linux-kernel - Re: [PATCH] sched/deadline: fix switching to -deadline

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Message-ID: <20170424101624.GA13394@e106622-lin>
Date:   Mon, 24 Apr 2017 11:16:24 +0100
From:   Juri Lelli <juri.lelli@....com>
To:     luca abeni <luca.abeni@...tannapisa.it>
Cc:     linux-kernel@...r.kernel.org,
        Peter Zijlstra <peterz@...radead.org>,
        Ingo Molnar <mingo@...hat.com>,
        Claudio Scordino <claudio@...dence.eu.com>,
        Steven Rostedt <rostedt@...dmis.org>,
        Tommaso Cucinotta <tommaso.cucinotta@...up.it>,
        Daniel Bristot de Oliveira <bristot@...hat.com>,
        Mathieu Poirier <mathieu.poirier@...aro.org>
Subject: Re: [PATCH] sched/deadline: fix switching to -deadline

On 21/04/17 21:08, Luca Abeni wrote:
> On Fri, 21 Apr 2017 11:26:59 +0100
> Juri Lelli <juri.lelli@....com> wrote:
> > On 21/04/17 11:59, Luca Abeni wrote:
> > > On Fri, 21 Apr 2017 10:47:29 +0100
> > > Juri Lelli <juri.lelli@....com> wrote:
> > > [...]  
> > > > > > > *dl_se, update_dl_entity(dl_se, pi_se);
> > > > > > >  	else if (flags & ENQUEUE_REPLENISH)
> > > > > > >  		replenish_dl_entity(dl_se, pi_se);
> > > > > > > +	else if ((flags & ENQUEUE_RESTORE) &&      
> > > > > > 
> > > > > > Not sure I understand how this works. AFAICT we are doing
> > > > > > __sched_setscheduler() when we want to catch the case of a new
> > > > > > dl_entity (SCHED_{OTHER,FIFO} -> SCHED_DEADLINE}, but
> > > > > > queue_flags (which are passed to enqueue_task()) don't seem
> > > > > > to have ENQUEUE_RESTORE set?    
> > > > > 
> > > > > I was under the impression sched_setscheduler() sets
> > > > > ENQUEUE_RESTORE...   
> > > > 
> > > > Oh, I think it works "by coincidence", as ENQUEUE_RESTORE ==
> > > > DEQUEUE_SAVE == 0x02 ? :)  
> > > 
> > > Not sure if this is a conincidence... By looking at the comments in
> > > sched/sched.h I got the impression the two values match by design
> > > (and __sched_setscheduler() is using this property to simplify the
> > > code :) 
> > 
> > Yep, right.
> > 
> > Do you think we might get into trouble with do_set_cpus_allowed()?
> > Can it happen that we change a task affinity while its deadline is in
> > the past?
> 
> Well, double thinking about it, this is an interesting problem... What
> do we want to do with do_set_cpus_allowed()? (I mean: what is the
> expected behaviour?)
> 
> With this patch, if a task is moved to a different runqueue when its
> deadline is in the past (because we are doing gEDF, or because of timer
> granularity issues) its scheduling deadline is reinitialized to current
> time + relative deadline... I think this makes perfect sense, doesn't
> it?
> 

Mmm, I don't think we will (with this patch) actually reinitialize the
deadline when a "normal" gEDF migration happen (push/pull), as
(de)activate_task() have no flag set. Which brings the question, should
we actually take care of this corner case (as what you say makes sense
to me too)?