How to detect which codes of c++ cause mode switch

Question

I'm trying to figure out which codes of c++ can cause the mode switch (user mode to kernel mode) on Linux. And I've known two things:

1. A system call cause the mode switch: Is mode switch occur switching from user thread to kernel thread?
2. The command strace can list all of the system calls.

Now I write such a code as below to do a test:

#include <iostream>
#include <chrono>
#include <thread>

int main()
{
    int i = 0;
    while (true) {
        std::this_thread::sleep_for(std::chrono::milliseconds(1000));
        i++;
    }
    return 0;
}

I compile it with the command g++ -std=c++11 test.cpp -lpthread and then I run strace -ttT ./a.out to list its system calls. Here is the output:

22:08:52.424382 prlimit64(0, RLIMIT_STACK, NULL, {rlim_cur=8192*1024, rlim_max=RLIM64_INFINITY}) = 0 <0.000009>
22:08:52.424611 brk(NULL)               = 0x55ca3968b000 <0.000009>
22:08:52.424725 brk(0x55ca396ac000)     = 0x55ca396ac000 <0.000011>
22:08:52.424887 futex(0x7f0aad83909c, FUTEX_WAKE_PRIVATE, 2147483647) = 0 <0.000010>
22:08:52.425070 futex(0x7f0aad8390a8, FUTEX_WAKE_PRIVATE, 2147483647) = 0 <0.000010>
22:08:52.425246 nanosleep({tv_sec=1, tv_nsec=0}, 0x7ffcae110cb0) = 0 <1.000142>
22:08:53.425553 nanosleep({tv_sec=1, tv_nsec=0}, 0x7ffcae110cb0) = 0 <1.000253>
22:08:54.425924 nanosleep({tv_sec=1, tv_nsec=0}, 0x7ffcae110cb0) = 0 <1.000138>
22:08:55.426177 nanosleep({tv_sec=1, tv_nsec=0}, 0x7ffcae110cb0) = 0 <1.000145>
22:08:56.426455 nanosleep({tv_sec=1, tv_nsec=0}, 0x7ffcae110cb0) = 0 <1.000141>
22:08:57.426710 nanosleep({tv_sec=1, tv_nsec=0}, 0x7ffcae110cb0) = 0 <1.000146>
22:08:58.426967 nanosleep({tv_sec=1, tv_nsec=0}, 0x7ffcae110cb0) = 0 <1.000146>

Now I have some questions:

1. Why is there futex? As far as I know, futex is the lower mechanism to implement to thread mutex in c++11, but I haven't used any mutex here so why is there futex?
2. We can see that there are infinite nanosleep, so does this mean that the c++11 code std::this_thread::sleep_for(std::chrono::milliseconds(1000)); will cause the mode switch from user mode to kernel mode?
3. Is there a better way to check which c++ codes/functions will cause the mode switch or I have to use the command strace?