Re: [RFC][PATCH 06/22] sched: SCHED_DEADLINE handles spacialkthreads

[Peter Zijlstra]

On Fri, 2010-10-29 at 08:31 +0200, Raistlin wrote:
> 
> There sometimes is the need of executing a task as if it would
> have the maximum possible priority in the entire system, i.e.,
> whenever it gets ready it must run! This is for example the case
> for some maintainance kernel thread like migration and (sometimes)
> watchdog or ksoftirq.
> 
> Since SCHED_DEADLINE is now the highest priority scheduling class
> these tasks have to be handled therein, but it is not obvious how
> to choose a runtime and a deadline that guarantee what explained
> above. Therefore, we need a mean of recognizing system tasks inside
> the -deadline class and always run them as soon as possible, without
> any kind of runtime and bandwidth limitation.
> 
> This patch:
>  - adds the SF_HEAD flag, which identify a special task that need
>    absolute prioritization among any other task;
>  - ensures that special tasks always preempt everyone else (and,
>    obviously, are not preempted by non special tasks);
>  - disables runtime and bandwidth checking for such tasks, hoping
>    that the interference they cause is small enough.
> 

Yet in the previous patch you had this hunk:

> +++ b/kernel/sched_stoptask.c
> @@ -81,7 +81,7 @@ get_rr_interval_stop(struct rq *rq, struct
> task_struct *task)
>   * Simple, special scheduling class for the per-CPU stop tasks:
>   */
>  static const struct sched_class stop_sched_class = {
> -       .next                   = &rt_sched_class,
> +       .next                   = &dl_sched_class,
>  
>         .enqueue_task           = enqueue_task_stop,
>         .dequeue_task           = dequeue_task_stop,

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