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

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Date:	Mon, 29 Apr 2013 02:25:34 -0400
From:	KOSAKI Motohiro <kosaki.motohiro@...il.com>
To:	Olivier Langlois <olivier@...llion01.com>
CC:	Peter Zijlstra <peterz@...radead.org>,
	Ingo Molnar <mingo@...hat.com>,
	Thomas Gleixner <tglx@...utronix.de>, schwidefsky@...ibm.com,
	Steven Rostedt <rostedt@...dmis.org>,
	Frederic Weisbecker <fweisbec@...il.com>,
	KOSAKI Motohiro <kosaki.motohiro@...il.com>,
	LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v2 3/3] process cputimer is moving faster than its corresponding
 clock

(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));
>                 }

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