linux-kernel - Re: nr_cpu_ids vs AMD 3970x(32 physical CPUs)

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Date:   Fri, 3 Jul 2020 19:38:14 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Uladzislau Rezki <urezki@...il.com>
Cc:     linux-kernel@...r.kernel.org, linux-mm@...ck.org,
        Andrew Morton <akpm@...ux-foundation.org>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        GregKroah-Hartmangregkh@...uxfoundation.org
Subject: Re: nr_cpu_ids vs AMD 3970x(32 physical CPUs)

On Fri, Jul 03, 2020 at 07:09:41PM +0200, Uladzislau Rezki wrote:
> On Fri, Jul 03, 2020 at 06:56:27PM +0200, Peter Zijlstra wrote:
> > On Fri, Jul 03, 2020 at 05:57:49PM +0200, Uladzislau Rezki wrote:
> > > Hello, folk.
> > > 
> > > I have a system based on AMD 3970x CPUs. It has 32 physical cores
> > > and 64 threads. It seems that "nr_cpu_ids" variable is not correctly
> > > set on latest 5.8-rc3 kernel. Please have a look below on dmesg output:
> > > 
> > > <snip>
> > > urezki@...38:~$ sudo dmesg | grep 128
> > > [    0.000000] IOAPIC[0]: apic_id 128, version 33, address 0xfec00000, GSI 0-23
> > > [    0.000000] smpboot: Allowing 128 CPUs, 64 hotplug CPUs
> > 
> > This is your BIOS saying it needs 128 ids, 64 of which are 'empty'.
> > 
> > I have a box like that as well, if it bothers you boot with:
> > "possible_cpus=64" or something.
> >
> OK, i got it. I thought that "cpu_possible_mask" strictly follows
> the rule: the number of CPUs in a system that physically are present.

Nah, it's based on ACPI (SRAT IIRC) tables. The case of
over-provisioning is useful for systems that support physical hotplug,
but I've seen boards without that capability do it too.

Just chalk it up to the foibles of BIOS.