linux-kernel - Re: [PATCH v4 06/10] powerpc/smp: Generalize 2nd sched domain

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Message-ID: <20200731092901.GH14603@linux.vnet.ibm.com>
Date:   Fri, 31 Jul 2020 14:59:01 +0530
From:   Srikar Dronamraju <srikar@...ux.vnet.ibm.com>
To:     Michael Ellerman <mpe@...erman.id.au>
Cc:     linuxppc-dev <linuxppc-dev@...ts.ozlabs.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Nicholas Piggin <npiggin@...il.com>,
        Anton Blanchard <anton@...abs.org>,
        "Oliver O'Halloran" <oohall@...il.com>,
        Nathan Lynch <nathanl@...ux.ibm.com>,
        Michael Neuling <mikey@...ling.org>,
        Gautham R Shenoy <ego@...ux.vnet.ibm.com>,
        Ingo Molnar <mingo@...nel.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Valentin Schneider <valentin.schneider@....com>,
        Jordan Niethe <jniethe5@...il.com>
Subject: Re: [PATCH v4 06/10] powerpc/smp: Generalize 2nd sched domain

* Michael Ellerman <mpe@...erman.id.au> [2020-07-31 17:45:37]:

> Srikar Dronamraju <srikar@...ux.vnet.ibm.com> writes:
> > 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.
> >
> > While setting up the "CACHE" domain, check if shared_cache is already
> > set.
> 
> PeterZ asked for some overview of what you're doing and why, you
> responded to his mail, but I was expecting to see that text incorporated
> here somewhere.
> 

Okay, do you want that as part of the code or documentation dir or the
changelog?

> He also asked for some comments, which I would also like to see.
> 
> 
> I'm also not clear why we want to rename it to "bigcore", that's not a
> commonly understood term, I don't think it's clear to new readers what
> it means.
> 
> Leaving it as the shared cache domain, and having a comment mentioning
> that "bigcores" share a cache, would be clearer I think.
> 

Today, Shared cache is equal to Big Core. However in not too distant future,
Shared cache domain and Big Core may not be the same. For example lets
assume that L2 cache were to Shrink per small core with the firmware
exposing the core as a bigcore. Then with the current design, we have a SMT
== SHARED CACHE, and a DIE. We would not have any domain at the publicised 8
thread level. Keeping the Bigcore as a domain and mapping the shared
cache, (I am resetting the domain name as CACHE if BIGCORE==SHARED_CACHE),
helps us in this scenario.

> cheers
> 

-- 
Thanks and Regards
Srikar Dronamraju