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

[Peter Zijlstra <peterz@infradead.org>]

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.

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;
 	}