Measuring CPU frequency with AT&T assembler

I'm trying to measure CPU frequency using AT&T assembler. My algorithm is:

t0 = rdtsc
sleep(1)    
t1 = rdtsc
f = t1-t0 [Hz]

(And I cannot do it using rdmsr as I'm not in ring 0)

Code:

.data
t0: .long 0
t1: .long 0     
.text                       
.globl  main           
.type   main, @function       
main:
    rdtsc
    movl %eax, t0
    mov $1, %eax
    push %eax
    call sleep
    rdtsc
    sub %eax, t0
    #printeax

    movl    $1, %eax  
    movl    $0, %ebx   
    int     $0x80
.size   main, . - main 

I wonder if this algorithm is a correct one. If so, what am I doing wrong because my results confuse me: they can vary e.g. from 4121650118 to 86456722 Hz.

lscpu | grep MHz returns:

CPU max MHz:           2300,0000
CPU min MHz:           800,0000

I'm also interested, if it is possible to get CPU frequency with AT&T the other way, like, using some syscall or something.