Re: [patch] Real-Time Preemption, -RT-2.6.10-rc2-mm3-V0.7.32-6

[Ingo Molnar <mingo@elte.hu>]

* K.R. Foley <kr@cybsft.com> wrote:

> [...] it looks to me like the whole scheduler is built on the premise
> of allowing RT tasks to be just like other tasks with a few
> exceptions, one of which is that RT tasks never hit the expired task
> array.

this is more or less correct, and we are trying to keep RT scheduling
'integrated' into the SCHED_NORMAL scheduler as long as it's practical.

but Lee is correct too in that the scheduling behavior of RT tasks and
SCHED_NORMAL tasks is completely different. But on the implementational
level the distinction is less stark and boils down to a few branches
here and there, while 90% of the scheduling code is shared.

to answer your question: it is true that if there is an RT task active
then we never switch the arrays. That's how RT tasks work: they run
until they want. That's why it needs privileges to use RT scheduling,
and that's why a buggy RT task can lock up the system. The 'array
switching' mechanism is part of the 10% of code that is only used by
non-RT tasks. SCHED_FIFO tasks dont have any timeslices, they run until
they deschedule voluntarily. SCHED_RR tasks have a notion of timeslices
but they only yield to RR tasks on their own priority level, which is
implemented without an array-switch. [the array-switch implements fair
scheduling between different-priority SCHED_NORMAL tasks - this is a
fundamentally harder problem which necessiates more work from the
scheduler.]

(btw., the 'global RT balancing' SMP code i recently added, and the
priority inheritance scheduler code increases the 10% of non-shared
scheduling code to perhaps 15% or so. Not always is it possible to share
code.)

	Ingo