Date: Fri, 21 Apr 2017 21:08:33 +0200
From: luca abeni <luca.abeni@santannapisa.it>
To: Juri Lelli <juri.lelli@arm.com>
Cc: linux-kernel@vger.kernel.org, Peter Zijlstra <peterz@infradead.org>,
        Ingo Molnar <mingo@redhat.com>,
        Claudio Scordino <claudio@evidence.eu.com>,
        Steven Rostedt <rostedt@goodmis.org>,
        Tommaso Cucinotta <tommaso.cucinotta@sssup.it>,
        Daniel Bristot de Oliveira <bristot@redhat.com>,
        Mathieu Poirier <mathieu.poirier@linaro.org>
Subject: Re: [PATCH] sched/deadline: fix switching to -deadline
Message-ID: <20170421210833.1837de6f@sweethome>
In-Reply-To: <20170421102659.GS23862@e106622-lin>
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        <20170421114240.0ef43522@nowhere>
        <20170421094729.GR23862@e106622-lin>
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        <20170421102659.GS23862@e106622-lin>
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On Fri, 21 Apr 2017 11:26:59 +0100
Juri Lelli <juri.lelli@arm.com> wrote:
> On 21/04/17 11:59, Luca Abeni wrote:
> > On Fri, 21 Apr 2017 10:47:29 +0100
> > Juri Lelli <juri.lelli@arm.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?


				Luca