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

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Date:   Mon, 7 Aug 2017 17:10:20 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     "Ofer Levi(SW)" <oferle@...lanox.com>
Cc:     "rusty@...tcorp.com.au" <rusty@...tcorp.com.au>,
        "vatsa@...ibm.com" <vatsa@...ibm.com>,
        "mingo@...hat.com" <mingo@...hat.com>,
        "Vineet.Gupta1@...opsys.com" <Vineet.Gupta1@...opsys.com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: hotplug support for arch/arc/plat-eznps platform

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?