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

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Date: Mon, 24 Jun 2024 23:24:11 +0200
From: Thomas Gleixner <tglx@...utronix.de>
To: Shrikanth Hegde <sshegde@...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>,
 "Nysal Jan K.A." <nysal@...ux.ibm.com>
Subject: Re: [PATCH 0/2] Skip offline cores when enabling SMT on PowerPC

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.

Thanks,

        tglx