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Date:   Tue, 30 Jul 2019 10:21:08 +0200
From:   Peter Zijlstra <peterz@infradead.org>
To:     Juri Lelli <juri.lelli@redhat.com>
Cc:     Dietmar Eggemann <dietmar.eggemann@arm.com>,
        Ingo Molnar <mingo@kernel.org>,
        Luca Abeni <luca.abeni@santannapisa.it>,
        Daniel Bristot de Oliveira <bristot@redhat.com>,
        Valentin Schneider <Valentin.Schneider@arm.com>,
        Qais Yousef <Qais.Yousef@arm.com>, linux-kernel@vger.kernel.org
Subject: Re: [PATCH 4/5] sched/deadline: Cleanup on_dl_rq() handling
Message-ID: <20190730082108.GJ31381@hirez.programming.kicks-ass.net>
References: <20190726082756.5525-1-dietmar.eggemann@arm.com>
 <20190726082756.5525-5-dietmar.eggemann@arm.com>
 <20190729164932.GN31398@hirez.programming.kicks-ass.net>
 <20190730064115.GC8927@localhost.localdomain>
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On Tue, Jul 30, 2019 at 08:41:15AM +0200, Juri Lelli wrote:
> On 29/07/19 18:49, Peter Zijlstra wrote:
> > On Fri, Jul 26, 2019 at 09:27:55AM +0100, Dietmar Eggemann wrote:
> > > Remove BUG_ON() in __enqueue_dl_entity() since there is already one in
> > > enqueue_dl_entity().
> > > 
> > > Move the check that the dl_se is not on the dl_rq from
> > > __dequeue_dl_entity() to dequeue_dl_entity() to align with the enqueue
> > > side and use the on_dl_rq() helper function.
> > > 
> > > Signed-off-by: Dietmar Eggemann <dietmar.eggemann@arm.com>
> > > ---
> > >  kernel/sched/deadline.c | 8 +++-----
> > >  1 file changed, 3 insertions(+), 5 deletions(-)
> > > 
> > > diff --git a/kernel/sched/deadline.c b/kernel/sched/deadline.c
> > > index 1fa005f79307..a9cb52ceb761 100644
> > > --- a/kernel/sched/deadline.c
> > > +++ b/kernel/sched/deadline.c
> > > @@ -1407,8 +1407,6 @@ static void __enqueue_dl_entity(struct sched_dl_entity *dl_se)
> > >  	struct sched_dl_entity *entry;
> > >  	int leftmost = 1;
> > >  
> > > -	BUG_ON(!RB_EMPTY_NODE(&dl_se->rb_node));
> > > -
> > >  	while (*link) {
> > >  		parent = *link;
> > >  		entry = rb_entry(parent, struct sched_dl_entity, rb_node);
> > > @@ -1430,9 +1428,6 @@ static void __dequeue_dl_entity(struct sched_dl_entity *dl_se)
> > >  {
> > >  	struct dl_rq *dl_rq = dl_rq_of_se(dl_se);
> > >  
> > > -	if (RB_EMPTY_NODE(&dl_se->rb_node))
> > > -		return;
> > > -
> > >  	rb_erase_cached(&dl_se->rb_node, &dl_rq->root);
> > >  	RB_CLEAR_NODE(&dl_se->rb_node);
> > >  
> > > @@ -1466,6 +1461,9 @@ enqueue_dl_entity(struct sched_dl_entity *dl_se,
> > >  
> > >  static void dequeue_dl_entity(struct sched_dl_entity *dl_se)
> > >  {
> > > +	if (!on_dl_rq(dl_se))
> > > +		return;
> > 
> > Why allow double dequeue instead of WARN?
> 
> As I was saying to Valentin, it can currently happen that a task could
> have already been dequeued by update_curr_dl()->throttle called by
> dequeue_task_dl() before calling __dequeue_task_dl(). Do you think we
> should check for this condition before calling into dequeue_dl_entity()?

Yes, that's what ->dl_throttled is for, right? And !->dl_throttled &&
!on_dl_rq() is a BUG.