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SubjectRe: [PATCH v4 09/10] Powerpc/smp: Create coregroup domain
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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@linux.vnet.ibm.com

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