Re: [PATCH 0/6] x86/cpu hotplug: Wake up offline CPU via mwait or nmi

[Paul E. McKenney]

On Tue, Jun 05, 2012 at 09:49:16PM +0200, Peter Zijlstra wrote:
> On Tue, 2012-06-05 at 21:43 +0200, Thomas Gleixner wrote:
> > Vs. the interrupt/timer/other crap madness:
> > 
> >  - We really don't want to have an interrupt balancer in the kernel
> >    again, but we need a mechanism to prevent the user space balancer
> >    trainwreck from ruining the power saving party.
> 
> What's wrong with having an interrupt balancer tied to the scheduler
> which optimistically tries to avoid interrupting nohz/isolated/idle
> cpus?

Such an interrupt balancer would be a good thing, but I don't believe
that it will be sufficient.

> >  - The timer issue is mostly solved by the existing nohz stuff
> >    (plus/minus the few bugs in there).
> 
> Its not.. if you create an isolated domain there's no way to expel
> existing timers from there.

OK, I'll bite...  Why not just use CPU hotplug to expel the timers?

(Sorry, but you just can't expect me to pass that one up!)

> >  - The other details (silly IPIs) and cross CPU timer arming) are way
> >    easier to solve by a proper prohibitive state than by chasing that
> >    nonsense all over the tree forever. 
> 
> But we need to solve all that without a prohibitibe state anyway for the
> isolation stuff to be useful.

I bet that we will end up having to do both.

							Thanx, Paul

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