linux-kernel - Re: [tip:core/futexes] futex: use current->time_slack

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Message-ID: <49B97FA8.1060504@us.ibm.com>
Date:	Thu, 12 Mar 2009 14:33:28 -0700
From:	Darren Hart <dvhltc@...ibm.com>
To:	Arjan van de Ven <arjan@...ux.intel.com>
CC:	Peter Zijlstra <peterz@...radead.org>,
	Thomas Gleixner <tglx@...utronix.de>, mingo@...hat.com,
	hpa@...or.com, linux-kernel@...r.kernel.org, rusty@...tcorp.com.au,
	mingo@...e.hu, linux-tip-commits@...r.kernel.org
Subject: Re: [tip:core/futexes] futex: use current->time_slack_ns for rt tasks
 too

Arjan van de Ven wrote:
> Peter Zijlstra wrote:
>>>> ---
>>>>
>>>> Subject: sched: adjust timer_slack_ns on scheduler policy change
>>>> From: Peter Zijlstra <a.p.zijlstra@...llo.nl>
>>>> Date: Thu Mar 12 15:01:02 CET 2009
>>>>
>>>> Ensure RT tasks have 0 timer slack.
>>>>
>>>> Signed-off-by: Peter Zijlstra <a.p.zijlstra@...llo.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;
>>>>      }
>>>>
>>
>> Looking at the default_timer_slack_ns stuff, do we want something like
>> the below?
> 
> the original idea was that you had a default slack that you got from 
> exec time,
> and then something you could just tweak around yourself independently....
> 
> this would throw that out of the window.

It seems to me that changing your scheduling class from userspace would 
be reasonable justification to change the timer slack to a default value 
for that class.  Thoughts?

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