Linux select 机制深入分析

Linux select 机制深入分析

     

     作为IO复用的实现方式。select是提高了抽象和batch处理的级别，不是传统方式那样堵塞在真正IO读写的系统调用上。而是堵塞在select系统调用上，等待我们关注的描写叙述符就绪。当然如今更好的方式是epoll，比方Java中的NIO底层就是用的epoll。这篇文章仅仅是为了搞懂select机制的原理。不看源代码就不能说懂这些IO复用手法。也在面试过程中体会到了，不去实践就会发现知道的永远是皮毛。面试问题：select的最大描写叙述符限制能够改动吗？（有待深入）

用户层API语法：

 /* According to POSIX.1-2001 */
       #include <sys/select.h>

       /* According to earlier standards */
       #include <sys/time.h>
       #include <sys/types.h>
       #include <unistd.h>

       int select(int nfds, fd_set *readfds, fd_set *writefds,
                  fd_set *exceptfds, struct timeval *timeout);

       void FD_CLR(int fd, fd_set *set);
       int  FD_ISSET(int fd, fd_set *set);
       void FD_SET(int fd, fd_set *set);
       void FD_ZERO(fd_set *set);

       #include <sys/select.h>
      int pselect(int nfds, fd_set *readfds, fd_set *writefds,
                   fd_set *exceptfds, const struct timespec *timeout,
                   const sigset_t *sigmask);

注：这里的API发生了变化（參见UNPv1 P127），timeout值是同意更新的，这在内核中有体现。

select系统调用的内核源代码主要流程是：sys_select() -> core_sys_select() -> do_select() -> poll_select_copy_remaining。

可代码能够一目了然。

/*
* SYSCALL_DEFINE5宏的作用就是将其转成系统调用的常见形式,
* asmlinkage long sys_select(int n, fd_set __user *inp, fd_set __user *outp,fd_set __user *exp, struct timeval __user *tvp);
*/
SYSCALL_DEFINE5(select, int, n, fd_set __user *, inp, fd_set __user *, outp,
          fd_set __user *, exp, struct timeval __user *, tvp)
{
     struct timespec end_time, *to = NULL;
     struct timeval tv;
     int ret;

     if (tvp) {//假设设置了超时阈值
          if (copy_from_user(&tv, tvp, sizeof(tv)))
               return -EFAULT;

          to = &end_time;
          // 从timeval(秒 微秒)转换为(秒 纳秒)  继而建立超时
          if (poll_select_set_timeout(to,
                    tv.tv_sec + (tv.tv_usec / USEC_PER_SEC),
                    (tv.tv_usec % USEC_PER_SEC) * NSEC_PER_USEC))
               return -EINVAL;
     }
     // 核心工作
     ret = core_sys_select(n, inp, outp, exp, to);
     //core_sys_select处理的fd_set 接下来更新timeout的值
     ret = poll_select_copy_remaining(&end_time, tvp, 1, ret);

     return ret;
}

/*
* We can actually return ERESTARTSYS instead of EINTR, but I'd
* like to be certain this leads to no problems. So I return
* EINTR just for safety.
*
* Update: ERESTARTSYS breaks at least the xview clock binary, so
* I'm trying ERESTARTNOHAND which restart only when you want to.
*/
int core_sys_select(int n, fd_set __user *inp, fd_set __user *outp,
                  fd_set __user *exp, struct timespec *end_time)
{
     // poll.h :fd_set_bits包装了6个long *，代表三个描写叙述表集的值-结果
     fd_set_bits fds;
     void *bits;
     int ret, max_fds;
     unsigned int size;
     struct fdtable *fdt;
     /* Allocate small arguments on the stack to save memory and be faster
     * 先是预分配256B的空间  大多数情况下可以满足须要 特殊情况在以下会分配空间
     */
     long stack_fds[SELECT_STACK_ALLOC/sizeof(long)];

     ret = -EINVAL;
     if (n < 0)
          goto out_nofds;

