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

[Randy Dunlap <randy.dunlap@oracle.com>]

Peter Zijlstra wrote:
> 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.

I know that my English is limited, but I don't know what this means:
  "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;
> 
> --


-- 
~Randy