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

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Date:   Fri, 3 Jul 2020 17:57:49 +0200
From:   Uladzislau Rezki <urezki@...il.com>
To:     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: nr_cpu_ids vs AMD 3970x(32 physical CPUs)

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
...
[    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