From: Selmon Yang <wolkayang@gmail.com>
To: dev@dpdk.org
Subject: [dpdk-dev] rte_eal_alarm_set() is affected by discontinuous jumps in the system time
Date: Wed, 3 Jun 2015 14:31:47 +0800 [thread overview]
Message-ID: <CA+bO-48xnexvauECRs=OCzJ1w6MtDThbinsPDceA+Yr4-zjT=w@mail.gmail.com> (raw)
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?
next reply other threads:[~2015-06-03 6:31 UTC|newest]
Thread overview: 7+ messages / expand[flat|nested] mbox.gz Atom feed top
2015-06-03 6:31 Selmon Yang [this message]
2015-06-03 12:54 ` Bruce Richardson
2015-06-03 13:07 ` Jay Rolette
2015-06-04 2:09 ` Selmon Yang
2015-06-04 2:29 ` Selmon Yang
2015-06-04 9:39 ` Bruce Richardson
2015-06-05 2:48 ` Selmon Yang
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