linux-kernel - Re: Question about prio_changed

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Message-ID: <20160225140149.GK6357@twins.programming.kicks-ass.net>
Date:	Thu, 25 Feb 2016 15:01:49 +0100
From:	Peter Zijlstra <peterz@...radead.org>
To:	luca abeni <luca.abeni@...tn.it>
Cc:	Juri Lelli <juri.lelli@....com>, linux-kernel@...r.kernel.org
Subject: Re: Question about prio_changed_dl()

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>
---
 kernel/sched/deadline.c | 3 +--
 1 file changed, 1 insertion(+), 2 deletions(-)

diff --git a/kernel/sched/deadline.c b/kernel/sched/deadline.c
index 57b939c81bce..c161c53d9424 100644
--- a/kernel/sched/deadline.c
+++ b/kernel/sched/deadline.c
@@ -1768,8 +1768,7 @@ static void prio_changed_dl(struct rq *rq, struct task_struct *p,
 		 */
 		resched_curr(rq);
 #endif /* CONFIG_SMP */
-	} else
-		switched_to_dl(rq, p);
+	}
 }
 
 const struct sched_class dl_sched_class = {