linux-kernel - Re: [RFC/PATCH] powerpc/smp: Add SD_SHARE_PKG

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Date:   Mon, 12 Apr 2021 11:54:36 +0530
From:   Srikar Dronamraju <srikar@...ux.vnet.ibm.com>
To:     "Gautham R. Shenoy" <ego@...ux.vnet.ibm.com>
Cc:     Michael Ellerman <mpe@...erman.id.au>,
        Michael Neuling <mikey@...ling.org>,
        Mel Gorman <mgorman@...hsingularity.net>,
        Rik van Riel <riel@...riel.com>,
        Valentin Schneider <valentin.schneider@....com>,
        Vincent Guittot <vincent.guittot@...aro.org>,
        Dietmar Eggemann <dietmar.eggemann@....com>,
        Nicholas Piggin <npiggin@...il.com>,
        Anton Blanchard <anton@...abs.org>,
        Parth Shah <parth@...ux.ibm.com>,
        Vaidyanathan Srinivasan <svaidy@...ux.vnet.ibm.com>,
        LKML <linux-kernel@...r.kernel.org>,
        linuxppc-dev@...ts.ozlabs.org
Subject: Re: [RFC/PATCH] powerpc/smp: Add SD_SHARE_PKG_RESOURCES flag to MC
 sched-domain

* Gautham R. Shenoy <ego@...ux.vnet.ibm.com> [2021-04-02 11:07:54]:

> 
> To remedy this, this patch proposes that the LLC be moved to the MC
> level which is a group of cores in one half of the chip.
> 
>       SMT (SMT4) --> MC (Hemisphere)[LLC] --> DIE
> 

I think marking Hemisphere as a LLC in a P10 scenario is a good idea.

> While there is no cache being shared at this level, this is still the
> level where some amount of cache-snooping takes place and it is
> relatively faster to access the data from the caches of the cores
> within this domain. With this change, we no longer see regressions on
> P10 for applications which require single threaded performance.

Peter, Valentin, Vincent, Mel, etal

On architectures where we have multiple levels of cache access latencies
within a DIE, (For example: one within the current LLC or SMT core and the
other at MC or Hemisphere, and finally across hemispheres), do you have any
suggestions on how we could handle the same in the core scheduler?

-- 
Thanks and Regards
Srikar Dronamraju