))
[-> kernel/include/linux/pid_namespace.h]
struct pidmap {
atomic_t nr_free; //可用pid的个数
void *page; //用于存放位图
};
pidmap->page的大小为4KB，每一个bit位代表一个进程pid的分配情况，那么4KB*8=32768， 这正好是pid可分配的上限，用nr_free代表该namespace下还有多少可用pid。
[-> pid.c]
#define find_next_offset(map, off) \ find_next_zero_bit((map)->page, BITS_PER_PAGE, off)
static inline int mk_pid(struct pid_namespace *pid_ns, struct pidmap *map, int off) {
return (map - pid_ns->pidmap)*BITS_PER_PAGE + off;
}
[-> kernel/include/linux/pid.h]
static inline pid_t pid_nr(struct pid *pid)
{
pid_t nr = 0;
if (pid)
nr = pid->numbers[0].nr;
return nr;
}
根据pid结构体找到真正的pid数值。
cat /proc/sys/kernel/pid_max，Android系统一般默认为32768。微信公众号 Gityuan | 微博 weibo.com/gityuan | 博客 留言区交流
原文：https://www.cnblogs.com/sky-heaven/p/13676322.html