linux-kernel - Re: [PATCH v4 07/10] Powerpc/numa: Detect support for coregroup

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Message-ID: <20200731091859.GG14603@linux.vnet.ibm.com>
Date:   Fri, 31 Jul 2020 14:48:59 +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 07/10] Powerpc/numa: Detect support for coregroup

* Michael Ellerman <mpe@...erman.id.au> [2020-07-31 17:49:55]:

> Srikar Dronamraju <srikar@...ux.vnet.ibm.com> writes:
> > Add support for grouping cores based on the device-tree classification.
> > - The last domain in the associativity domains always refers to the
> > core.
> > - If primary reference domain happens to be the penultimate domain in
> > the associativity domains device-tree property, then there are no
> > coregroups. However if its not a penultimate domain, then there are
> > coregroups. There can be more than one coregroup. For now we would be
> > interested in the last or the smallest coregroups.
> 
> This still doesn't tell me what a coregroup actually represents.
> 
> I get that it's a grouping of cores, and that the device tree specifies
> it for us, but grouping based on what?
> 

We have just abstracted the fact that we are creating a sub-group of cores
within a DIE. We are limiting to one sub-group per core. However this would
allow the firmware the flexibility to vary the grouping. Once the firmware
starts using this group, we could add more code to detect the type of
grouping and adjust the sd domain flags accordingly.

> I think the answer is we aren't being told by firmware, it's just a
> grouping based on some opaque performance characteristic and we just
> have to take that as given.
> 

This is partially true. At this time, we dont have firmwares that can
exploit this code. Once the firmwares start using this grouping, we could
add more code to align the grouping to the scheduler topology.

> But please explain that clearly in the change log and the code comments.
> 

Okay, I will do the needful.

> cheers
> 

-- 
Thanks and Regards
Srikar Dronamraju