Bug 709170

Description of problem:
clock_gettime( CLOCK_PROCESS_CPUTIME_ID, & ts ) sometimes skips 0.2 ms, as if it was wall clock time rather than CPU time.

Version-Release number of selected component (if applicable):
glibc-2.13-1

How reproducible:
Always

Steps to Reproduce:
1. Run this with: cc test.c -lrt && ./a.out

#include <time.h>
#include <stdio.h>

double get_cpu_time( void ) {
  int ret;
  struct timespec ts;

  ret = clock_gettime( CLOCK_PROCESS_CPUTIME_ID, & ts );

  return ret ? 0 : ts.tv_nsec * 1e-9 + ts.tv_sec;
} /* end function get_cpu_time */

int main() {
  unsigned long count, lim;
  double delta, mi, ma, t1, t2, total;

  count = 0; lim = 1000;
  mi = 1e99; ma	= -1e99; total = 0;
  for ( ; ; ) {
    t1 = get_cpu_time();
    do { t2 = get_cpu_time(); } while ( t2 == t1 );
    delta = t2 - t1;
    ++ count;
    total += delta;
    if ( delta < mi ) mi = delta;
    if ( delta > ma ) ma = delta;
    if ( count >= lim ) {
      printf( "After %lu iters, min = %g ns, max = %g ns, mean = %g ns\n", count, mi * 1e9, ma * 1e9, total * 1e9 / count );
      lim *= 2;
    } /* end if */
  } /* end for */

  return 0;
} /* end function main */


Actual results:
After 1000 iters, min = 185 ns, max = 1772 ns, mean = 471.542 ns
After 2000 iters, min = 1 ns, max = 1772 ns, mean = 471.359 ns
After 4000 iters, min = 1 ns, max = 3061 ns, mean = 471.933 ns
After 8000 iters, min = 1 ns, max = 3061 ns, mean = 471.637 ns
After 16000 iters, min = 1 ns, max = 3061 ns, mean = 471.296 ns
After 32000 iters, min = 1 ns, max = 3061 ns, mean = 470.765 ns
After 64000 iters, min = 1 ns, max = 208869 ns, mean = 473.554 ns
After 128000 iters, min = 1 ns, max = 208869 ns, mean = 472.718 ns
[...]


Expected results:
Max delay in same ballpark as mean.

Additional info: Same with taskset -c 0 ./a.out