Philippe Gerum rpm at xenomai.org
Fri May 21 16:41:17 CEST 2021

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.


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