RE: [PATCH] Fix hotplug cpu on x86_64

["Protasevich, Natalie" <Natalie.Protasevich@UNISYS.com>]

> > sure convenient. I though at the time it would be great if
> > alloc_percpu() mechanism was able to dynamically re-create all the 
> > per_cpu's for new processors, that way cpu_possible_map woun't 
> > probably even be needed. Or is it too much trouble for too 
> little gain...
> 
> The problem is to tell all subsystems to reinitialize the 
> data for possible CPUs (essentially the concept of "possible" 
> CPUs would need to go)  It would need an reaudit of a lot of 
> code. Probably quite some work.

You know Andi, I was imagining something like bitmap or linked list of
all per_cpu vars (dynamically updated) and just going through this
list... Or something like that (maybe some registration mechanism).
There are not too many of them - about two dozens, mostly all sorts of
accounting.

> I think it is better to try to figure out how many hotplug 
> CPUs are supported, otherwise use a small default.

Exactly - such as on ES7000, it can support 32, 64, 128 etc. processors
depending on what configuration the customer actually ordered :)... it
should be something for that, then NR_CPUS could be either defined as
boot parameter or belong to subarchs.
Thanks,
--Natalie
  
> My 
> current code uses disabled CPUs as a heuristic, otherwise 
> half of available CPUs and it can be overwritten by the user.
> 
> -Andi
> 
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