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

[Luca Abeni <luca.abeni@unitn.it>]

On 01/21/2014 01:33 PM, Peter Zijlstra wrote:
> 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.
Well, but it does happen in reality :)
I mean: people might want to use SCHED_DEADLINE to schedule mplayer (or similar).
There are even scientific papers showing the advantage of doing so...
And if you try to use ftrace/kernelshark to check the wake-up times and similar
you will notice that even a single-threaded player like mplayer blocks and wakes-up
many times inside a job.

On the other hand, I agree with you that a hard real-time task should be designed
not to do things like this. But SCHED_DEADLINE is flexible enough to be used on
many different kinds of tasks (hard real-time, soft real-time, etc...).

> 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.
Ok.

> 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
> applies.
>
> 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.
Ok... My point was that it would be better (IMHO) to first explain how
sched_deadline works (and no notion of job/instance, etc is needed for this),
and then explain how this applies to the real-time task model (and here, of
course all the formal notation can be introduced).

Do you think this can be reasonable?

> 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
> proofs.
>
> Mention CBS and the ramification etc..
Ok.
I guess some of these details can be added incrementally, with additional
patches?


> 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).
Ok.



				Thanks,
					Luca