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

[Selmon Yang]

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?