Commit-ID: 60aa605dfce2976e54fa76e805ab0f221372d4d9
Gitweb: http://git.kernel.org/tip/60aa605dfce2976e54fa76e805ab0f221372d4d9
Author: Peter Zijlstra <a.p.zijlstra@xxxxxxxxx>
AuthorDate: Tue, 5 May 2009 17:50:21 +0200
Committer: Ingo Molnar <mingo@xxxxxxx>
CommitDate: Tue, 5 May 2009 20:07:57 +0200
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@xxxxxxxxxxxxx>
Signed-off-by: Peter Zijlstra <a.p.zijlstra@xxxxxxxxx>
LKML-Reference: <20090505155436.863098054@xxxxxxxxx>
Signed-off-by: Ingo Molnar <mingo@xxxxxxx>
---
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..eb74b01 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 54d67b9..2a43a58 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;
--
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