lost some call trace for sleep function

[yuzhoujian]

Hi, all.
I find a strange problem. Perf cannot record call stack which contains sleep 
functions.
The last function of the call trace is always "__GI___libc_nanosleep" for each 
sample.
one of the sample for perf script is below:

test_sleep 12275 185233.961287:          1 cycles:ppp: 
        ffffffff8100add0 intel_bts_enable_local ([kernel.kallsyms])
        ffffffff81008f20 intel_pmu_enable_all ([kernel.kallsyms])
        ffffffff810057ec x86_pmu_enable ([kernel.kallsyms])
        ffffffff81173e57 perf_pmu_enable ([kernel.kallsyms])
        ffffffff81175404 __perf_event_task_sched_in ([kernel.kallsyms])
        ffffffff810c1aa8 finish_task_switch ([kernel.kallsyms])
        ffffffff81690e00 __schedule ([kernel.kallsyms])
        ffffffff81691409 schedule ([kernel.kallsyms])
        ffffffff816902d6 do_nanosleep ([kernel.kallsyms])
        ffffffff810b747b hrtimer_nanosleep ([kernel.kallsyms])
        ffffffff810b75be sys_nanosleep ([kernel.kallsyms])
        ffffffff8169c749 system_call_fastpath ([kernel.kallsyms])
                   bf190 __GI___libc_nanosleep (/usr/lib64/libc-2.17.so)

Below is the source code of test_sleep:

void f2()
{
        sleep(1);
}
void f1()
{
        f2();
}
int main()
{
        while(1)
                f1();
        return 0;
}

I think the right call stack should contain the __sleep function in glibc, just 
as follow

test_sleep 12275 185233.961287:          1 cycles:ppp: 
        ffffffff8100add0 intel_bts_enable_local ([kernel.kallsyms])
        ffffffff81008f20 intel_pmu_enable_all ([kernel.kallsyms])
        ffffffff810057ec x86_pmu_enable ([kernel.kallsyms])
        ffffffff81173e57 perf_pmu_enable ([kernel.kallsyms])
        ffffffff81175404 __perf_event_task_sched_in ([kernel.kallsyms])
        ffffffff810c1aa8 finish_task_switch ([kernel.kallsyms])
        ffffffff81690e00 __schedule ([kernel.kallsyms])
        ffffffff81691409 schedule ([kernel.kallsyms])
        ffffffff816902d6 do_nanosleep ([kernel.kallsyms])
        ffffffff810b747b hrtimer_nanosleep ([kernel.kallsyms])
        ffffffff810b75be sys_nanosleep ([kernel.kallsyms])
        ffffffff8169c749 system_call_fastpath ([kernel.kallsyms])
                   bf190 __GI___libc_nanosleep (/usr/lib64/libc-2.17.so)
                   bef70 __sleep (/usr/lib64/libc-2.17.so)
                   5a1 f2 (/home/test_sleep)
                   5c1 f1 (/home/test_sleep)
                   5d1 main (/home/test_sleep)
                   21c05 __libc_start_main (/usr/lib64/libc-2.17.so)

Is it a bug for perf record ??