[PATCH] sched: rt: document the risk of small values in the bandwidth settings

[Peter Zijlstra <peterz@infradead.org>]

Thomas noted that we should disallow sysctl_sched_rt_runtime == 0 for (!
RT_GROUP) since the root group always has some RT tasks in it.

Further, update the documentation to inspire clue.

Signed-off-by: Peter Zijlstra <a.p.zijlstra@chello.nl>
Acked-by: Thomas Gleixner <tglx@linutronix.de>
---
 Documentation/scheduler/sched-rt-group.txt |   18 ++++++++++++++++++
 kernel/sched.c                             |    7 +++++++
 2 files changed, 25 insertions(+), 0 deletions(-)

diff --git a/Documentation/scheduler/sched-rt-group.txt b/Documentation/scheduler/sched-rt-group.txt
index 5ba4d3f..cfd51c0 100644
--- a/Documentation/scheduler/sched-rt-group.txt
+++ b/Documentation/scheduler/sched-rt-group.txt
@@ -4,6 +4,7 @@
 CONTENTS
 ========
 
+0. WARNING
 1. Overview
   1.1 The problem
   1.2 The solution
@@ -14,6 +15,23 @@ CONTENTS
 3. Future plans
 
 
+0. WARNING
+==========
+
+Fiddling with these settings can result in an unusable system, the knobs are
+root only and assume root knows what he's on about.
+
+Most notable:
+ 
+ * very small values in sched_rt_period_us can result in an unstable
+   system when the period is smaller than either the available hrtimer
+   resolution, or the time it takes to handle the budget refresh itself.
+
+ * very small values in sched_rt_runtime_us can result in an unstable
+   system when the runtime is so small the system has difficulty making
+   forward progress (NOTE: the migration thread and kstopmachine both
+   are real-time processes).
+
 1. Overview
 ===========
 
diff --git a/kernel/sched.c b/kernel/sched.c
index 11f6da4..c5ef114 100644
--- a/kernel/sched.c
+++ b/kernel/sched.c
@@ -10021,6 +10021,13 @@ static int sched_rt_global_constraints(void)
 	if (sysctl_sched_rt_period <= 0)
 		return -EINVAL;
 
+	/*
+	 * There's always some RT tasks in the root group
+	 * -- migration, kstopmachine etc..
+	 */
+	if (sysctl_sched_rt_runtime == 0)
+		return -EBUSY;
+
 	spin_lock_irqsave(&def_rt_bandwidth.rt_runtime_lock, flags);
 	for_each_possible_cpu(i) {
 		struct rt_rq *rt_rq = &cpu_rq(i)->rt;