Preemption on MV78100 (ARMv5TE)?

[Ian Lepore]

On Sat, 2013-03-23 at 11:15 -0700, MagnusNilsson wrote:
> I am looking for information about FreeBSD 8.2 on Marvell MV78100 (Discovery
> SOC) - an ARMv5TE - with preemption enabled.
> My kernel is stable, but if I enable preemption it promptly crashes - during
> or within minutes of boot.
> 
> My configuration is similar to:
> http://svn.freebsd.org/base/head/sys/arm/conf/DB-78XXX
> I notice that this configuration does not have the PREEMPTION option - not
> even commented - whereas many other configurations in:
> http://svn.freebsd.org/base/head/sys/arm/conf/
> do have it.
> 
> I am interested to hear if anyone has been able to run MV78100 (or similar)
> with preemption enabled - and in that case what your configuration and
> FreeBSD version is - or if anyone can confirm that preemption is not
> implemented for this processor.
> 
> Thank you.

Can you provide any information about the crashes?  In my experience
with 8.2 on arm (extensive, we use it at work, but with a variety of
fixes pulled in from 9 and 10), the main source of problems has been
busdma cache coherency problems.  A variety of such problems have been
fixed since 8.2, but I'm not sure all the fixes have been merged back to
the 8-stable branch.

I've never tried enabling PREEMPTION, I'll give that a try.

I'm not sure why you're enabling it, but something to try that may give
you most of the same benefits would be to compile with HZ=1000 or even
higher, to shorten the scheduling quantum.  Most of our ARM configs have
HZ=100, but I've successfully run with HZ=4000 even on a low-end at91
chip running at 180mhz (just to see if I could, we don't normally do
that on our products at work).

-- Ian