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Commit 60aa605d authored by Peter Zijlstra's avatar Peter Zijlstra Committed by Ingo Molnar
Browse files

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: default avatarThomas Gleixner <tglx@linutronix.de>
Signed-off-by: default avatarPeter Zijlstra <a.p.zijlstra@chello.nl>
LKML-Reference: <20090505155436.863098054@chello.nl>
Signed-off-by: default avatarIngo Molnar <mingo@elte.hu>
parent 6e29ec57

Documentation/scheduler/sched-rt-group.txt

+18 −0
Original line number Diff line number Diff line
@@ -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

===========

kernel/sched.c

+7 −0
Original line number Diff line number Diff line
@@ -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;