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

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Message-ID: <87r1sr53b2.fsf@mpe.ellerman.id.au>
Date:   Fri, 31 Jul 2020 21:31:13 +1000
From:   Michael Ellerman <mpe@...erman.id.au>
To:     Srikar Dronamraju <srikar@...ux.vnet.ibm.com>
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

Srikar Dronamraju <srikar@...ux.vnet.ibm.com> writes:
> * 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.

OK. That's good info to have in the change log.

>> 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.

Thanks.

cheers