Re: hotplug support for arch/arc/plat-eznps platform

[Peter Zijlstra]

On Mon, Aug 07, 2017 at 01:41:38PM +0000, Ofer Levi(SW) wrote:
> > You've failed to explain why you think hotplug should be a performance
> > critical path.
> 1. hotplug bring up of 4K cpus takes 40 minutes.  Way too much for any user.
> 2. plat-eznps is a network processor, where bring up time is sensitive.

But who is doing actual hotplug? Why would you ever unplug or plug a CPU
in a time critical situation?

> > I'm also not seeing how it would be different from boot; you'd be
> > looking at a similar cost for SMP bringup.
> bring up time of 4k cpus during kernel boot takes 4.5 minutes. 
> The function in question is performed only when smp init was performed. 
> If I understand correctly, whatever this function is doing is performed after all cpus 
> were brought up during kernel boot.

Doesn't make sense. If you look at smp_init() boot brings up the CPUs
one at a time.

So how can boot be different than hot-pugging them?