SCHED_SPORADIC in Xenomai 3

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??

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...

Jan

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