Subject: Re: [PATCH v4 09/10] Powerpc/smp: Create coregroup domain

From: Valentin Schneider <[email-masked]>
Date: Tue, 28 Jul 2020 16:03:11 +0100

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]: [email-masked]