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

Thomas Gleixner
2009-03-12T14:27:05Z

On Thu, 12 Mar 2009, 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..

Yup, that's what I meant when I hollered about the timer_slack_ns
hackery in futex.c

Thanks,

tglx

> ---
>
> 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;
> }
>