Re: [PATCH v2 3/3] process cputimer is moving faster than its corresponding clock

KOSAKI Motohiro
2013-04-29T06:25:38Z

(4/27/13 12:41 AM), Olivier Langlois wrote:
>
>
> Add thread group delta to cpu timer sample when computing a timer expiration.
>
> This is mandatory to make sure that the posix cpu timer does not fire too
> soon relative to the process cpu clock which do include the task group delta.
>
> test case to validate the patch is glibc-2.17/rt/tst-cputimer1.c

First, I could reproduce this issue. thanks. Second, actually, this issue is not
cause by race. This just occur by timer initialization mistake. I'll show you
the smallest fix.

> @@ -697,7 +755,8 @@ static int posix_cpu_timer_set(struct k_itimer *timer, int flags,
> if (CPUCLOCK_PERTHREAD(timer->it_clock)) {
> cpu_clock_sample(timer->it_clock, p, &val);
> } else {
> - cpu_timer_sample_group(timer->it_clock, p, &val);
> + cpu_timer_sample_group(timer->it_clock, p, &val,
> + CPUTIMER_NEED_DELTA);

POSIX says,

http://pubs.opengroup.org/onlinepubs/7908799/xsh/timer_gettime.html
> If the argument ovalue is not NULL, the function timer_settime() stores,
> in the location referenced by ovalue, a value representing the previous
> amount of time before the timer would have expired or zero if the timer
> was disarmed, together with the previous timer reload value. The members
> of ovalue are subject to the resolution of the timer, and they are the
> same values that would be returned by a timer_gettime() call at that point in time.
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

but your posix_cpu_timer_set() and posix_cpu_timer_get() are not consistent. I'm worry
about this.

> }
>
> if (old) {
> @@ -845,7 +904,8 @@ static void posix_cpu_timer_get(struct k_itimer *timer, struct itimerspec *itp)
> read_unlock(&tasklist_lock);
> goto dead;
> } else {
> - cpu_timer_sample_group(timer->it_clock, p, &now);
> + cpu_timer_sample_group(timer->it_clock, p, &now,
> + CPUTIMER_NO_DELTA);
>
> clear_dead = (unlikely(p->exit_state) &&
> thread_group_empty(p));
> }