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

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Message-ID: <20200703192651.GA5207@pc636>
Date:   Fri, 3 Jul 2020 21:26:51 +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 07:38:14PM +0200, Peter Zijlstra wrote:
> 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.
>
Yes, i see that such information is propagated by the BIOS to
the kernel, at least for x86 systems. Thad sad if i have single
socket system then we do not have any physical hotplug ability,
thus there is no need in over-provisioning.

Agree that it can be hard to fix, since all that depends on ACPI
interface. Like you stated in another mail. 

--
Vlad Rezki