Re: [PATCH] sched: don't allow rt_runtime_us to be zero for groups having rt tasks

[Peter Zijlstra <a.p.zijlstra@chello.nl>]

On Wed, 2008-02-27 at 16:38 +0530, Dhaval Giani wrote:
> This patch checks if we can set the rt_runtime_us to 0. If there is a
> realtime task in the group, we don't want to set the rt_runtime_us to 0
> otherwise bad things will happen.

I had considered this a: don't do that then, thing. But sure, helping
the admin seems like a valid option.

> Signed-off-by: Dhaval Giani <dhaval@linux.vnet.ibm.com>
> ---
>  kernel/sched.c |   19 +++++++++++++++++++
>  1 files changed, 19 insertions(+)
> 
> Index: linux-2.6.25-rc3/kernel/sched.c
> ===================================================================
> --- linux-2.6.25-rc3.orig/kernel/sched.c
> +++ linux-2.6.25-rc3/kernel/sched.c
> @@ -7960,6 +7960,17 @@ static int __rt_schedulable(struct task_
>  	return total + to_ratio(period, runtime) < global_ratio;
>  }
>  
> +/* Must be called with tasklist_lock held */
> +static inline int tg_has_rt_tasks(struct task_group *tg)
> +{
> +	struct task_struct *p;
> +	for_each_process(p) {
> +		if (rt_task(p) && rt_rq_of_se(&p->rt)->tg == tg)
> +			return 1;
> +	}
> +	return 0;
> +}
> +
>  int sched_group_set_rt_runtime(struct task_group *tg, long rt_runtime_us)
>  {
>  	u64 rt_runtime, rt_period;
> @@ -7971,6 +7982,13 @@ int sched_group_set_rt_runtime(struct ta
>  		rt_runtime = rt_period;
>  
>  	mutex_lock(&rt_constraints_mutex);
> +	read_lock(&tasklist_lock);
> +	if (rt_runtime_us == 0) {
> +		if (tg_has_rt_tasks(tg)) {

&&

> +			err = -EINVAL;

-EBUSY perhaps?

> +			goto unlock;
> +		}
> +	}
>  	if (!__rt_schedulable(tg, rt_period, rt_runtime)) {
>  		err = -EINVAL;
>  		goto unlock;
> @@ -7979,6 +7997,7 @@ int sched_group_set_rt_runtime(struct ta
>  		rt_runtime = RUNTIME_INF;
>  	tg->rt_runtime = rt_runtime;
>   unlock:
> +	read_unlock(&tasklist_lock);
>  	mutex_unlock(&rt_constraints_mutex);
>  
>  	return err;