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Subject: Re: periods and deadlines in SCHED_DEADLINE
From: Raistlin <raistlin@linux.it>
To: Luca Abeni <lucabe72@email.it>
Cc: linux-kernel <linux-kernel@vger.kernel.org>,
       Song Yuan <song.yuan@ericsson.com>,
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Date: Sat, 10 Jul 2010 11:20:47 +0200
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On Sat, 2010-07-10 at 09:09 +0200, Luca Abeni wrote:
> >  - do you think it could be useful to have a different syscall to deal
> >    with the period parameter (if it's different from deadline), e.g.,
> >    something useful to make the task periodic as you have (if I remembe=
r
> >    well) in Xenomai or RTAI?
> Maybe I am confused because I missed the initial part of the discussion,
> but here I think there is the risk to mix two different concepts: the
> "reservation period" (that is, the period P used to postpone the
> scheduling deadline when the budget arrives to 0), and the "task
> period" (which has to do with the periodicity of tasks activations).=20
>
Agree, this is also what I fear...

> For
> implementing a periodic behaviour in the task (this is, AFAIK, what RTAI
> similar API provide), no new syscall is needed: clock_nanosleep() is
> enough. See http://www.disi.unitn.it/~abeni/RTOS/rtapi.pdf for a
> (stupid) example.
>
Yep, agree again, just wanted to see what other folks' thoughts
were. :-)

> >    If you think it's worth doing that, do you think the
> >    task_wait_interval() syscall that we already added could/should do
> >    the job?
> I do not remember what task_wait_interval() does :)
> Is it the syscall you added to indicate the end of a job?
>=20
Yep. And it can be useful for that purpose, or not being used at all.

> I think if you want a different P_i and D_i you can use D_i for
> generating new scheduling deadlines on task arrivals as "d =3D t + D_i",
> and P_i to postpone the scheduling deadlines as "d =3D d + T_i" when the
> budget is 0.
>
Yes, that's exactly what we wanted to do, considered it's also a very
small and easy to achieve behavioural change...

> Depending on the replenishment amount you use, you might need to modify
> the admission test as "Sum_i(Q_i/min{P_i,D_i}) < 1" or not (if you
> always replenish to Q_i, then you need a modified admission test;
> otherwise, you can compute the replenishment amount so that the
> admission test is unaffected).
>=20
Well, as I tried to point out in the other e-mails, there also are other
problems with the admission test... Let's see if we find a consensus on
how to proceed... :-)

Thanks and regards,
Dario

--=20
<<This happens because I choose it to happen!>> (Raistlin Majere)
----------------------------------------------------------------------
Dario Faggioli, ReTiS Lab, Scuola Superiore Sant'Anna, Pisa  (Italy)

http://blog.linux.it/raistlin / raistlin@ekiga.net /
dario.faggioli@jabber.org

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