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Subject: Re: [PATCH] cpu-topology: warn if NUMA configurations conflicts with
 lower layer
To:     Morten Rasmussen <morten.rasmussen@arm.com>
Cc:     "Zengtao (B)" <prime.zeng@hisilicon.com>,
        Sudeep Holla <sudeep.holla@arm.com>,
        Linuxarm <linuxarm@huawei.com>,
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        "Rafael J. Wysocki" <rafael@kernel.org>,
        "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>
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From:   Valentin Schneider <valentin.schneider@arm.com>
Message-ID: <f973e77b-9c0a-6506-da97-f7a0ea1829fd@arm.com>
Date:   Sun, 12 Jan 2020 13:22:02 +0000
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On 09/01/2020 10:52, Morten Rasmussen wrote:
>> AFAIA what matters here is memory controllers, less so LLCs. Cores within
>> a single die could have private LLCs and separate memory controllers, or
>> shared LLC and separate memory controllers.
> 
> Don't confuse cache boundaries, packages and nodes :-)
> 
> core_siblings are cpus in the same package and doesn't say anything
> about cache boundaries. It is not given that there is sched_domain that
> matches the core_sibling span.
> 
> The MC sched_domain is supposed to match the LLC span which might
> different for core_siblings. So the about example should be valid for a
> NUMA-in-package system with one package containing two nodes.
> 

Right, the point I was trying to make is that node boundaries can be pretty
much anything, so nodes can span over LLCs, or LLCs can span over nodes,
which is why we need checks such as the one in arch_topology() that lets us
build up a usable domain hierarchy (which cares about LLCs, at least at some
level).

> Morten
>