linux-kernel - Re: [PATCH linux-next] sched: Fix broken setscheduler()

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Date:	Thu, 6 Mar 2014 12:58:25 +0100
From:	Peter Zijlstra <peterz@...radead.org>
To:	Steven Rostedt <rostedt@...dmis.org>
Cc:	LKML <linux-kernel@...r.kernel.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Sebastian Andrzej Siewior <bigeasy@...utronix.de>,
	Ingo Molnar <mingo@...nel.org>,
	Juri Lelli <juri.lelli@...il.com>
Subject: Re: [PATCH linux-next] sched: Fix broken setscheduler()

On Wed, Mar 05, 2014 at 11:29:31PM -0500, Steven Rostedt wrote:
> I decided to run my tests on linux-next, and my wakeup_rt tracer was
> broken. After running a bisect, I found that the problem commit was:
> 
>    linux-next commit c365c292d059
>    "sched: Consider pi boosting in setscheduler()"
> 
> And the reason the wake_rt tracer test was failing, was because it had
> no RT task to trace. I first noticed this when running with
> sched_switch event and saw that my RT task still had normal SCHED_OTHER
> priority. Looking at the problem commit, I found:
> 
>  -       p->normal_prio = normal_prio(p);
>  -       p->prio = rt_mutex_getprio(p);
> 
> With no
> 
>  +       p->normal_prio = normal_prio(p);
>  +       p->prio = rt_mutex_getprio(p);
> 
> Reading what the commit is suppose to do, I realize that the p->prio
> can't be set if the task is boosted with a higher prio, but the
> p->normal_prio still needs to be set regardless, otherwise, when the
> task is deboosted, it wont get the new priority.
> 
> The p->prio has to be set before "check_class_changed()" is called,
> otherwise the class wont be changed.

So Juri had a different patch for this problem:

  http://lkml.kernel.org/r/20140301191838.d15d03112b2598a671dac22c@gmail.com

> Signed-off-by: Steven Rostedt <rostedt@...dmis.org>
> ---
> diff --git a/kernel/sched/core.c b/kernel/sched/core.c
> index 4600bca..b1cc871 100644
> --- a/kernel/sched/core.c
> +++ b/kernel/sched/core.c
> @@ -3198,6 +3198,7 @@ static void __setscheduler_params(struct task_struct *p,
>  	 * getparam()/getattr() don't report silly values for !rt tasks.
>  	 */
>  	p->rt_priority = attr->sched_priority;
> +	p->normal_prio = normal_prio(p);
>  	set_load_weight(p);
>  }

Now; if I'm reading things right, normal_prio is the unboosted priority
of a task. And we should indeed keep setting that, otherwise the unboost
doesn't know where it should go.

Juri put that in __setscheduler(), but I think that's wrong because the
rt_mutex_check_prio() case in __sched_setscheduler() still needs to
update this.

> @@ -3207,6 +3208,8 @@ static void __setscheduler(struct rq *rq, struct task_struct *p,
>  {
>  	__setscheduler_params(p, attr);
>  
> +	p->prio = rt_mutex_getprio(p);
> +
>  	if (dl_prio(p->prio))
>  		p->sched_class = &dl_sched_class;
>  	else if (rt_prio(p->prio))
> 

And when we call this we're sure to not be boosted; so this is
effectively the same as Juri has:

  p->prio = p->normal_prio = normal_prio(p)

Seeing how rt_mutex_getprio() and normal_prio() are the same under these
conditions.


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