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Subjectprocess time < thread time?

If someone who understands our thread/process time implementation can
look into this, I'd appreciate it.

Attached below is a watered-down version of rt/tst-cpuclock2.c from
GLIBC. Just build it with "gcc -o test test.c -lpthread -lrt" or

Run it several times, and you will see cases where the main thread
will measure a process clock difference before and after the nanosleep
which is smaller than the cpu-burner thread's individual thread clock
difference. This doesn't make any sense since the cpu-burner thread
is part of the top-level process's thread group.

I've reproduced this on both x86-64 and sparc64 (using both 32-bit and
64-bit binaries).

For example:

[davem@boricha build-x86_64-linux]$ ./test
process: before(0.001221967) after(0.498624371) diff(497402404)
thread: before(0.000081692) after(0.498316431) diff(498234739)
self: before(0.001223521) after(0.001240219) diff(16698)
[davem@boricha build-x86_64-linux]$

The diff of 'process' should always be >= the diff of 'thread'.

I make sure to wrap the 'thread' clock measurements the most tightly
around the nanosleep() call, and that the 'process' clock measurements
are the outer-most ones.

I suspect this might be some kind of artifact of how the partial
runqueue ->clock and ->clock_task updates work? Maybe some weird
interaction with ->skip_clock_update?

Or is this some known issue?


#include <unistd.h>
#include <stdio.h>
#include <stdlib.h>
#include <time.h>
#include <fcntl.h>
#include <string.h>
#include <errno.h>
#include <pthread.h>

static pthread_barrier_t barrier;

static void *chew_cpu(void *arg)
while (1)
__asm__ __volatile__("" : : : "memory");
return NULL;

int main(void)
clockid_t process_clock, my_thread_clock, th_clock;
struct timespec process_before, process_after;
struct timespec me_before, me_after;
struct timespec th_before, th_after;
struct timespec sleeptime;
unsigned long diff;
pthread_t th;
int err;

err = clock_getcpuclockid(0, &process_clock);
if (err)
return 1;

err = pthread_getcpuclockid(pthread_self(), &my_thread_clock);
if (err)
return 1;

pthread_barrier_init(&barrier, NULL, 2);
err = pthread_create(&th, NULL, chew_cpu, NULL);
if (err)
return 1;

err = pthread_getcpuclockid(th, &th_clock);
if (err)
return 1;


err = clock_gettime(process_clock, &process_before);
if (err)
return 1;

err = clock_gettime(my_thread_clock, &me_before);
if (err)
return 1;

err = clock_gettime(th_clock, &th_before);
if (err)
return 1;

sleeptime.tv_sec = 0;
sleeptime.tv_nsec = 500000000;
nanosleep(&sleeptime, NULL);

err = clock_gettime(th_clock, &th_after);
if (err)
return 1;

err = clock_gettime(my_thread_clock, &me_after);
if (err)
return 1;

err = clock_gettime(process_clock, &process_after);
if (err)
return 1;

diff = process_after.tv_nsec - process_before.tv_nsec;
printf("process: before(%lu.%.9lu) after(%lu.%.9lu) diff(%lu)\n",
process_before.tv_sec, process_before.tv_nsec,
process_after.tv_sec, process_after.tv_nsec, diff);
diff = th_after.tv_nsec - th_before.tv_nsec;
printf("thread: before(%lu.%.9lu) after(%lu.%.9lu) diff(%lu)\n",
th_before.tv_sec, th_before.tv_nsec,
th_after.tv_sec, th_after.tv_nsec, diff);
diff = me_after.tv_nsec - me_before.tv_nsec;
printf("self: before(%lu.%.9lu) after(%lu.%.9lu) diff(%lu)\n",
me_before.tv_sec, me_before.tv_nsec,
me_after.tv_sec, me_after.tv_nsec, diff);

return 0;

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