Re: [PATCH v2 3/7] cpu/hotplug: Add dynamic parallel bringup states before CPUHP_BRINGUP_CPU

[David Woodhouse]

On Tue, 2021-12-14 at 14:24 +0000, Mark Rutland wrote:
> On Tue, Dec 14, 2021 at 12:32:46PM +0000, David Woodhouse wrote:
> > From: David Woodhouse <
> > dwmw@xxxxxxxxxxxx
> > >
> > 
> > If the platform registers these states, bring all CPUs to each registered
> > state in turn, before the final bringup to CPUHP_BRINGUP_CPU. This allows
> > the architecture to parallelise the slow asynchronous tasks like sending
> > INIT/SIPI and waiting for the AP to come to life.
> > 
> > There is a subtlety here: even with an empty CPUHP_BP_PARALLEL_DYN step,
> > this means that *all* CPUs are brought through the prepare states and to
> > CPUHP_BP_PREPARE_DYN before any of them are taken to CPUHP_BRINGUP_CPU
> > and then are allowed to run for themselves to CPUHP_ONLINE.
> > 
> > So any combination of prepare/start calls which depend on A-B ordering
> > for each CPU in turn, such as the X2APIC code which used to allocate a
> > cluster mask 'just in case' and store it in a global variable in the
> > prep stage, then potentially consume that preallocated structure from
> > the AP and set the global pointer to NULL to be reallocated in
> > CPUHP_X2APIC_PREPARE for the next CPU... would explode horribly.
> > 
> > We believe that X2APIC was the only such case, for x86. But this is why
> > it remains an architecture opt-in. For now.
> 
> It might be worth elaborating with a non-x86 example, e.g.
> 
> >  We believe that X2APIC was the only such case, for x86. Other architectures
> >  have similar requirements with global variables used during bringup (e.g.
> >  `secondary_data` on arm/arm64), so architectures must opt-in for now.
> 
> ... so that we have a specific example of how unconditionally enabling this for
> all architectures would definitely break things today.

I do not have such an example, and I do not know that it would
definitely break things to turn it on for all architectures today.

The x2apic one is an example of why it *might* break random
architectures and thus why it needs to be an architecture opt-in.

> FWIW, that's something I would like to cleanup for arm64 for general
> robustness, and if that would make it possible for us to have parallel bringup
> in future that would be a nice bonus.

Yes. But although I lay the groundwork here, the arch can't *actually*
do parallel bringup without some arch-specific work, so auditing the
pre-bringup states is the easy part. :)

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