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

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Message-Id: <1236869314.22914.3697.camel@twins>
Date:	Thu, 12 Mar 2009 15:48:34 +0100
From:	Peter Zijlstra <peterz@...radead.org>
To:	Thomas Gleixner <tglx@...utronix.de>
Cc:	Arjan van de Ven <arjan@...ux.intel.com>, mingo@...hat.com,
	hpa@...or.com, dvhltc@...ibm.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


> > ---
> > 
> > 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?

---
diff --git a/kernel/sys.c b/kernel/sys.c
index 7306f94..6d8a84d 100644
--- a/kernel/sys.c
+++ b/kernel/sys.c
@@ -1811,11 +1811,13 @@ SYSCALL_DEFINE5(prctl, int, option, unsigned
long, arg2, unsigned long, arg3,
 			error = current->timer_slack_ns;
 			break;
 		case PR_SET_TIMERSLACK:
-			if (arg2 <= 0)
+			if (arg2 <= 0) {
 				current->timer_slack_ns =
 					current->default_timer_slack_ns;
-			else
-				current->timer_slack_ns = arg2;
+			} else {
+				current->default_timer_slack_ns =
+					current->timer_slack_ns = arg2;
+			}
 			error = 0;
 			break;
 		default:

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