Re: [PATCH] sched/deadline: Add sched_dl documentation

From: Peter Zijlstra
Date: Tue Jan 21 2014 - 07:34:29 EST

On Tue, Jan 21, 2014 at 12:35:27PM +0100, Luca Abeni wrote:
> >In a system, we typically look at a set of tasks. In Linux-kernel
> >terminology, a particular task is normally a thread. When a thread is
> >ready to run, we say that a *job* of that task is running.
> This would be true in the original Liu&Layland model (where a task blocks
> only when a job finishes), but I do not think it is correct in a real system...
> For example: (notice: this discussion might be slightly off-topic, and I do not
> think this should go in the document... I am writing just to clarify my point
> of view)
> - Let's consider a (over simplified) video decoder as an example of task
> - The task periodically read a video frame (from disk or network), decodes it,
> and displays it
> - So, each job starts when the frame is read, and finishes when the frame is
> displayed. And jobs are (in this case) activated periodically
> - During the execution of a job, the task might invoke a blocking system call,
> and block... When it wakes up, it is still in the same job (decoding the same
> video frame), and not in a different one.
> This is (IMHO) where all the confusion comes from.

I would strongly urge you not to use that as an example, because its
dead wrong design. An RT thread (be it RR,FIFO or DL) should _NEVER_ do
blocking IO.

Have !RT tasks read the stuff from disk into a buffer, then let the RT
task read data from the buffer and flip frames and such.

If you want to mention blocking, then please use the most common one:
blocking on a (hopefully PI) mutex.

On the other subject; I wouldn't actually mind if it grew into a proper
(academic or not) summary of deadline scheduling theory and how it

Sure, refer to actual papers for all the proofs and such, but it would
be very good to go over all the bits and pieces that make up the system.

So cover the periodic, sporadic and aperiodic model like henr_k
suggested, please do cover the job/instance idiom as it is used all over
the place.

Then also treat schedulability tests and their ramification, explain
what laxity is, what tardiness is, that GEDF doesn't have 0 tardiness
but does have bounded tardiness.

Maybe even mention the actual bounds -- but refer to papers for their

Mention CBS and the ramification etc..

Yes this is all a bit much, but I feel it is important, after all how
can you properly use something you don't understand? (and yes I know its
a very popular thing to not want to understand how things work but still
use them :-/).

I mean, I'm the kind of idiot that actually goes out and read a bunch of
papers, but many people simply cannot read those things, or are not
given the time to, even if they wanted and could (arguably they have
bigger problems).

so think of this as the idiot guide to deadline scheduling :-)
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