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

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Message-ID: <20231019125459.GG2194132@linux.vnet.ibm.com>
Date:   Thu, 19 Oct 2023 18:24:59 +0530
From:   Srikar Dronamraju <srikar@...ux.vnet.ibm.com>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     Michael Ellerman <mpe@...erman.id.au>,
        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> [2023-10-19 09:48:28]:

> 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?
> 

Each independent thread group acts as a SMT4 core or a small core. A set of
2 thread groups form a SMT8 core aka big core. PowerVM aka pHYp schedules
at a big core granularity

So if we have 2 LPARS, each spanning 2 big cores, aka 16 CPUs, and if at
somepoint, each LPAR has only 2 threads to run, we are exploring if we can
run both the threads on just one big core, so that PhyP can schedule both
LPARS at the same time and avoid having to switch/multiplex between these
two LPARS.

-- 
Thanks and Regards
Srikar Dronamraju