linux-kernel - Re: [PATCH 0/2] Skip offline cores when enabling SMT on PowerPC

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Date: Mon, 1 Jul 2024 10:24:49 +0530
From: Shrikanth Hegde <sshegde@...ux.ibm.com>
To: Thomas Gleixner <tglx@...utronix.de>,
        "Nysal Jan K.A."
 <nysal@...ux.ibm.com>
Cc: Tyrel Datwyler <tyreld@...ux.ibm.com>,
        Michal Suchanek
 <msuchanek@...e.de>,
        Nicholas Piggin <npiggin@...il.com>,
        Christophe Leroy <christophe.leroy@...roup.eu>,
        "Naveen N. Rao" <naveen.n.rao@...ux.ibm.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Laurent Dufour
 <ldufour@...ux.ibm.com>, linuxppc-dev@...ts.ozlabs.org,
        linux-kernel@...r.kernel.org, Michael Ellerman <mpe@...erman.id.au>
Subject: Re: [PATCH 0/2] Skip offline cores when enabling SMT on PowerPC



On 6/25/24 2:54 AM, Thomas Gleixner wrote:
> On Tue, Jun 25 2024 at 00:41, Shrikanth Hegde wrote:
>> On 6/24/24 1:44 AM, Thomas Gleixner wrote:
>>> Right. So changing it not to online a thread when the full core is
>>> offline should not really break stuff.
>>>
>>> OTH, the mechanism to figure that out on x86 is definitely different and
>>> more complicated than on power because the sibling threads are not
>>> having consecutive CPU numbers.
>>
>> wouldn't topology_sibling_cpumask have this info? 
>> If the mask is empty does it mean the core is offline? 
> 
> The mask is not yet available for the to be brought up CPU. That's
> established when the CPU boots. It might work because all threads are
> brought up during early boot for !~*&^!@% reasons, but then it won't
> work under all circumstances, e.g. 'maxcpus=$N'.
> 
> We could fix that now with the new topology enumeration code, but that's
> a larger scale project.

Ok. 

Can we please document the behavior on different platforms i.e on x86 and PowerPC? 
May be in ABI/testing/sysfs-devices-system-cpu?