Re: calibrate_migration_costs takes ages on s390

[Heiko Carstens <heiko.carstens@de.ibm.com>]

> > The boot sequence on s390 sometimes takes ages and we spend a very 
> > long time (up to one or two minutes) in calibrate_migration_costs. The 
> > time spent there differs from boot to boot. Also the calculated costs 
> > differ a lot. I've seen differences by up to a factor of 15 (yes, 
> > factor not percent). Also I doubt that making these measurements make 
> > much sense on a completely virtualized architecture where you cannot 
> > tell how much cpu time you will get anyway. Is there any workaround or 
> > fix available so we can avoid seeing this?
> 
> which is the precise kernel version used? We toned down calibration a 
> bit recently.

2.6.16-rc3.

> The immediate workaround would be to use the migration_cost=0 boot 
> parameter.
> 
> Generally, i agree that it makes sense to not calibrate at all on 
> virtual platforms. Does the patch below help?  It gives virtual 
> platforms a way to provide a default migration cost and thus avoid the 
> boot-time calibration altogether. (I have tested it on x86, it does the 
> expected thing.) This needs to hit v2.6.16 too.

Yes, calibrate_migration_costs is very fast now. But it turned out that
this was just hiding the real problem: if we have CONFIG_PREEMPT disabled
the kernel gets (sometimes) unbelievably slow.
I think this happened somewhere between rc1 and rc3. Maybe Hannes knows
more exactly when this happened the first time, since I always run with
CONFIG_PREEMPT enabled.

Heiko