linux-kernel - Re: [PATCH v2 2/6] powerpc/smp: Enable Asym packing for cores on shared processor

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Date:   Thu, 19 Oct 2023 22:50:43 +1100
From:   Michael Ellerman <mpe@...erman.id.au>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     Srikar Dronamraju <srikar@...ux.vnet.ibm.com>,
        linuxppc-dev <linuxppc-dev@...ts.ozlabs.org>,
        Christophe Leroy <christophe.leroy@...roup.eu>,
        Nicholas Piggin <npiggin@...il.com>,
        Rohan McLure <rmclure@...ux.ibm.com>,
        Valentin Schneider <vschneid@...hat.com>,
        Josh Poimboeuf <jpoimboe@...nel.org>,
        Mark Rutland <mark.rutland@....com>,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2 2/6] powerpc/smp: Enable Asym packing for cores on
 shared processor

Peter Zijlstra <peterz@...radead.org> writes:
> On Thu, Oct 19, 2023 at 03:38:40PM +1100, Michael Ellerman wrote:
>> Srikar Dronamraju <srikar@...ux.vnet.ibm.com> writes:
>> > If there are shared processor LPARs, underlying Hypervisor can have more
>> > virtual cores to handle than actual physical cores.
>> >
>> > Starting with Power 9, a core has 2 nearly independent thread groups.
>> 
>> You need to be clearer here that you're talking about "big cores", not
>> SMT4 cores as seen on bare metal systems.
>
> What is a 'big core' ? I'm thinking big.LITTLE, but I didn't think Power
> went that route (yet?).. help?

No it's not big.LITTLE :)

It means we have two SMT4 cores glued together that behave as a single
SMT8 core, a system is either in "big core" mode or it's not, it's never
heterogeneous.

If you grep for "big_core" there's some code in the kernel for dealing
with it, though it's probably not very illuminating.

Possibly we should switch to using the "thread group" terminology, so
that it doesn't confuse folks about big.LITTLE. Also the device tree
property that we use to discover if we're using big cores is called
ibm,thread-groups.

cheers