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

Olivier Langlois
2013-04-29T17:16:34Z

On Mon, 2013-04-29 at 02:25 -0400, KOSAKI Motohiro wrote:
> (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.
>
>
Great!
>
> > @@ -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));
> > }
>
> --
I have tried to minimize rq locks contention to strict minimum. If to
remain POSIX compliant, it is required to also use CPUTIMER_NEED_DELTA,
so be it. I have no objections.