How to understand "/proc/[pid]/stack"?

According to proc manual:

/proc/[pid]/stack (since Linux 2.6.29)

This file provides a symbolic trace of the function calls in this process's kernel stack. This file is provided only if the kernel was built with the CONFIG_STACKTRACE configuration option.

So I write a program to test:

#include <stdio.h>#include <sys/wait.h>#include <unistd.h>#include <pthread.h>void *thread_func(void *p_arg){        pid_t pid = fork();        if (pid > 0) {            wait(NULL);            return 0;        } else if (pid == 0) {            sleep(1000);            return 0;        }        return NULL;}int main(void){        pthread_t t1, t2;        pthread_create(&t1, NULL, thread_func, "Thread 1");        pthread_create(&t2, NULL, thread_func, "Thread 2");        sleep(1000);        return 0;}

After running, use pstack to check the threads of progress:

linux-uibj:~ # pstack 24976Thread 3 (Thread 0x7fd6e4ed5700 (LWP 24977)):#0  0x00007fd6e528d3f4 in wait () from /lib64/libpthread.so.0#1  0x0000000000400744 in thread_func ()#2  0x00007fd6e52860a4 in start_thread () from /lib64/libpthread.so.0#3  0x00007fd6e4fbb7fd in clone () from /lib64/libc.so.6Thread 2 (Thread 0x7fd6e46d4700 (LWP 24978)):#0  0x00007fd6e528d3f4 in wait () from /lib64/libpthread.so.0#1  0x0000000000400744 in thread_func ()#2  0x00007fd6e52860a4 in start_thread () from /lib64/libpthread.so.0#3  0x00007fd6e4fbb7fd in clone () from /lib64/libc.so.6Thread 1 (Thread 0x7fd6e569f700 (LWP 24976)):#0  0x00007fd6e4f8d6cd in nanosleep () from /lib64/libc.so.6#1  0x00007fd6e4f8d564 in sleep () from /lib64/libc.so.6#2  0x00000000004007b1 in main ()

At the same time, check /proc/24976/stack:

linux-uibj:~ # cat /proc/24976/stack[<ffffffff804ba1a7>] system_call_fastpath+0x16/0x1b[<00007fd6e4f8d6cd>] 0x7fd6e4f8d6cd[<ffffffffffffffff>] 0xffffffffffffffff

The 24976 process has 3 threads, and they all block on system call(nanosleep and wait), so all 3 threads now work in kernel space, and turn into kernel threads now, right? If this is true, there should be 3 stacks in /proc/[pid]/stack file. But it seems there is only 1 stack in /proc/[pid]/stack file.

How should I understand /proc/[pid]/stack?