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

[Selmon Yang <wolkayang@gmail.com>]

Hi, Bruce, Jay,

I am sorry that the patch in last mail is the wrong one,
please replace it with the attached one in this mail.

2015-06-04 10:09 GMT+08:00 Selmon Yang <wolkayang@gmail.com>:
> 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@infiniteio.com>:
>>
>> On Wed, Jun 3, 2015 at 7:54 AM, Bruce Richardson
>> <bruce.richardson@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