Re: Attempted summary of "RT patch acceptance" thread

["Paul E. McKenney" <paulmck@us.ibm.com>]

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
comparison.

						Thanx, Paul