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

Question

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 24976
Thread 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.6
Thread 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.6
Thread 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?

Answer 1

How should I understand /proc/[pid]/stack ?
Taken from the man pages for proc:
There are additional helpful pseudo-paths:
[stack]
The initial process's (also known as the main thread's) stack.
Just below this, you can find:
[stack:[tid]] (since Linux 3.4)
A thread's stack (where the [tid] is a thread ID).
It corresponds to the /proc/[pid]/task/[tid]/path.
Which seems to be what you are looking for.

Answer 2

Nan Xiao is right.
Thread kernel mode stack is under /proc/[PID]/task/[TID]/stack.
you are checking /proc/[PID]/stack, that's the main thread stack so you have only 1. Others are under task folder.

Answer 3

That is for sleep locks. You might also look at perf -g to see spin locks including high system time.