Re: [Xenomai-core] [RFC] shadow threads with prio 0 / SCHED_NORMAL

[Philippe Gerum <rpm@xenomai.org>]

Gilles Chanteperdrix wrote:
> Philippe Gerum wrote:
>  > > What about keeping SCHED_RR as the default scheduling policy and
>  > > requiring users to manually select SCHED_NORMAL in thread creation
>  > > attributes in order to create hybrid threads with pthread_create ?
>  > > 
>  > 
>  > No objection a priori, but what would this buy us?
> 
> As a user, I would expect pthread_create and pthread_setschedparam to
> allow the same scheduling policies and priorities.
> 

Is your concern about having the POSIX skin in kernel space currently 
assuming that SCHED_OTHER == SCHED_RR, which would be different than 
Linux's perception in that case?

However, I really think that we should not rely on implicit rules when 
we could avoid it (e.g. SCHED_RR as default), especially when those 
rules are going to be most often overlooked (people are rather used to 
default to SCHED_NORMAL/OTHER in the Linux sense).

What I suggested was to let people create normal threads using 
pthread_create (likely conforming to the SCHED_OTHER policy), then use 
the redirected pthread_setschedparam syscall (i.e. always applied to the 
current thread) to promote them as Xenomai shadows, but leave them in 
their original scheduling class. The same goes for rt_task_shadow. This 
would be explicit actions that would not leave much room for "surprises".

-- 

Philippe.