Marco Barletta barlettamarco8 at gmail.com
Sat May 22 00:04:57 CEST 2021

I want to thank you for time and patience as well!

Il giorno ven 21 mag 2021 alle ore 17:13 Jan Kiszka <jan.kiszka at siemens.com>
ha scritto:

> On 21.05.21 16:41, Philippe Gerum via Xenomai wrote:
> >
> > Marco Barletta via Xenomai <xenomai at xenomai.org> writes:
> >
> >> Hi everyone, related to the sched_quota patch I posted about one week
> ago.
> >> I'm trying to understand why xnsched_kick is needed, in which situation
> is
> >> called. Reading the code it seems it's called when there's a xeno thread
> >> that must be downgraded and scheduled in Linux, but I didn't understand
> why
> >> there's the need to run to complention without respecting group budget.
> Can
> >> you help me?
> >> Best regards.
> >
> > When a thread is forcibly kicked out of oob context by the core, this
> > means that it ought to move quickly to in-band context in order to
> > respond to a pending kernel event, such as handling a signal. e.g. we
> > use xnthread_kick() to force a thread which is being sent a SIGTRAP
> > signal by a debugger, to receive that signal - otherwise bad things may
> > happen kernel-wise.
> >
> > With that in mind, what would happen if a SCHED_QUOTA thread is kicked
> > out, but belongs to a group which is given no runtime credit? Typically,
> > the user might set the quota limit to 0% for any group. In that case,
> > any thread from that group would be prevented from resuming, therefore
> > could not honor the request for switching back to in-band mode asap,
> > which is the only way to secure the handling of a pending kernel event
> > (again, such as a pending signal). To prevent this, every scheduler
> > module which might cause a thread to have no execution time due to its
> > policy must implement the sched_kick handler so that we have a bypass
> > method for that particular case.
> >
> > SCHED_TP has no sched_kick handler because under such a policy, there is
> > no provision for permanently depriving a thread of execution time. At
> > worst, that thread would have to wait for a complete global time frame
> > to elapse before receiving renewed runtime credit. This implies that we
> > do expect the global time frame not to last for an unreasonably long
> > time, which has been a safe bet for SCHED_TP so far.
> >
> Thanks for the explanation! Would someone like to write a comment patch
> for the quota sched handler?
> Jan
> --
> Siemens AG, T RDA IOT
> Corporate Competence Center Embedded Linux

Marco Barletta

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