     /* max_fds can increase, so grab it once to avoid race */
     rcu_read_lock();
     // 获得打开文件描写叙述符表(指针析取)
     fdt = files_fdtable(current->files);
     max_fds = fdt->max_fds;
     rcu_read_unlock();
     if (n > max_fds)
          n = max_fds;//參数修正

     /*
     * 如今要监视的描写叙述符个数个size*8个对于每个都须要6个位来标示
     * 它是否可以读写异常而且把结果写在res_in res_out res_exp中
     * 所以构成了以下的内存布局(见图1)
     */
     size = FDS_BYTES(n);
     bits = stack_fds;
     if (size > sizeof(stack_fds) / 6) {
          /* Not enough space in on-stack array; must use kmalloc */
          ret = -ENOMEM;
          bits = kmalloc(6 * size, GFP_KERNEL);
          if (!bits)
               goto out_nofds;
     }
     fds.in      = bits;
     fds.out     = bits +   size;
     fds.ex      = bits + 2*size;
     fds.res_in  = bits + 3*size;
     fds.res_out = bits + 4*size;
     fds.res_ex  = bits + 5*size;

     // 从用户空间得到这些fd sets
     if ((ret = get_fd_set(n, inp, fds.in)) ||
         (ret = get_fd_set(n, outp, fds.out)) ||
         (ret = get_fd_set(n, exp, fds.ex)))
          goto out;
     // 初始化这些结果參数为0
     zero_fd_set(n, fds.res_in);
     zero_fd_set(n, fds.res_out);
     zero_fd_set(n, fds.res_ex);

     // 到这里 一切准备工作都就绪了.....
     ret = do_select(n, &fds, end_time);

     if (ret < 0)
          goto out;
     if (!ret) {
          ret = -ERESTARTNOHAND;
          if (signal_pending(current))
               goto out;
          ret = 0;
     }
     // do_select正确返回后 通过copy_to_user将fds中的描写叙述符就绪结果參数
     // 反馈到用户空间
     if (set_fd_set(n, inp, fds.res_in) ||
         set_fd_set(n, outp, fds.res_out) ||
         set_fd_set(n, exp, fds.res_ex))
          ret = -EFAULT;

out:
     if (bits != stack_fds)
          kfree(bits);
out_nofds:
     return ret;
}

// select 的核心工作
int do_select(int n, fd_set_bits *fds, struct timespec *end_time)
{
     ktime_t expire, *to = NULL;
     struct poll_wqueues table;
     poll_table *wait;
     int retval, i, timed_out = 0;
     unsigned long slack = 0;
     unsigned int busy_flag = net_busy_loop_on() ? POLL_BUSY_LOOP : 0;
     unsigned long busy_end = 0;

     // 得到Select要监測的最大的描写叙述符值
     rcu_read_lock();
     retval = max_select_fd(n, fds);
     rcu_read_unlock();

     if (retval < 0)
          return retval;
     n = retval;

     poll_initwait(&table);
     wait = &table.pt;
     // 定时器值(秒 纳秒)为0的话标示不等待
     if (end_time && !end_time->tv_sec && !end_time->tv_nsec) {
          wait->_qproc = NULL;
          timed_out = 1;
     }

     if (end_time && !timed_out)
          slack = select_estimate_accuracy(end_time);

     // 以下会用到这个变量统计就绪的描写叙述符个数 所以先清0
     retval = 0;
     for (;;) {
          unsigned long *rinp, *routp, *rexp, *inp, *outp, *exp;
          bool can_busy_loop = false;

          inp = fds->in; outp = fds->out; exp = fds->ex;
          rinp = fds->res_in; routp = fds->res_out; rexp = fds->res_ex;

          for (i = 0; i < n; ++rinp, ++routp, ++rexp) {
               unsigned long in, out, ex, all_bits, bit = 1, mask, j;
               unsigned long res_in = 0, res_out = 0, res_ex = 0;

