Re: [PATCH] mm: Print the correct method to disable automatic numa migration

[Mel Gorman <mgorman@suse.de>]

On Thu, Apr 11, 2013 at 04:04:25PM +0200, Andi Kleen wrote:
> > As David pointed out, CONFIG_NUMA_BALANCING_DEFAULT_ENABLED only comes
> > into play when CONFIG_NUMA_BALANCING is set and CONFIG_NUMA_BALANCING
> > will default to N for make oldconfig. I think it's sensible to enable it
> > by default if it's configured in.
> 
> I've got reports from users who got it unexpected and it messed 
> everything up for them.
> 

They enabled the option to have the feature and were then surprised it
was enabled? That surprises me.

> > 
> > David has also already pointed out the problems with NO_NUMA vs -NUMA and
> > the fact that the option only exists if CONFIG_SCHED_DEBUG which I agree
> > is unfortunate. Ends up with this sort of mess
> 
> We just need the sysctl. Are you adding one or should I send
> another patch with it?
> 

I hadn't planned on it in the short term at least. Originally there was
a sysctl to control the NUMA auto balancing behaviour but it was one of
the points of contention that got dropped along the way. As SCHED_DEBUG
is enabled in some distribution configs at least, it was expected the
option would generally be available even though the documentation for
/sys/kernel/debug/sched_features is non-existent. I had hoped to have
revisited NUMA balancing a long time ago but too many bugs have been
getting in the way.

-- 
Mel Gorman
SUSE Labs

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