Re: SCHED_SPORADIC in Xenomai 3

On 11.06.26 09:33, Philippe Gerum wrote:
> 
> Hi Jan,
> 
> Jan Kiszka <[email protected]> writes:
> 
>> Hi Philippe,
>>
>> while trying to port the signal-while-suspended fix to Xenomai 3, I ran
>> into XNHELD, a state only existing there. I suppose that was once
>> forward-ported as EVL_T_HALT. The only user of XNHELD in Xenomai 3 is
>> SCHED_SPORADIC - so let's dive into that scheduling class.
>>
>> Turned out it was never documented, not even linked to the POSIX
>> standard. But it also slightly differs from it (low_prio = -1 -> suspend
>> on depletion). There is also no test case, so I asked an AI for one.
>> That worked fairly well as it seems to have revealed an issue:
>>
>> Could it be that we are not properly suspending the budget tracking when
>> a higher-prio task from a different scheduling class is preempting a
>> sporadic thread? It looks like that xnsched_sporadic_pick is not invoked
>> if a thread is selected from a higher-prio class first, namely sched-rt
>> with its weight 4 vs. 3 if sched-sporadic. Or is that an (undocumented)
>> limitation/misconfiguration? Is that issue even affecting other
>> time-slicing classes as well??
> 
> Yes, this is a systemic bug. We need to tell classes that some thread of
> theirs is scheduling out.
> 

OK, then please have a look of my proposal from today. If it makes
sense, I'm happy to submit it for x3 as well as linux-evl patch.

>>
>> That furthermore makes me wonder if we actually have users of
>> sched-sporadic. Likely a hard to answer question, as usual. But such a
>> limitation should have been observed earlier under real workload...
>>
> 
> I only know of one user, back in 2009. Never received any feedback since
> then.
> 

Hmm, I'm inclined to retire it then.
@all: If there are users today, please speak up now!

Jan

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