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Subject: Re: periods and deadlines in SCHED_DEADLINE
From: Raistlin <raistlin@linux.it>
To: Peter Zijlstra <peterz@infradead.org>
Cc: linux-kernel <linux-kernel@vger.kernel.org>,
       Song Yuan <song.yuan@ericsson.com>,
       Dmitry Adamushko <dmitry.adamushko@gmail.com>,
       Thomas Gleixner <tglx@linutronix.de>,
       Nicola Manica <nicola.manica@disi.unitn.it>,
       Luca Abeni <lucabe72@email.it>,
       Claudio Scordino <claudio@evidence.eu.com>,
       Harald Gustafsson <harald.gustafsson@ericsson.com>,
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       Giuseppe Lipari <lipari@retis.sssup.it>
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On Fri, 2010-07-09 at 16:32 +0200, Peter Zijlstra wrote:
> On Fri, 2010-07-09 at 15:38 +0200, Raistlin 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?
> >    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?=20
>=20
> Again, I'm afraid I need some extra education here in order to make a
> sensible comment.
>=20
Hey, fine, where's the problem? :-P

> What are the exact semantics of this extra proposed syscall?
>=20
Right now, it is:
task_wait_interval(t) --> "wake me up at the first instant after t when=20
			   you can give me my full runtime"

> What exactly are the benefits over not having it, and simply rely on the
> task to not wake up more often, but if it does have it run into the lack
> of budget and sort it that way?
>
What you're saying obviously will always work, and it is actually a
quite common usage pattern (we use it like that a lot! :-)).

The new syscall might help when it is important for a task to
synchronize with the budget provisioning mechanism. It might be
uncommon, but there could be situations --more in hard than in soft
scenarios-- where you want to be sure that you're next job (and all the
subsequent ones, if you behave well) will get its full runtime, even if
this means waiting a little bit.

what I was wondering was if this semantic should be modified by the
introduction of the "period", but I also agree with Luca that we must do
our best to avoid confusion!

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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