Re: [patch 1/5] sched: remove degenerate domains

[Nick Piggin <nickpiggin@yahoo.com.au>]

Ingo Molnar wrote:
> * Nick Piggin <nickpiggin@yahoo.com.au> wrote:
> 
> 
>>This is Suresh's patch with some modifications.
> 
> 
>>Remove degenerate scheduler domains during the sched-domain init.
> 
> 
> actually, i'd suggest to not do this patch. The point of booting with a 
> CONFIG_NUMA kernel on a non-NUMA box is mostly for testing, and the 
> 'degenerate' toplevel domain exposed conceptual bugs in the 
> sched-domains code. In that sense removing such 'unnecessary' domains 
> inhibits debuggability to a certain degree. If we had this patch earlier 
> we'd not have experienced the wrong decisions taken by the scheduler, 
> only on the much rarer 'really NUMA' boxes.
> 

True. Although I'd imagine it may be something distros may want.
For example, a generic x86-64 kernel for both AMD and Intel systems
could easily have SMT and NUMA turned on.

I agree with the downside of exercising less code paths though.

What about putting as a (default to off for 2.6) config option in
the config embedded menu?

> is there any case where we'd want to simplify the domain tree? One more 
> domain level is just one (and very minor) aspect of CONFIG_NUMA - i'd 
> not want to run a CONFIG_NUMA kernel on a non-NUMA box, even if the 
> domain tree got optimized. Hm?
> 

I guess there is the SMT issue too, and even booting an SMP kernel
on a UP system. Also small ia64 NUMA systems will probably have one
redundant NUMA level.

If/when topologies get more complex (for example, the recent Altix
discussions we had with Paul), it will be generally easier to set
up all levels in a generic way, then weed them out using something
like this, rather than put the logic in the domain setup code.

Nick

-- 
SUSE Labs, Novell Inc.