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

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.

[ Impact: exclude corner-case sysctl_sched_rt_runtime value ]

Reported-by: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Peter Zijlstra <a.p.zijlstra@chello.nl>
LKML-Reference: <20090505155436.863098054@chello.nl>
Signed-off-by: Ingo Molnar <mingo@elte.hu>

2 files changed, 25 insertions, 0 deletions

diff --git a/Documentation/scheduler/sched-rt-group.txt b/Documentation/scheduler/sched-rt-group.txt
index 5ba4d3fc625a..eb74b014a3f8 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 unstable system, the knobs are
+ root only and assumes root knows what he is doing.
+
+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 54d67b94f1a9..2a43a581ead3 100644
--- a/kernel/sched.c
+++ b/kernel/sched.c
@@ -9917,6 +9917,13 @@ 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;