Re: [dpdk-dev] rte_eal_alarm_set() is affected by discontinuous jumps in the system time

[Bruce Richardson <bruce.richardson@intel.com>]

On Wed, Jun 03, 2015 at 02:31:47PM +0800, Selmon Yang wrote:
> Hi,
> 
> I found that, in dpdk 2.0, rte_eal_alarm_set() is affected by
> discontinuous jumps in the system time because eal_alarm_callback()
> and rte_eal_alarm_set() use gettimeofday() to get the current time.
> 
> Here is what I encountered.
> I set up a rte eal alarm as below, and I like it to be triggered every second.
>         #define USE_PER_S 1000 * 1000
>         void my_alarm_cb(void *arg)
>         {
>                 /* send heartbeat signal out, etc. */
> 
>                 rte_eal_alarm_set(1 * US_PER_S, my_alarm_cb, NULL);
>                 return;
>         }
> 
>         int main(void)
>         {
>                 /* ..., do something */
>                 rte_eal_alarm_set(1 * US_PER_S, my_alarm_cb, NULL);
>                 /* ... do something else */
>         }
> 
> It works fine in most of time.
> However, if I change system time manually, it is possible that rte alarm
> function works out of my expectation.
> Suppose that current time is 11:00:00 AM, and eal_alarm_callback()
> is triggered because I executed
> rte_eal_alarm_set(1 * US_PER_S, my_alarm_cb, NULL) at 10:59:59 AM.
> eal_alarm_callback() gets the current time (11:00:00 AM)
> and calls my_alarm_cb() as below.
>         while ((ap = LIST_FIRST(&alarm_list)) !=NULL &&
>                       gettimeofday(&now, NULL) == 0 &&
>                       (ap->time.tv_sec < now.tv_sec ||
> (ap->time.tv_sec == now.tv_sec &&
>                                               ap->time.tv_usec <=
> now.tv_usec))){
>                 ap->executing = 1;
>                 ap->executing_id = pthread_self();
>                 rte_spinlock_unlock(&alarm_list_lk);
> 
>                 ap->cb_fn(ap->cb_arg);
> 
>                 rte_spinlock_lock(&alarm_list_lk);
> 
>                 LIST_REMOVE(ap, next);
>                 rte_free(ap);
>         }
> 
> In my_alarm_cb(), rte_eal_alarm_set() is called again.
> rte_eall_alarm_set() gets the current time (11:00:00 AM), plus 1 second,
> and adds the new alarm entry to alarm_list.
>         /* use current time to calculate absolute time of alarm */
>         gettimeofday(&now, NULL);
> 
>         new_alarm->cb_fn = cb_fn;
>         new_alarm->cb_arg = cb_arg;
>         new_alarm->time.tv_usec = (now.tv_usec + us) % US_PER_S;
>         new_alarm->time.tv_sec = now.tv_sec + ((now.tv_usec + us) / US_PER_S);
> 
>         rte_spinlock_lock(&alarm_list_lk);
>         if (!handler_registered) {
>                 ret |= rte_intr_callback_register(&intr_handle,
>                                 eal_alarm_callback, NULL);
>                 handler_registered = (ret == 0) ? 1 : 0;
>         }
> 
>         if (LIST_EMPTY(&alarm_list))
>                 LIST_INSERT_HEAD(&alarm_list, new_alarm, next);
>         else {
>                 LIST_FOREACH(ap, &alarm_list, next) {
>                         if (ap->time.tv_sec > new_alarm->time.tv_sec ||
>                                         (ap->time.tv_sec ==
> new_alarm->time.tv_sec &&
> 
> ap->time.tv_usec > new_alarm->time.tv_usec)){
>                                 LIST_INSERT_BEFORE(ap, new_alarm, next);
>                                 break;
>                         }
>                         if (LIST_NEXT(ap, next) == NULL) {
>                                 LIST_INSERT_AFTER(ap, new_alarm, next);
>                                 break;
>                         }
>                 }
>         }
> 
> After the new alarm entry is added to alarm_list, if current time is
> set to 8:00:00 AM manually, the current time in eal_alarm_callback()
> will be updated to 8:00:00 AM, too.
> Then the new alarm entry will be triggered after 3 hours and 1 second.
> 
> I think rte alarm should not be affected by discontinuous jumps in
> the system time.
> I tried to replace gettimeofday() with clock_gettime(CLOCK_MONOTONIC_RAW, &now),
> and it looks work fine.
> What do you think about this modification?
> Will you consider to modify rte_alarm functions to be not affected
> by discontinuous jumps in the system time?

I agree with you that the alarm functionality should not be affected by jumps
in system time. If you have a patch that fixes this bug, it would be great if
you could upstream it here.

Thanks,
/Bruce