[Xenomai] non-blocking rt_task_suspend(NULL)

Petr Cervenka grugh at centrum.cz
Wed Apr 16 18:02:05 CEST 2014

> Od: Gilles Chanteperdrix <gilles.chanteperdrix at xenomai.org>
> CC: "Xenomai" <xenomai at xenomai.org>
>On 04/16/2014 04:20 PM, Petr Cervenka wrote:
>>> Od: Gilles Chanteperdrix <gilles.chanteperdrix at xenomai.org>
>>> CC: "Xenomai" <xenomai at xenomai.org> On 04/16/2014 02:22 PM, Petr
>>> Cervenka wrote:
>>>>> Od: Gilles Chanteperdrix <gilles.chanteperdrix at xenomai.org>
>>>>> CC: "Xenomai" <xenomai at xenomai.org> On 04/15/2014 02:42 PM,
>>>>> Petr Cervenka wrote:
>>>>>> Hello I have a problem with the rt_task_suspend(NULL) call.
>>>>>> I'm using it for synchronization of two (producer / consumer
>>>>>> like) tasks. 1) When the consumer task has no work to do, it
>>>>>> stops itself by calling of the rt_task_suspend(NULL). 2) When
>>>>>> the producer creates new work for consumer, it wakes it up by
>>>>>> calling of rt_task_resume(&consumerTask). The problem is,
>>>>>> that consumer seldom switches to a state, that it sleeps by
>>>>>> rt_task_suspend no more. And the task then takes all the CPU
>>>>>> time. The return code is 0. But I already have seen couple of
>>>>>> -4 (-EINTR) values in the past also. Consumer task status was
>>>>>> 00300380 before and 00300184 (if there is small safety sleep
>>>>>> present). I can use for example RT_EVENT variable instead,
>>>>>> but I'm curious if you by chance don't know, what is
>>>>>> happening? Xenomai 2.6.3, Linux 3.5.7
>>>>> Could you post the example of code you are using to get this
>>>>> issue?
>>>> It's and application with many threads, mutexes and others. It's
>>>> also special measuring HW dependent. I can post here some
>>>> simplified example. But I don't think it would be possible to
>>>> reproduce the same behavior easily. It happens in my
>>>> configuration only probably once per day and very unpredictably.
>>>> But I have more details. I replaced rt_task_suspend /
>>>> rt_task_resume by rt_event_wait / rt_event_signal. It failed
>>>> similar way, but this time the result of wait was -4 (-EINTR).
>>>> And (after several millions of invocations) it recovered itself.
>>> -EINTR is a valid return value for both rt_event_wait and
>>> rt_task_suspend. In case you get this error, you should loop to
>>> call rt_event_wait again, and not call rt_event_clear, as you risk
>>> clearing an event which has been signaled afterwards.
>> You are right. It was just very quick replace of waiting and
>> waking-up functions. But I'm checking the "work queue" anyway and it
>> also doesn't need exact timing here. My problem it that the slow
>> consumer task seems to be "interrupted by signal" (or whatever) for
>> several minutes. I mean, that it doesn't wait for the event anymore
>> and it always returns immediately (with -EINTR return code).
>Are you running inside gdb? Does the task receive the SIGDEBUG signal? 
>Do you have the XNWARNSW bit armed?

gdb: No.
SIGDEBUG, XNWARNSW: I don't even know what it is ;-).

>> I also
>> already got one such situation half an hour ago. But the return code
>> was 0 that time. Could you give me some advice what to check when
>> such situation happens again?
>Well the task status should help.
Normaly the task status is (from /proc/xenomai/stat): 00300182.
(XNFPU | XNSHADOW| XNMAPPED | XNSTARTED | XNPEND - waiting for an event)
Task status from the last issue (from /proc/xenomai/stat) was 00300380.
CPU load of the task was 23% (and more than 4mil. MSW/CSW). Perhaps sending of UDP packets (used for debugging) caused some sleep and prevented the computer from total freeze.

After next issue I will have more precise information from rt_task_inquire.


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