Re: [tip:core/futexes] futex: use current->time_slack_ns for rt tasks too

[Darren Hart <dvhltc@us.ibm.com>]

Peter Zijlstra wrote:
> On Thu, 2009-03-12 at 06:53 -0700, Arjan van de Ven wrote:
>> Darren Hart wrote:
>>> Commit-ID:  16f4993f4e9860715918efd4eeac928f8de1218b
>>> Gitweb:     http://git.kernel.org/tip/16f4993f4e9860715918efd4eeac928f8de1218b
>>> Author:     "Darren Hart" <dvhltc@us.ibm.com>
>>> AuthorDate: Thu, 12 Mar 2009 00:55:59 -0700
>>> Commit:     Ingo Molnar <mingo@elte.hu>
>>> CommitDate: Thu, 12 Mar 2009 11:20:57 +0100
>>>
>>> futex: use current->time_slack_ns for rt tasks too
>>>
>>> RT tasks should set their timer slack to 0 on their own.  This
>>> patch removes the 'if (rt_task()) slack = 0;' block in
>>> futex_wait.
>> Hi,
>>
>> can you explain the rationale for this reasoning?
> 
> Yeah, I found it iffy as well, I think we want something like the below
> instead though..
> 
> ---
> 
> Subject: sched: adjust timer_slack_ns on scheduler policy change
> From: Peter Zijlstra <a.p.zijlstra@chello.nl>
> Date: Thu Mar 12 15:01:02 CET 2009
> 
> Ensure RT tasks have 0 timer slack.

Fork takes care of this by setting the child's default_timer_slack_ns to 
current->timer_slack_ns.  This change takes care of tasks that are 
converted to rt by the user.

What about tasks that are demoted from RT to SCHED_NORMAL?  I'm not sure 
setting it to the default_timer_slack_ns is the right thing since that 
could have been the timer_slack_ns of the rt process it forked from. 
Perhaps heach scheduler class needs to have a default_timer_slack_ns set 
in the class?

> 
> Signed-off-by: Peter Zijlstra <a.p.zijlstra@chello.nl>
> ---
>  kernel/sched.c |    2 ++
>  1 file changed, 2 insertions(+)
> 
> Index: linux-2.6/kernel/sched.c
> ===================================================================
> --- linux-2.6.orig/kernel/sched.c
> +++ linux-2.6/kernel/sched.c
> @@ -5511,10 +5511,12 @@ __setscheduler(struct rq *rq, struct tas
>  	case SCHED_NORMAL:
>  	case SCHED_BATCH:
>  	case SCHED_IDLE:
> +		p->timer_slack_ns = p->default_timer_slack_ns;
>  		p->sched_class = &fair_sched_class;
>  		break;
>  	case SCHED_FIFO:
>  	case SCHED_RR:
> +		p->timer_slack_ns = 0;
>  		p->sched_class = &rt_sched_class;
>  		break;
>  	}
> 


-- 
Darren Hart
IBM Linux Technology Center
Real-Time Linux Team