[patchlet] sched: fix rt throttle runtime borrowing

[Mike Galbraith <efault@gmx.de>]

Greetings,

The RT throttle leaves a bit to be desired as a protection mechanism.
With default settings, the thing won't save your bacon if you start a
single hog as RT on SMP box, or if your normally sane app goes nuts.

With the below, my box will limp along so I can kill the RT hog.  May
not be the best solution, but works for me.. modulo bustage I haven't
noticed yet of course.

sched: fix rt throttle runtime borrowing

If allowed to borrow up to rt_period, the throttle has no effect on an out
of control RT task, allowing it to consume 100% CPU indefinitely, blocking
system critical SCHED_NORMAL threads indefinitely.

Signed-off-by: Mike Galbraith <efault@gmx.de>

---
 kernel/sched_rt.c |    2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

Index: linux-2.6/kernel/sched_rt.c
===================================================================
--- linux-2.6.orig/kernel/sched_rt.c
+++ linux-2.6/kernel/sched_rt.c
@@ -354,7 +354,7 @@ static int do_balance_runtime(struct rt_
 	weight = cpumask_weight(rd->span);
 
 	raw_spin_lock(&rt_b->rt_runtime_lock);
-	rt_period = ktime_to_ns(rt_b->rt_period);
+	rt_period = ktime_to_ns(rt_b->rt_period) - 1;
 	for_each_cpu(i, rd->span) {
 		struct rt_rq *iter = sched_rt_period_rt_rq(rt_b, i);
 		s64 diff;