               in = *inp++; out = *outp++; ex = *exp++;
               all_bits = in | out | ex;
               // 要一次轮询这些这些位图 定位到某个有我们关心的fd的区间
               // 否则以32bits步长前进
               if (all_bits == 0) {
                    i += BITS_PER_LONG;
                    continue;
               }
               // 当前这个区间有我们关心的fd 所以深入细节追踪（图2）
               for (j = 0; j < BITS_PER_LONG; ++j, ++i, bit <<= 1) {
                    struct fd f;
                    if (i >= n)
                         break;
                    if (!(bit & all_bits))
                         continue;
                    // 假设发现了当前区间的某一个bit为1 则说明相应的fd须要我们处理
                    // 此时此刻的i正是文件描写叙述符值
                    f = fdget(i);
                    if (f.file) {
                         const struct file_operations *f_op;
                         f_op = f.file->f_op;
                         mask = DEFAULT_POLLMASK;
                         //详细到文件操作结果中的poll函数指针  对于
                         if (f_op->poll) {
                              wait_key_set(wait, in, out,
                                        bit, busy_flag);
                              mask = (*f_op->poll)(f.file, wait);// TODO
                         }
                         // 上面的fdget添加了file引用计数 所以这里恢复
                         fdput(f);
                         /* 推断关注的描写叙述符是否就绪 就绪的话就更新到结果參数中
                         * 而且添加就绪个数
                         */
                         if ((mask & POLLIN_SET) && (in & bit)) {
                              res_in |= bit;
                              retval++;
                              wait->_qproc = NULL;
                         }
                         if ((mask & POLLOUT_SET) && (out & bit)) {
                              res_out |= bit;
                              retval++;
                              wait->_qproc = NULL;
                         }
                         if ((mask & POLLEX_SET) && (ex & bit)) {
                              res_ex |= bit;
                              retval++;
                              wait->_qproc = NULL;
                         }
                         /* got something, stop busy polling
                         * 停止忙循环
                         */
                         if (retval) {
                              can_busy_loop = false;
                              busy_flag = 0;

                         /*
                         * only remember a returned
                         * POLL_BUSY_LOOP if we asked for it
                         */
                         } else if (busy_flag & mask)
                              can_busy_loop = true;

                    }
               }
               // 这一轮的区间遍历完之后 更新结果參数
               if (res_in)
                    *rinp = res_in;
               if (res_out)
                    *routp = res_out;
               if (res_ex)
                    *rexp = res_ex;
               /* 进行一次调度 同意其它进程执行
               * 后面有等待队列唤醒
               */
               cond_resched();
          }
          // 一轮轮询之后
          wait->_qproc = NULL;
          // 假设有描写叙述符就绪 或者设置了超时 或者有待处理信号 则退出这个死循环
          if (retval || timed_out || signal_pending(current))
               break;
          if (table.error) {
               retval = table.error;
               break;
          }

          /* only if found POLL_BUSY_LOOP sockets && not out of time */
          if (can_busy_loop && !need_resched()) {
               if (!busy_end) {
                    busy_end = busy_loop_end_time();
                    continue;
               }
               if (!busy_loop_timeout(busy_end))
                    continue;
          }
          busy_flag = 0;

          /* 假设设置超时 而且这是首次循环(to==NULL) */
          if (end_time && !to) {
               // 从timespec转化为ktime类型(64位的有符号值)
               expire = timespec_to_ktime(*end_time);
               to = &expire;
          }
          /*设置该进程状态TASK_INTERRUPTIBLE 睡眠直到超时
          * 返回到这里后进程 TASK_RUNNING
          */
          if (!poll_schedule_timeout(&table, TASK_INTERRUPTIBLE, to, slack))
               timed_out = 1;
     }

     // 释放该poll wait queue
     poll_freewait(&table);

     return retval;
}

附图1：

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附图2：

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參考：

（1）Linux kernel 3.18 source code 

（2）Linux man page

（3）UNPv1

耗时：3h