Re: 2.6 preemptive kernel on mips

[Jun Sun <jsun@mvista.com>]

On Wed, Aug 04, 2004 at 02:51:40PM -0700, Tom Rini wrote:
> On Tue, Aug 03, 2004 at 12:40:48PM -0700, Jun Sun wrote:
> 
> > On Tue, Aug 03, 2004 at 12:22:44PM -0700, Song Wang wrote:
> > > Hi,
> > > 
> > > Has anyone tried to enable kernel preemption on
> > > Linux mips 2.6 kernel (mips32) and test it? If
> > > so, which version does it work?
> > > 
> > > I tried on 2.6.3 and it didn't work.
> > > 
> > 
> > Try the latest kernel.  I checked preemption around 2.6.5 time
> > and I believe all the obvious problems are fixed then.
> > 
> > There are still some issues with both SMP and PREEMPT, but most
> > people won't see them in normal cases.
> 
> MIPS or generic?  It's claimed, at least, that SMP&&PREEMPT have no
> fatal, generic, issues now (I forget if that was the case around 2.6.5).
> 

It is MIPS specific problems I was referring to (such as unsafe 
smp_processor_id() reference etc).

If you think about it the real problem is that kernel has non-migratable
regions, a section where process should not migrate from one CPU to
another.  Before preemtible kernel is introduced such non-migratable
regions are not a problem because they can't migrate during those
regions.

So a potentially better solution is to introduce non-migratable
regions during which scheduler promises not to migrate the processes.
Under such promises a process can actually be preempted while
it is in such a region.

Jun