linux-kernel - Re: [PATCH v4 09/10] Powerpc/smp: Create coregroup domain

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Message-ID: <jhjr1sviswg.mognet@arm.com>
Date:   Tue, 28 Jul 2020 16:03:11 +0100
From:   Valentin Schneider <valentin.schneider@....com>
To:     Srikar Dronamraju <srikar@...ux.vnet.ibm.com>
Cc:     Michael Ellerman <mpe@...erman.id.au>,
        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>,
        Jordan Niethe <jniethe5@...il.com>
Subject: Re: [PATCH v4 09/10] Powerpc/smp: Create coregroup domain


Hi,

On 27/07/20 06:32, Srikar Dronamraju wrote:
> Add percpu coregroup maps and masks to create coregroup domain.
> If a coregroup doesn't exist, the coregroup domain will be degenerated
> in favour of SMT/CACHE domain.
>

So there's at least one arm64 platform out there with the same "pairs of
cores share L2" thing (Ampere eMAG), and that lives quite happily with the
default scheduler topology (SMT/MC/DIE). Each pair of core gets its MC
domain, and the whole system is covered by DIE.

Now arguably it's not a perfect representation; DIE doesn't have
SD_SHARE_PKG_RESOURCES so the highest level sd_llc can point to is MC. That
will impact all callsites using cpus_share_cache(): in the eMAG case, only
pairs of cores will be seen as sharing cache, even though *all* cores share
the same L3.

I'm trying to paint a picture of what the P9 topology looks like (the one
you showcase in your cover letter) to see if there are any similarities;
from what I gather in [1], wikichips and your cover letter, with P9 you can
have something like this in a single DIE (somewhat unsure about L3 setup;
it looks to be distributed?)

     +---------------------------------------------------------------------+
     |                                  L3                                 |
     +---------------+-+---------------+-+---------------+-+---------------+
     |       L2      | |       L2      | |       L2      | |       L2      |
     +------+-+------+ +------+-+------+ +------+-+------+ +------+-+------+
     |  L1  | |  L1  | |  L1  | |  L1  | |  L1  | |  L1  | |  L1  | |  L1  |
     +------+ +------+ +------+ +------+ +------+ +------+ +------+ +------+
     |4 CPUs| |4 CPUs| |4 CPUs| |4 CPUs| |4 CPUs| |4 CPUs| |4 CPUs| |4 CPUs|
     +------+ +------+ +------+ +------+ +------+ +------+ +------+ +------+

Which would lead to (ignoring the whole SMT CPU numbering shenanigans)

NUMA     [                                                   ...
DIE      [                                             ]
MC       [         ] [         ] [         ] [         ]
BIGCORE  [         ] [         ] [         ] [         ]
SMT      [   ] [   ] [   ] [   ] [   ] [   ] [   ] [   ]
         00-03 04-07 08-11 12-15 16-19 20-23 24-27 28-31  <other node here>

This however has MC == BIGCORE; what makes it you can have different spans
for these two domains? If it's not too much to ask, I'd love to have a P9
topology diagram.

[1]: 20200722081822.GG9290@...ux.vnet.ibm.com