xnsched_kick

xnsched_kick

[Marco Barletta]

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.

-- 
Marco Barletta

Re: xnsched_kick

[Jan Kiszka]

On 21.05.21 13:21, Marco Barletta via Xenomai wrote:
> 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?

I think that is a valid question because sched-tp does not implement
such a scheme. Basic information can be found in

https://source.denx.de/Xenomai/xenomai/-/commit/cee27b2945f92857308dcde619a3e64607b5327e

I would forward this for further background to the designer...

Jan

-- 
Siemens AG, T RDA IOT
Corporate Competence Center Embedded Linux

Re: xnsched_kick

[Philippe Gerum]

Marco Barletta via Xenomai <xenomai@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.

-- 
Philippe.

Re: xnsched_kick

[Jan Kiszka]

On 21.05.21 16:41, Philippe Gerum via Xenomai wrote:
> 
> Marco Barletta via Xenomai <xenomai@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

Re: xnsched_kick

[Marco Barletta]

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@siemens.com>
ha scritto:

> On 21.05.21 16:41, Philippe Gerum via Xenomai wrote:
> >
> > Marco Barletta via Xenomai <xenomai@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