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

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Date:   Fri, 3 Jul 2020 18:56:27 +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 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.