linux-kernel - Re: [PATCH 1/2] xen/x86: Convert to hotplug state machine

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Date:   Wed, 31 Aug 2016 18:15:06 +0200
From:   Sebastian Andrzej Siewior <bigeasy@...utronix.de>
To:     Boris Ostrovsky <boris.ostrovsky@...cle.com>
Cc:     david.vrabel@...rix.com, jgross@...e.com,
        xen-devel@...ts.xenproject.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 1/2] xen/x86: Convert to hotplug state machine

On 2016-08-26 15:37:38 [-0400], Boris Ostrovsky wrote:
> > If you do find the time, you might manage to rework the code to avoid
> > using the _nocalls() function. If see this right, you use
> > xen_setup_vcpu_info_placement() for the init in the first place. This
> > uses for_each_possible_cpu macro. The cpuhp_setup_state() function would
> > perform the init for all CPUs before they come up.
> 
> I am not sure I see what this would buy us.
> 
> Besides, cpuhp_setup_state() uses for_each_present_cpu().

Correct. So you would avoid running the init code on CPUs which are
within the for_each_possible_cpu() set but not in for_each_present_cpu().

Assuming a NUMA box with two CPUs, 8 cores each gives you 32 CPUs in
Linux with hyper threading. BIOS may report 240 CPUs as the upper limit
(possible CPUs) but if you never deploy them you don't need to
initialize them… Should they be plugged physically then the
for_each_present_cpu() loop will cover them once they come up.

Sebastian