linux-kernel - Re: [PATCH] sched: Make sure task has correct sched

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Message-ID: <4ABA173B.4040106@bigpond.net.au>
Date:	Wed, 23 Sep 2009 22:40:27 +1000
From:	Peter Williams <pwil3058@...pond.net.au>
To:	Peter Zijlstra <a.p.zijlstra@...llo.nl>
CC:	Ingo Molnar <mingo@...e.hu>, Mike Galbraith <efault@....de>,
	Linux Kernel Mailing <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] sched: Make sure task has correct sched_class after policy
 change

On 23/09/09 16:32, Peter Zijlstra wrote:
> On Wed, 2009-09-23 at 02:21 +0000, Peter Williams wrote:
>>  From the code in rt_mutex_setprio(), it is evident that the intention
>> is that task's with a RT 'prio' value as a consequence of receiving a PI
>> boost also have their 'sched_class' field set to '&rt_sched_class'.
>> However, the code in __setscheduler() could result in this intention
>> being frustrated.
>>
>> This patch fixes this problem.
>>
>> Signed-off-by: Peter Williams<pwil3058@...pond.net.au>
>
> I think you're right, but the problem seems to be that it sets
> sched_class based on policy, which seems fragile in the face of PI.
>
> How about the alternative below?

Yes, that's what I meant to do i.e. use p->prio in the if condition.  I 
sent a second patch with a fix but your solution is neater :-).

>
> ---
>   kernel/sched.c |   17 ++++-------------
>   1 files changed, 4 insertions(+), 13 deletions(-)
>
> diff --git a/kernel/sched.c b/kernel/sched.c
> index 91843ba..753a52c 100644
> --- a/kernel/sched.c
> +++ b/kernel/sched.c
> @@ -6129,23 +6129,14 @@ __setscheduler(struct rq *rq, struct task_struct *p, int policy, int prio)
>   {
>   	BUG_ON(p->se.on_rq);
>
> -	p->policy = policy;
> -	switch (p->policy) {
> -	case SCHED_NORMAL:
> -	case SCHED_BATCH:
> -	case SCHED_IDLE:
> -		p->sched_class =&fair_sched_class;
> -		break;
> -	case SCHED_FIFO:
> -	case SCHED_RR:
> -		p->sched_class =&rt_sched_class;
> -		break;
> -	}
> -
>   	p->rt_priority = prio;
>   	p->normal_prio = normal_prio(p);
>   	/* we are holding p->pi_lock already */
>   	p->prio = rt_mutex_getprio(p);
> +	if (rt_prio(p->prio))
> +		p->sched_class =&rt_sched_class;
> +	else
> +		p->sched_class =&fair_sched_class;
>   	set_load_weight(p);
>   }

Peter
-- 
Peter Williams                                   pwil3058@...pond.net.au

"Learning, n. The kind of ignorance distinguishing the studious."
  -- Ambrose Bierce
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