Re: [ANNOUNCE] Linux 2.6 Real Time Kernel

[Karim Yaghmour <karim@opersys.com>]

Lee Revell wrote:
> In theory, I think yes, if all IRQs on the system run in threads except
> the saw interrupt, and the RT task that controls the saw runs at a
> higher priority than all the IRQ threads.  You can guarantee that other
> interrupts won't delay the saw, because the saw irq is the only thing on
> the system that runs in interrupt context.  With the current VP
> implementation you are still bounded by the longest non-preemptible code
> path in the kernel AKA the longest time that a spinlock is held. 
> Replacing most spinlocks with mutexes reduces this to less than 20 code
> paths according to Mvista, which then can be individually audited for
> RT-safeness. 
> 
> That being said, no way would I put my hand under the saw with the
> current implementation.  But, unless I am missing something, it seems
> like this kind of determinism is possible with the Mvista design.

It may be a question of taste, but even if that did work, which I am
not convinced of, it seems to me that it's awfully convoluted.
With the current interrupt pipeline mechanism part of Adeos, on
which RTAI and RTAI fusion are built, I can give you absolute hard-rt
deterministic guarantees while keeping the spinlocks intact, and not
having to check for the rt-safeness of any part of the kernel. You
just write the time-sensitive saw driver int handler in front of
Linux in the ipipe and you're done: 100% deterministic hard-rt,
regardless of the application load and the driver set.

> I will check that out, I have not looked at RTAI in over a year.

Here are some interesting links:

RTAI/fusion presentation by Philipppe Gerum last July (see slide 25
for some interesting numbers):
http://www.enseirb.fr/~kadionik/rmll2004/presentation/philippe_gerum.pdf
Here's a thread that explains the details about RTAI/fusion:
https://mail.rtai.org/pipermail/rtai/2004-June/thread.html#7909
Here's the ipipe core API:
http://home.gna.org/adeos/doc/api/interface_8h.html

Karim
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