Re: [Regression] 2.6.24-git9: RT sched mishandles artswrapper (bisected)

[Peter Zijlstra <a.p.zijlstra@chello.nl>]

On Fri, 2008-02-01 at 08:44 +0100, Peter Zijlstra wrote:
> On Fri, 2008-02-01 at 03:04 +0100, Rafael J. Wysocki wrote:
> > On Friday, 1 of February 2008, Rafael J. Wysocki wrote:
> > > Hi,
> > > 
> > > This is related to the problem I reported earlier this week:
> > > http://lkml.org/lkml/2008/1/30/554
> > > 
> > > Apparently artswrapper, run by KDE in openSUSE 10.3 with a real time priority,
> > > is mishandled by the scheduler.  The problem is that after the user logs out,
> > > artswrapper stays in TASK_RUNNING forever and prevents other tasks from being
> > > scheduled on the CPU occupied by it.  In this state it also breaks suspend and
> > > hibernation (it cannot be frozen).
> > > 
> > > Since the problem is 100% reproducible on my test boxes, I carried out a
> > > bisection which turned out the following commit:
> > > 
> > > commit 6f505b16425a51270058e4a93441fe64de3dd435
> > > Author: Peter Zijlstra <a.p.zijlstra@chello.nl>
> > > Date:   Fri Jan 25 21:08:30 2008 +0100
> > > 
> > >     sched: rt group scheduling
> > > 
> > > I'm now checking if the problem disappears after reverting this patch (along a
> > > couple of dependent ones).
> > 
> > Yes, it does.
> > 
> > Please let me know what I can do to debug it further.
> 
> It arts run as root, or does it use RLIMIT_RTPRIO to allow users to
> execute realtime tasks?
> 

If the latter, does this help:

diff --git a/kernel/sched.c b/kernel/sched.c
index ba4c880..bb76cbc 100644
--- a/kernel/sched.c
+++ b/kernel/sched.c
@@ -4563,6 +4563,15 @@ recheck:
 			return -EPERM;
 	}
 
+#ifdef CONFIG_FAIR_GROUP_SCHED
+	/*
+	 * Do not allow realtime tasks into groups that have no runtime
+	 * assigned.
+	 */
+	if (rt_policy(policy) && task_group(p)->rt_ratio == 0)
+		return -EPERM;
+#endif
+
 	retval = security_task_setscheduler(p, policy, param);
 	if (retval)
 		return retval;