Re: [PATCH v2] sched/deadline: remove useless param from setup_new_dl_entity

[Juri Lelli]

On 06/07/16 10:44, Luca Abeni wrote:
> On Tue, 5 Jul 2016 17:58:30 +0100
> Juri Lelli <juri.lelli@xxxxxxx> wrote:
> 
> > On 05/07/16 12:47, Steven Rostedt wrote:
> > > On Tue, 5 Jul 2016 15:39:33 +0100
> > > Juri Lelli <juri.lelli@xxxxxxx> wrote:
> > > 
> > > 		return;  
> > > > > >  
> > > > > >  	/*
> > > > > > +	 * Use the scheduling parameters of the top
> > > > > > pi-waiter task,
> > > > > > +	 * if we have one from which we can inherit a
> > > > > > deadline.
> > > > > > +	 */
> > > > > > +	if (pi_task && dl_se->dl_boosted &&
> > > > > > dl_prio(pi_task->normal_prio))
> > > > > > +		pi_se = &pi_task->dl;
> > > > > > +    
> > > > > 
> > > > > OK, I'm micro-optimizing now, but hey, isn't this a fast path?
> > > > > 
> > > > > What about changing the above to:
> > > > > 
> > > > > 	struct task_struct *pi_task;
> > > > > 	[...]
> > > > > 
> > > > > 	if (dl_se->dl_boosted && dl_prio(pi_task->normal_prio
> > > > > &&    
> > > >                                     ^
> > > > OK, we need to reorder these two
> > > >                                     V  
> > > > > 	    (pi_task = rt_mutex_get_top_task(dl_task_of(dl_se)))
> > > > > 		pe_se = &pi_task->dl;  
> > > 
> > > Opps, you're right.
> > >   
> > > > > 
> > > > > This way we don't need to do any work of looking at
> > > > > rt_mutex_get_top_task() for the normal case.
> > > > >     
> > > > 
> > > > But, yes. Looks good to me. I'll shoot a v3 ASAP.  
> > > 
> > > I have to ask, should there be any check if the dl_se has a shorter
> > > deadline than the pi one?
> > >   
> > 
> > Yeah. I wondered the same actually. I convinced myself that, since the
> > task is boosted, we assume that the donor will have a shorter
> > deadline.
> 
> Do you mean relative deadline (dl_se->dl_deadline) or absolute
> (scheduling) dealine (dl_se->deadline)?
> 
> If I understand well, here we are in setup_new_dl_entity(), right?
> This should be called only from switched_to_dl(); so, dl_se is from a
> task that is switching to -deadline. If it is dl_boosted, it means that
> it is switching from SCHED_OTHER (or RT) to -deadline because of
> inheritance... So, it is very likely that dl_se->dl_deadline is not
> meaningful.
> 

Right, very same thought I also had (and forgot to mention). So, we
cannot really do here the check Steve was wondering about.

> Moreover, setup_new_dl_entity() is only called if the current
> scheduling deadline of the task is not usable (that is, if
> "dl_time_before(p->dl.deadline, rq_clock(rq)"). So, dl_se->deadline
> will be surely smaller than pi_se->deadline... But the inheritance has
> to happen anyway.
> 
> 
> > We seem to be doing the same elsewhere, but Luca was saying
> > some time ago that the DI thing my have some problems and needs to be
> > revised.
> 
> My doubts regarding the inheritance code currently used for -deadline
> tasks are due to the fact that it is not clear which kind of
> inheritance algorithm is used...
> I think it should use deadline inheritance, that, AFAIK, says that when
> task T1 block waiting for task T2, T2 can inherit T1's _absolute_
> deadline - if it is earlier than T2's one.
> But the current code seems to be using relative deadlines (dl_deadline)
> to decide the inheritance...
> 

True. Problem is however that, even if enforcing is disabled for a
boosted task, we keep postponing the task's deadline when it depletes
its runtime (soft-CBS). So, which one should we use to do so?

At the instant of time the task gets a new potential donor this donor
might have a shorter absolute deadline. But the task's relative deadline
might be better (shorter w.r.t. donor's relative) to postpone the
absolute one when needed.

> Having a better look at this is in my TODO list... But I still need to
> find some time :)
> 

Same here. No spare cycles right now to have a thorough look at this. :(

Thanks,

- Juri