linux-kernel - Re: [PATCH] sched/deadline: remove useless param from setup_new_dl

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Date:	Fri, 17 Jun 2016 22:36:12 +0200
From:	luca abeni <luca.abeni@...tn.it>
To:	Juri Lelli <juri.lelli@....com>
Cc:	Steven Rostedt <rostedt@...dmis.org>, linux-kernel@...r.kernel.org,
	peterz@...radead.org, mingo@...hat.com
Subject: Re: [PATCH] sched/deadline: remove useless param from
 setup_new_dl_entity

On Fri, 17 Jun 2016 22:15:18 +0200
luca abeni <luca.abeni@...tn.it> wrote:

> On Fri, 17 Jun 2016 17:28:37 +0100
> Juri Lelli <juri.lelli@....com> wrote:
> [...]
> > True, but we were practically already using the same parameter, under a
> > different name though, after
> > 
> > 2f9f3fdc928 "sched/deadline: Remove dl_new from struct sched_dl_entity"
> > 
> > as we currently do:
> > 
> >   setup_new_dl_entity(&p->dl, &p->dl)
> >   
> > > This patch reverts part of the change done in
> > > commit 2d3d891d334 "sched/deadline: Add SCHED_DEADLINE inheritance
> > > logic"
> > >     
> > 
> > Before Luca's change we were doing
> > 
> >  setup_new_dl_entity(dl_se, pi_se)
> > 
> > in update_dl_entity() for a dl_se->new entity. So, I guess the question
> > is actually why we wanted to use pi_se's parameters (the potential PI
> > donor) for setting up a new entity?  
> That's a good question :)
> 
> > Maybe we broke the situation where a
> > task is currently boosted by a DEADLINE waiter and we swich the holder
> > to DEADLINE?  
> I remember I tested this setup (using linaro's version of rt-app), and
> it seemed to work correctly...
> 
> Re-reading the code now, I actually wonder why my patch did not break
> inheritance in this situation...
Ok; I think I know why inheritance is not broken (or, at least, it does
not appear to be broken when testing it with rt-app):
- When a -deadline task blocks on a mutex that is held by a SCHED_OTHER
  or SCHED_FIFO task, such a task is promoted to -deadline
- setup_new_dl_entity() is invoked, and it sets the tasks' deadline to
  rq_clock(rq) (+ 0), so the task holding the lock is immediately
  scheduled
- as soon as update_curr_dl() is invoked (in the worst case at the next
  tick), the task's deadline and runtime are set to the "desired values"
  (using pi_se)

So, the behaviour is not changed too much respect to the previous one.



				Luca