Re: [PATCH]cpuset: add new API to change cpuset top group's cpus

[Andi Kleen]

On Wed, May 20, 2009 at 03:41:55PM +0200, Peter Zijlstra wrote:
> On Wed, 2009-05-20 at 15:13 +0200, Andi Kleen wrote:
> > Thanks for the explanation.
> > 
> > My naive reaction would be to fail if the socket to be taken out
> > is the only member of some cpuset. Or maybe break affinities in this case.
> 
> Right, breaking affinities would go against the policy of the admin, I'm
> not sure we'd want to go there. 

> We could start generating msgs about how
> we're in thermal trouble and the given configuration is obstructing
> counter measures etc..

Makes sense.

> 
> Currently hot-unplug does break affinities, but that's an explicit
> action by the admin himself, so he gets what he asks for (and we do

I have some code which can do it implicitely too in mcelog (not yet out).
Basically the CPU can detect when its caches have a problem and the reaction
is then to offline the affected CPUs. But that's a very obscure case
and the alternative is to die.

> generate complaints in syslog about it).

One possible alternative would be also "weak breaking", as in remembering
the old affinities and reinstating them once the CPU becomes online again.

> [ Same scenario for the HPC guys who affinity fix all their threads to
>   specific cpus, there's really nothing you can do there. Then again
>   such folks generally run their machines at 100% so they'd better
>   be able to deal with their thermal peak capacity anyway. ]

Yes. Same for real time. These guys are really not expected to use
these advanced power management features. 

> > So it's a bit more than a hint; it's more like a command "or else"
> > 
> > So it's a good idea to react or at least make at least a reasonable attempt 
> > to react.
> 
> Sure, does the thing give more than a: 'react now, or else' impulse?
> That is, can we see it coming, or will we have to deal with it when
> we're there?
> 
> The latter also has the problem that you have to react very quickly.

My understanding it is a quite strong hint: "do the best you can"
So yes doing it quickly would be good.

> 
> > > The thing is, you cannot simply rip cpus out from under a system, people
> > > might rely on them being there and have policy attached to them -- esp.
> > > people touching cpusets should know that a machine isn't configured
> > > homogeneous and any odd cpu will do.
> > 
> > Ok, so do you think it's possible to figure out based on the cpuset
> > graph / real time runqueue if a socket can be taken out? 
> 
> Right, so all of this depends on a number of things, how frequent and
> how fast would these situations occur?
> 
> I would think they'd be rare events, otherwise you really messed up your

My assumption too.

> infrastructure. I also think reaction times should be in the seconds,
> otherwise you're cutting it way to close.

Yep.

> I was hoping we could control the situation with that. But for that to
> work we need some gradual information in order to make that
> thermal<->overload feedback work.
> 
> 
> A single: idle a core now (< 'n' sec) or die, isn't really helpful.

That's what you get unfortuantely.

-Andi
-- 
ak@xxxxxxxxxxxxxxx -- Speaking for myself only.
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