Subject: Re: periods and deadlines in SCHED_DEADLINE
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From: Bjoern Brandenburg <bbb@email.unc.edu>
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Date: Sun, 11 Jul 2010 08:15:23 +0200
Cc: Peter Zijlstra <peterz@infradead.org>,
       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>, bastoni@cs.unc.edu,
       Giuseppe Lipari <lipari@retis.sssup.it>
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To: Raistlin <raistlin@linux.it>
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On Jul 10, 2010, at 9:50 AM, Raistlin wrote:

>> 
>> What are the exact semantics of this extra proposed syscall?
>> 
> Right now, it is:
> task_wait_interval(t) --> "wake me up at the first instant after t when 
> 			   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.

Isn't this basically sched_yield? Don't run me now, give lower-priority work a chance to complete, let me run again when my budget is replenished.

Otherwise, what are the semantics of sched_yield under EDF? Cycle through equal-deadline jobs? Given sporadic task activations, this is probably not very useful.

- Bj?rn

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