Re: [PATCH v2 1/5] x86: replace timeouts when booting secondary CPU with infinite wait loop

[Andi Kleen]

On Wed, Apr 02, 2014 at 11:29:56PM +0200, Igor Mammedov wrote:
> On Wed, 02 Apr 2014 10:15:29 -0700
> Andi Kleen <andi@xxxxxxxxxxxxxx> wrote:
> 
> > Igor Mammedov <imammedo@xxxxxxxxxx> writes:
> > 
> > > Hang is observed on virtual machines during CPU hotplug,
> > > especially in big guests with many CPUs. (It reproducible
> > > more often if host is over-committed).
> > >
> > > It happens because master CPU gives up waiting on
> > > secondary CPU and allows it to run wild. As result
> > > AP causes locking or crashing system. For example
> > > as described here: https://lkml.org/lkml/2014/3/6/257
> > >
> > > If master CPU have sent STARTUP IPI successfully,
> > > make it wait indefinitely till AP boots.
> > 
> > 
> > But what happens on a real machine when the other CPU is dead?
> One possible way to boot such machine would be to disable dead CPU
> in kernel parameters.

That would need explicit user action. It's much better to recover 
automatically, even if somewhat crippled.

-Andi

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