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

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Date:   Fri, 21 Apr 2017 10:47:29 +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 11:42, Luca Abeni wrote:
> On Fri, 21 Apr 2017 10:39:26 +0100
> Juri Lelli <juri.lelli@....com> wrote:
> 
> > Hi Luca,
> > 
> > On 20/04/17 21:30, Luca Abeni wrote:
> > > From: Luca Abeni <luca.abeni@...tannapisa.it>
> > > 
> > > When switching to -deadline, if the scheduling deadline of a task is
> > > in the past then switched_to_dl() calls setup_new_entity() to
> > > properly initialize the scheduling deadline and runtime.
> > > 
> > > The problem is that the task is enqueued _before_ having its
> > > parameters initialized by setup_new_entity(), and this can cause
> > > problems. For example, a task with its out-of-date deadline in the
> > > past will potentially be enqueued as the highest priority one;
> > > however, its adjusted deadline may not be the earliest one.
> > > 
> > > This patch fixes the problem by initializing the task's parameters
> > > before enqueuing it.
> > > 
> > > Signed-off-by: Luca Abeni <luca.abeni@...tannapisa.it>
> > > Reviewed-by: Daniel Bristot de Oliveira <bristot@...hat.com>
> > > ---
> > >  kernel/sched/deadline.c | 12 ++++--------
> > >  1 file changed, 4 insertions(+), 8 deletions(-)
> > > 
> > > diff --git a/kernel/sched/deadline.c b/kernel/sched/deadline.c
> > > index a2ce590..ec53d24 100644
> > > --- a/kernel/sched/deadline.c
> > > +++ b/kernel/sched/deadline.c
> > > @@ -950,6 +950,10 @@ enqueue_dl_entity(struct sched_dl_entity
> > > *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 ? :)