Re: SCHED_SPORADIC in Xenomai 3

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.

>
> 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.

-- 
Philippe.