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

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Date:   Fri, 3 Jul 2020 21:05:12 +0200
From:   peter enderborg <peter.enderborg@...y.com>
To:     Uladzislau Rezki <urezki@...il.com>,
        <linux-kernel@...r.kernel.org>, <linux-mm@...ck.org>
CC:     Andrew Morton <akpm@...ux-foundation.org>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        <GregKroah-Hartmangregkh@...uxfoundation.org>,
        <peterz@...radead.org>
Subject: Re: nr_cpu_ids vs AMD 3970x(32 physical CPUs)

On 7/3/20 5:57 PM, 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
> [    0.000000] setup_percpu: NR_CPUS:512 nr_cpumask_bits:512 nr_cpu_ids:128 nr_node_ids:1

My 3950 do


[    0.005271] ACPI: SSDT 0x00000000CA1F5000 0003F1 (v02 ALASKA CPUSSDT  01072009 AMI  01072009)
[    0.108266] smpboot: Allowing 32 CPUs, 0 hotplug CPUs
[    0.111384] setup_percpu: NR_CPUS:8192 nr_cpumask_bits:32 nr_cpu_ids:32 nr_node_ids:1

(Fedora F32  5.6.18-300) What motherboard and BIOs do you use?


> ...
> [    0.000000] SLUB: HWalign=64, Order=0-3, MinObjects=0, CPUs=128, Nodes=1
> [    0.000000] rcu:     RCU restricting CPUs from NR_CPUS=512 to nr_cpu_ids=128.
> [    0.000000] rcu: Adjusting geometry for rcu_fanout_leaf=16, nr_cpu_ids=128
> urezki@...38:~$
> <snip>
>
> For example SLUB thinks that it deals with 128 CPUs in the system what is
> wrong if i do not miss something. Since nr_cpu_ids is broken(?), thus the
> "cpu_possible_mask" does not correspond to reality as well.
>
> Any thoughts?
>
> Thanks!
>
> --
> Vlad Rezki