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

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Message-ID: <20200703170941.GA4372@pc636>
Date:   Fri, 3 Jul 2020 19:09:41 +0200
From:   Uladzislau Rezki <urezki@...il.com>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     Uladzislau Rezki <urezki@...il.com>, 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 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.

Thanks, Peter!

--
Vlad Rezki