[BUG] rebuild_sched_domains considered dangerous

[Peter Zijlstra]

On Wed, 2011-03-09 at 14:15 +0100, Martin Schwidefsky wrote:
> On Wed, 09 Mar 2011 12:33:49 +0100
> Peter Zijlstra <peterz at infradead.org> wrote:
> 
> > On Wed, 2011-03-09 at 11:19 +0100, Peter Zijlstra wrote:
> > > > It appears that this corresponds to one CPU deciding to rebuild the
> > > > sched domains. There's various reasons why that can happen, the typical
> > > > one in our case is the new VPNH feature where the hypervisor informs us
> > > > of a change in node affinity of our virtual processors. s390 has a
> > > > similar feature and should be affected as well.
> > > 
> > > Ahh, so that's triggering it :-), just curious, how often does the HV do
> > > that to you? 
> > 
> > OK, so Ben told me on IRC this can happen quite frequently, to which I
> > must ask WTF were you guys smoking? Flipping the CPU topology every time
> > the HV scheduler does something funny is quite insane. And you did that
> > without ever talking to the scheduler folks, not cool.
> > 
> > That is of course aside from the fact that we have a real bug there that
> > needs fixing, but really guys, WTF!
> 
> Just for info, on s390 the topology change events are rather infrequent.
> They do happen e.g. after an LPAR has been activated and the LPAR
> hypervisor needs to reshuffle the CPUs of the different nodes.

But if you don't also update the cpu->node memory mappings (which I
think it near impossible) what good is it to change the scheduler
topology?