Re: [RFD] sched/deadline: Support single CPU affinity

[luca abeni]

Hi Peter,

On Thu, 10 Nov 2016 11:59:18 +0100
Peter Zijlstra <peterz@xxxxxxxxxxxxx> wrote:

[...]  
> > > MIXED CRITICALITY SCHEDULING
> > > 
> > > Since we want to provide better guarantees for single CPU affine
> > > tasks than the G-EDF scheduler provides for the single CPU tasks,
> > > we need to somehow alter the scheduling algorithm.
> > > 
> > > The trivial layered EDF/G-EDF approach is obviously flawed in
> > > that it will result in many unnecessary deadline misses. The
> > > trivial example is having a single CPU task with a deadline after
> > > a runnable global task. By always running single CPU tasks over
> > > global tasks we can make the global task miss its deadline even
> > > though we could easily have ran both within the alloted time.  
> 
> > Ok; the layered approach clearly causes some unneeded deadline
> > misses on global tasks... But those tasks risk to miss deadlines
> > anyway (with the corrent scheduler, they are guaranteed to have a
> > limited tardiness, not to respect all of the deadlines). I think
> > this is related to the question about which guarantees are provided
> > to the global tasks.  
> 
> Right, so I wanted to limit the impact. Basically, given a stronger
> admission test for the GEDF scheduler that would guarantee deadlines,
> I would like the new scheduling function to still guarantee all
> deadlines.
Ok; this is interesting... I do not know if it is possible, but it is
definitly something interesting to look at.


> > > Therefore we must use a more complicated scheme. By adding a
> > > second measure present in the sporadic task model to the
> > > scheduling function we can try and distinguish between the
> > > constraints of handling the two cases in a single scheduler.
> > > 
> > > We define the time to fail as:
> > > 
> > >   ttf(t) := t_d - t_b; where
> > > 
> > > 	t_d is t's absolute deadline
> > > 	t_b is t's remaining budget  
> 
> > Ok; I think this is similar to what is called "pseudo-deadline" in
> > some papers.
> > If I understand well, it is equal to the current time + the task
> > "laxity" (or slack time). So, scheduling the task with the smaller
> > ttf is equivalent to the "least laxity first" (LLF) algorithm.
> > Giving precedence to tasks with 0 laxity is a technique that is
> > often used to improve the schedulability on multi-processor
> > systems.  
> 
> Right, similar to LLF. Initially I was using the term laxity here, but
> Hendrik convinced me that this is called time-to-fail. I'll let him
> explain :-)
Well, if I understand well "time-to-fail" is equal to "laxity + current
time"... So, they are two different quantities but the final scheduling
algorithm is the same (and using ttf simplifies the implementation a
lot :).


> But yes, a pure TTF based scheduler should be equivalent to LLF which
> itself is fairly similar to EDF in that its optimal for UP etc.. (I
> think).
Right

> > I _suspect_ that the migration rules must also be changed (when a
> > task migrates from a runqueue, it currently selects as a target the
> > runqueue having the largest earliest deadline... Maybe it should
> > also consider the presence of rq-local tasks - or the LLF-ordered
> > heap - on the target?)  
> 
> Quite possible, I didn't think about that.
> 
> > Did you find this scheduling strategy on some paper? Otherwise, I
> > think we need to develop some theoretical analysis for it... (which
> > is of course another interesting thing! :)  
> 
> No, I cooked this up myself. There might of course still be a paper on
> this, but if so, I'm blissfully unaware.
Ok; I'll try to have a look at the theoretical analysis.



			Thanks,
				Luca