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SubjectRe: Attempted summary of "RT patch acceptance" thread
On Sun, Jun 12, 2005 at 09:35:15PM -0400, Karim Yaghmour wrote:
> Paul E. McKenney wrote:
> > This could potentially address the need for version-synchronization
> > between RTAI-Fusion and the Linux kernel. Would you really want two
> > separate builds, or is there some reasonable way of producing a single
> > kernel binary that has both? And if there is some reasonable way of
> > doing this, is it the right thing to do?
> No, single build is what I'm looking for. Nothing precludes the
> fusion parts from being built during the same kernel build ...
> as modules. If you don't load 'em, you don't need to worry about
> 'em.

OK... Then the idea is to dynamically redirect the symbolic link
to include/linux-srt or include/linux-srt that you mentioned in your
previous email, or is the symlink serving some other purpose?

> > The single-binary approach could potentially reduce the
> > dual-OS-administration load associated with RTAI-Fusion. However,
> > handling all the interactions between the deterministic and
> > non-deterministic system calls could get hairy. No big deal for
> > scheduling primitives, but things could get interesting for I/O and
> > networking protocols.
> Again, if you don't load 'em, you don't get 'em. If you use it
> and it's broken, then you're doing rt and you need to sync up
> with the maintainer. Nothing different here from the standard
> run of the mill "I'm using subsystem X and it doesn't work"
> posting to LKML.

So your focus is on system calls that can have totally separate
realtime and non-realtime implementations? Or am I missing some
trick here?

> > So, one can use the following types of combination:
> >
> > o single source tree, multiple kernels (which is what I now
> > think that you are getting at above).
> >
> > o straight merge, as between PREEMPT and PREEMPT_RT.
> >
> > o single kernel, multiple syscall implementations for
> > some syscalls (deterministic vs. non-deterministic).
> >
> > o side-by-side combination, as with dual-OS/dual-core and
> > pretty much any other approach.
> I'm not sure how you'd fit what I'm trying to suggest above, but
> let me rephrase it with the above in mind:
> What I'm suggesting is that all rt components be included, but
> in separate directories within mainline. That may or may not
> mean additional schedulers/services. In the case where the
> new layout would include both PREEMPT_RT and fusion, what
> we'd get is that the user would have access to these configs:
> - Plain Linux, no PREEMPT_RT, no ipipe, no fusion.
> - Linux with PREEMPT_RT, no fusion: ints are threaded and locks
> are mutexes like now (however without the code intrusiveness
> given the use of separate directories.) May or may not include
> ipipe.
> - Linux with fusion, no PREEMPT_RT: the fusion modules are built
> and installed with the rest of the modules. ipipe must be
> enabled.
> - Linux with fusion and PREEMPT_RT: combination of the previous
> two.
> - Linux with ipipe, no fusion or PREEMPT_RT: the soft-cli stuff
> is built into the kernel and loaded drivers can get
> deterministic response times, but there are no fancy rt
> services offered to anyone.

Single kernel, multiple implementations for some syscalls, more
or less, anyway.

> Practically, linux/hard-rt/ would contain both the code for
> PREEMPT_RT and the code for fusion. The actual layout in that
> directory would still need to be detailed, but the desired
> effect is that both PREEMPT_RT and fusion share as much code
> as possible.
> Hope this clarifies what I'm suggesting a little bit more. Of
> course, all this would need to be rehashed a number of times,
> and most importantly, the PREEMPT_RT folks and the fusion
> effort would need to agree to join forces. From the fusion
> POV, it's clear that the door is open for collaboration. As
> proof, Philippe has been publishing combo patches with Adeos
> and PREEMPT_RT for some time. I can't speak for the PREEMPT_RT
> POV, though. I might be mistaken, but it seems that the feedback
> I've seen from some PREEMPT_RT backers does seem to indicate
> some openess. We'll see how things go.

My guess is that there are enough people in the PREEMPT_RT camp that
it might not make sense to ascribe a single point of view to them ;-)

How are you and Kristian looking to benchmark/compare the various
combinations you call out above? Seems like one would have to look
at more than straight scheduling/interrupt latency to make a reasonable

Thanx, Paul
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