preemption stable under 5.3?

[Stephan Uphoff]

On Mon, 2004-11-08 at 16:11, Julian Elischer wrote: 
> Robert Watson wrote:
> 
> >On Mon, 8 Nov 2004, Mipam wrote:
> >
> >  
> >
> >>Thanks for your reply, okay, then i'd like to enable preemption.  I
> >>noticed it's not in the GENERIC kernel config file.  So: options
> >>PREEMPTION would suffice to enable it i guess?  Any experience with
> >>preemption. noticable changes?  So the problem: "PREEMPTION triggers
> >>frequent hangs" is resolved?  Btw, is RELENG_5 also stable or only for
> >>early adopters?  I really would like to see ule working stable in
> >>combination with preemption, but in 5.3 it won't happen. Maybe ule will
> >>be enabled later in the 5 series? 
> >>    
> >>
> >
> >There was a series of bugs in the scheduler which got tickled by
> >preemption; I'm unclear as to whether they were all resolved before 5.3 or
> >whether they require fixes in HEAD that haven't yet been merged.  It may
> >well be safe, but I make no promises.  Hopefully we can trick Julian or
> >John into responding to this thread. :-)  Having it off by default on 5.3
> >is certainly the more conservative (and reasonable) position, but if it
> >helps your environment and appears stable, there should be no reason not
> >to turn it on.  It should substantially improve latency in interrupt
> >processing as well as packet processing.
> >
> 
> I think that PREEMPTION with SCHED_4BSD might be ok..
> It's hard to say because it's always harder to prove something correct 
> than to prove it broken :-)

I agree - PREEMPTION should be ok (On head and on 5.3) as far as the
SCHED_4BSD is concerned. ( I ran most tests with PREEMPTION and
FULL_PREEMPTION)

However running with PREEMPTION may expose locking problems elsewhere. I
think at least PREEMPTION (and FULL_PREEMPTION?) should be made standard
in head for the archs that support them.
( Otherwise locking will never really get exercised on UP systems) 

> Hopefully with the rush off, we can sit down and try "prove it ok" and 
> take some cleanup passes over it.
> I still owe my wife a significant "chunk-o-time" (TM) however so count 
> me out for a while .
> Hopefully however ups@ is coming online again this week.

I recovered enough to at least debug problems and will resume coding
tomorrow. Have fun - I will try to hold the ford ;-)

	Stephan