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

[Selmon Yang]

Hi, Bruce,

Thank you very much for your positive response.
The attachment is the patch.
Please have a look.

Hi, Jay,

Thank you so much for your kindly reminding.
However, due to I really like the alarm functionalities not to be affected.
I guess I need to replace CLOCK_MONOTONIC_RAW with CLOCK_MONOTONIC
and do more tests to see if it also works for me.


2015-06-03 21:07 GMT+08:00 Jay Rolette <rolette at infiniteio.com>:
>
> On Wed, Jun 3, 2015 at 7:54 AM, Bruce Richardson
> <bruce.richardson at intel.com> wrote:
>>
>> 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
>
>
> I haven't looked through the RTE alarm code, but one thing to consider is
> whether you want to use CLOCK_MONOTONIC_RAW or just CLOCK_MONOTONIC. The RAW
> version is ~10x slower than the CLOCK_MONOTONIC.
>
> CLOCK_MONOTONIC isn't completely protected from NTP frequency adjustments,
> but it won't have discontinuities.
>
> We've found the rte_eal_alarm calls to be surprisingly (ie., we hadn't
> bothered to look at the implementation yet) variable and intermittently
> slow, so the 10x difference on the call to clock_gettime() may be relevant.
>
> Jay