linux-kernel - Re: Question about prio_changed

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Message-ID: <20160225141659.GB18792@e106622-lin>
Date:	Thu, 25 Feb 2016 14:16:59 +0000
From:	Juri Lelli <juri.lelli@....com>
To:	Peter Zijlstra <peterz@...radead.org>
Cc:	luca abeni <luca.abeni@...tn.it>, linux-kernel@...r.kernel.org
Subject: Re: Question about prio_changed_dl()

On 25/02/16 15:01, Peter Zijlstra wrote:
> On Fri, Feb 19, 2016 at 01:43:45PM +0100, luca abeni wrote:
> > Hi,
> > 
> > when playing with the __dl_{add,sub}_ac() stuff recently posted by
> > Juri, I found something that looks strange in prio_changed_dl():
> > 
> > static void prio_changed_dl(struct rq *rq, struct task_struct *p,
> > 			    int oldprio)
> > {
> > 	if (task_on_rq_queued(p) || rq->curr == p) {
> > 		[...]
> > 	} else
> > 		switched_to_dl(rq, p);
> > }
> > but switched_to_dl() does:
> > static void switched_to_dl(struct rq *rq, struct task_struct *p)
> > {
> > 	if (task_on_rq_queued(p) && rq->curr != p) {
> > 		[...]
> > 	}
> > }
> > 
> > so, prio_changed_dl() invokes switched_to_dl() if task_on_rq_queued()
> > is false, but in this case switched_to_dl() does nothing... Am I
> > missing something, or the
> > 	} else
> > 		switched_to_dl(rq, p);
> > is useless?
> 
> Agreed, see below.
> 
> > (BTW, it seems to me that switched_to_dl() is never invoked, for some
> > reason...)
> 
> Hmm, it should be invoked if you do sched_setattr() to get
> SCHED_DEADLINE.
> 
> ---
> Subject: sched/deadline: Remove superfluous call to switched_to_dl()
> 
> 	if (A || B) {
> 
> 	} else if (A && !B) {
> 
> 	}
> 
> If A we'll take the first branch, if !A we will not satisfy the second.
> Therefore the second branch will never be taken.
> 
> Cc: Juri Lelli <juri.lelli@....com>
> Reported-by: luca abeni <luca.abeni@...tn.it>
> Signed-off-by: Peter Zijlstra (Intel) <peterz@...radead.org>

Indeed!

Acked-by: Juri Lelli <juri.lelli@....com>

Thanks,

- Juri