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

[Juri Lelli <juri.lelli@gmail.com>]

On 01/21/2014 02:55 PM, Peter Zijlstra wrote:
> On Tue, Jan 21, 2014 at 01:50:41PM +0100, Luca Abeni wrote:
>> On 01/21/2014 01:33 PM, Peter Zijlstra wrote:
> 
>>>> - 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 :)
> 
> Yeah, I know, my point was more about not encouraging people to do this
> by explicitly mentioning it.
> 
>> 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...).
> 
> At which point I feel obliged to mention the work Jim did on statistical
> bounded tardiness and a potential future option:
> SCHED_FLAG_DL_AVG_RUNTIME, where we would allow tasks to somewhat exceed
> their runtime budget provided that they meet their budget on average.
> 
> A possible implementation could be to track the unused budget of
> previous instances and keep a decaying sum (such that we're guaranteed
> this unused budget < 2*runtime). And then allow runtime overruns upto
> this limit.
> 
> Another possibly extension; one proposed by Ingo; is to demote tasks to
> SCHED_OTHER once they exceed their budget instead of the full block they
> get now -- we could possibly call this SCHED_FLAG_DL_CBS_SOFT or such.
> 
> And of course SCHED_FLAG_DL_CBS_SIGNAL, where the task gets a signal
> delivered if it exceeded the runtime -- I think some of the earlier
> patches had things like this, no?
>

Yes, they both got removed along the way. But we can pick them back if we
realize that they are needed for some scenario.

>>> 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?
> 
> Sure, I think that's 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?
> 
> Oh sure, all of this will always be a work in progress anyway ;-)
> 

Ok, we are working on a first update.

Thanks,

- Juri