linux-kernel - Re: Question about prio_changed

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Message-ID: <20160225152558.2fbac9e5@utopia>
Date:	Thu, 25 Feb 2016 15:25:58 +0100
From:	luca abeni <luca.abeni@...tn.it>
To:	Peter Zijlstra <peterz@...radead.org>
Cc:	Juri Lelli <juri.lelli@....com>, linux-kernel@...r.kernel.org
Subject: Re: Question about prio_changed_dl()

Hi Peter,

On Thu, 25 Feb 2016 15:01:49 +0100
Peter Zijlstra <peterz@...radead.org> wrote:
[...]
> > 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.

Sorry, that was me being confused...
It is prio_changed_dl() that is not invoked when the deadline
parameters are changed (I am testing a change to fix this - it actually
is included in the "Move the remaining __dl_{sub,add}_ac() calls from
core.c to deadline.c" patch I posted on Monday; I am going to extract
it in a separate patch).
switched_to_dl() is correctly invoked.

> 
> ---
> Subject: sched/deadline: Remove superfluous call to switched_to_dl()

This is what I was thinking about, yes :)


			Thanks,
				Luca

> 
> 	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 = {
> 
>