linux-kernel - Re: [PATCH v2 06/10] powerpc/smp: Generalize 2nd sched domain

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Date:   Wed, 22 Jul 2020 13:48:22 +0530
From:   Srikar Dronamraju <srikar@...ux.vnet.ibm.com>
To:     peterz@...radead.org
Cc:     Michael Ellerman <michaele@....ibm.com>,
        linuxppc-dev <linuxppc-dev@...ts.ozlabs.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Ingo Molnar <mingo@...nel.org>,
        Valentin Schneider <valentin.schneider@....com>,
        Nick Piggin <npiggin@....ibm.com>,
        Oliver OHalloran <oliveroh@....ibm.com>,
        Nathan Lynch <nathanl@...ux.ibm.com>,
        Michael Neuling <mikey@...ux.ibm.com>,
        Anton Blanchard <anton@....ibm.com>,
        Gautham R Shenoy <ego@...ux.vnet.ibm.com>,
        Vaidyanathan Srinivasan <svaidy@...ux.ibm.com>,
        Jordan Niethe <jniethe5@...il.com>
Subject: Re: [PATCH v2 06/10] powerpc/smp: Generalize 2nd sched domain

* peterz@...radead.org <peterz@...radead.org> [2020-07-22 09:46:24]:

> On Tue, Jul 21, 2020 at 05:08:10PM +0530, Srikar Dronamraju wrote:
> > Currently "CACHE" domain happens to be the 2nd sched domain as per
> > powerpc_topology. This domain will collapse if cpumask of l2-cache is
> > same as SMT domain. However we could generalize this domain such that it
> > could mean either be a "CACHE" domain or a "BIGCORE" domain.
> 
> What's the whole smallcore vs bigcore thing?
> 
> Would it make sense to have an actual overview of the various topology
> layers somewhere? Because I'm getting lost and can't really make sense
> of the code due to that.

To quote with an example: using (Power 9)

Power 9 is an SMT 8 core by design. However this 8 thread core can run as 2
independent core with threads 0,2,4 and 6 acting like a core, while threads
1,3,5,7 acting as another core.  

The firmware can decide to showcase them as 2 independent small cores or as
one big core. However the LLC (i.e last level of cache) is shared between
all the threads of the SMT 8 core. Future power chips, LLC might change, it
may be expanded to share with another SMT 8 core or it could be made
specific to SMT 4 core.

The smt 8 core is also known as fused core/ Big core.
The smaller smt 4 core is known as small core.

So on a Power9 Baremetal, the firmware would show up as SMT4 core.
and we have a CACHE level at SMT 8. CACHE level would be very very similar
to MC domain in X86.

Hope this is clear.

-- 
Thanks and Regards
Srikar Dronamraju