linux-kernel - Re: Early boot regression from f0551af0213 ("x86/topology: Ignore non-present APIC IDs in a present package")

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Date: Thu, 2 May 2024 05:33:28 -0500
From: Mario Limonciello <mario.limonciello@....com>
To: Thomas Gleixner <tglx@...utronix.de>, Lyude Paul <lyude@...hat.com>,
 Borislav Petkov <bp@...en8.de>
Cc: x86@...nel.org, linux-kernel@...r.kernel.org
Subject: Re: Early boot regression from f0551af0213 ("x86/topology: Ignore
 non-present APIC IDs in a present package")

On 4/25/2024 16:42, Thomas Gleixner wrote:
> Lyude!
> 
> On Thu, Apr 25 2024 at 11:56, Lyude Paul wrote:
>> On Thu, 2024-04-25 at 04:11 +0200, Thomas Gleixner wrote:
>>>
>>> Can you please boot a kernel with the commit in question reverted and
>>> add 'possible_cpus=8' to the kernel command line?
>>>
>>> In theory this should fail too.
>>
>> Yep - tried booting a kernel with f0551af0213 reverted and
>> possible_cpus=8, it definitely looks like that crashes things as well
>> in the same way.
> 
> Good. That means it's a problem which existed before but went unnoticed.
> 
>> Also - it scrolled off the screen before I had a chance to write it
>> down, but I'm -fairly- sure I saw some sort of complaint about "16 [or
>> some double digit number] processors exceeds max number of 8". Which
>> is quite interesting, as this is definitely just a quad core ryzen
>> processor with hyperthreading - so there should only be 8 threads.
> 
> Right, that's what we saw with the debug patch. The ACPI/MADT table
> is clearly bonkers. The effect of it is that it pretends that the system
> has 16 possible CPUs:
> 
>      [    0.089381] CPU topo: Allowing 8 present CPUs plus 8 hotplug CPUs
> 
> Which in turn changes the sizing of the per CPU data and affects some
> other details which depend on the number of possible CPUs.

At least this aspect of this I suspect is caused by commit 
fed8d8773b8ea68ad99d9eee8c8343bef9da2c2c.

If you try reverting that I expect the "hotplug CPUs" disappear.

> 
> But that should not matter at all because the system scaling should be
> sufficient with 8 CPUs, but it does not for some completely non-obvious
> reasons.
> 
> Can you please try to increase possible_cpus=N on the command line one
> by one and check when it actually starts to "work" again.
> 
> One other thing to try is to boot with 'possible_cpus=8' and
> 'intremap=off' and see whether that makes a difference.
> 
> I really have no idea where to look and not having the early boot
> messages in case of the fail is not helpful as I can't add meaningful
> debug to it.
> 
> I just checked: the motherboard has a serial port, so it would be
> extremly helpful to hook up a serial cable to this thing and enable
> serial console on the kernel command line. That way we might eventually
> see information which is emitted before it fails to validate the timer
> interrupt.
> 
> Thanks,
> 
>          tglx
>