How to interpret the report of perf

I'm learning how to use the tool perf to profile my c++ project. Here is my code:

#include <iostream>
#include <thread>
#include <mutex>
#include <vector>


std::mutex mtx;
long long_val = 0;

void do_something(long &val)
{
    std::unique_lock<std::mutex> lck(mtx);
    for(int j=0; j<1000; ++j)
        val++;
}


void thread_func()
{
    for(int i=0; i<1000000L; ++i)
    {
        do_something(long_val);
    }
}


int main(int argc, char* argv[])
{
    std::vector<std::unique_ptr<std::thread>> threads;
    for(int i=0; i<100; ++i)
    {
        threads.push_back(std::move(std::unique_ptr<std::thread>(new std::thread(thread_func))));
    }
    for(int i=0; i<100; ++i)
    {
        threads[i]->join();
    }
    threads.clear();
    std::cout << long_val << std::endl;
    return 0;
}

To compile it, I run g++ -std=c++11 main.cpp -lpthread -g and then I get the executable file named a.out.

Then I run perf record --call-graph dwarf -- ./a.out and wait for 10 seconds, then I press Ctrl+c to interrupt the ./a.out because it needs too much time to execute.

Lastly, I run perf report -g graph --no-children and here is the output:

My goal is to find which part of the code is the heaviest. So it seems that this output could tell me do_something is the heaviest part(46.25%). But when I enter into do_something, I can not understand what it is: std::_Bind_simple, std::thread::_Impl etc.

So how to get more useful information from the output of perf report? Or we can't get more except the fact that do_something is the heaviest?